2019-094 2019 Mosquito SeasonDate: May 3, 2019 Report No. 2019-094
INFORMAL STAFF REPORT TO MAYOR AND CITY COUNCIL SUBJECT: Provide information concerning the upcoming 2019 Mosquito Season EXECUTIVE SUMMARY: The City of Denton’s mosquito season officially starts the first week in May. Mosquito monitoring for 2019 will begin May 6, and will continue on an approximately weekly basis until the end of the season in late fall. Monitoring provides information on mosquito populations and the presence of mosquito-borne viruses. Staff uses this information to establish risk levels
in accordance with the protocols in the City of Denton’s Mosquito Surveillance and Response Plan, and each Risk level triggers a specific public communication and mosquito control efforts. Although the program is designed to assess and mitigate the risk from West Nile Virus, staff continues to devote efforts to assessing risks from emerging mosquito borne diseases such as dengue fever, chikungunya virus, and Zika virus.
BACKGROUND: A copy of the City of Denton’s Mosquito Surveillance and Response Plan (MSRP) is provided as an exhibit. The MSRP outlines a systematic approach for tracking mosquito populations and assessing risk. This approach is based on mosquito population dynamics, detection of viruses in
captured mosquitoes, and human case information. The MSRP also includes public information, larviciding activities, and procedural issues associated with spraying for adult mosquito control (“adulticiding”). This approach ensures appropriate response levels by combining principles of integrated pest management, data from a comprehensive mosquito monitoring network, and procedural issues associated with a given risk level.
The MSRP includes 5 Risk Levels, with each level indicating progressively greater human health risks from WNV. Each Risk Level is enacted when specific trigger conditions are met. Once enacted, the Risk Level outlines a series of responses that include recommendations for mosquito surveillance, public information and education, and mosquito control measures. Responses
increase in scope and magnitude as Risk Levels increase. Although the MSRP is systematic in its approach, it is important to note that the MSRP is only intended to provide a decision-making framework for a complex human health issue involving many interacting factors, including the biology / ecology of mosquitoes, birds, and humans, as well as weather patterns. Flexibility is built into the MSRP to account for these variables / uncertainties.
Risk levels in the current plan are: Risk Level 1 Normal Response: Probability of human outbreak is remote Risk Level 2 Enhanced Response: Probability of human outbreak is low
Risk Level 3 Public Health Concern: Probability of human outbreak is low to moderate Risk Level 4 Public Health Warning: Probability of human outbreak is moderate to high Risk Level 5 Public Health Alert: Human outbreak is confirmed
Date: May 3, 2019 Report No. 2019-094
Currently, the mosquito season begins at Risk Level 2. Risk Level 1 is based on the condition of “normal mosquito activity with no evidence of disease during the past 3 years, and it is
unlikely that we will see this condition again in the region. Progressively greater levels of public education and outreach, source reduction, and larviciding using Bacillus thuringiensis israelensis (BTI) are the main responses for Risk Levels 2 through 4. The Risk Level 3 trigger is a single WNV positive mosquito sample. Risk level 4 is triggered by multiple mosquito samples collected at different times and locations testing positive for WNV, or a single human
case of WNV. Risk Level 5 is triggered by more than one human WNV case, and states that the City Council will recommend whether to implement ground level ultra-low volume (ULV) adulticiding as a response. The decision to spray is made by a Resolution of the City Council of Denton prior to the first ULV application, for each mosquito season. Based on information
available at the time of the Council decision, the Resolution may also include specific
requirements in addition to those outlined in the plan. Over the last 15 years of monitoring, a single genus of mosquito, Culex, has been responsible for approximately 99 percent of the WNV detected in Denton mosquito samples. A single species
of Culex (Culex quinquefasciatus) represents almost all of the WNV positive Culex mosquitoes
captured in the City. Although the monitoring program is designed to capture all mosquito species, it emphasizes capturing Culex quinquefasciatus as the “target mosquito”. Monitoring continues approximately weekly through the end of the season, which usually occurs in late October to mid –November. Monitoring is comprised of two components: “routine monitoring”
and monitoring for local WNV testing.
All mosquitoes collected during routine monitoring events are shipped to the Texas Department of State Health Services (TDSHS) for species identification and virus screening (which includes WNV, Dengue, Chikungunya, Zika, and several other virus). In 2013, the City of Denton and
University of North Texas (UNT) researchers implemented an additional monitoring and testing
program using a Rapid Analyte Measurement Platform (RAMP) system. The RAMP system provides more rapid and flexible local West Nile Virus testing capabilities. Although the services provided by the TDSHS are extremely valuable to the overall monitoring program, due to the State’s workload it can take a week or more before virus testing results are finalized and
reported. The RAMP approach allows local researchers to produce WNV test results within one
day of collection, which is extremely beneficial for quickly adjusting local responses. In a “normal” season, 16 traps are deployed each week for routine monitoring, and 8 traps are deployed each week to collect mosquitoes for RAMP testing. Trap results are provided as a time
series animated map on the City of Denton web page. The City of Denton web page also
contains a large amount of additional public information, including the current Risk Level, general information on mosquitoes emphasizing the importance of mosquito control around the home and personal protection, various methods for citizens to receive notifications, FAQs, and the MSRP itself.
Date: May 3, 2019 Report No. 2019-094
At the beginning of each mosquito season, a bilingual “general information” campaign covering mosquito and WNV issues is implemented by the Public Communication Office (PCO). The information focuses on what citizens can do to help control mosquito populations and how
citizens can protect themselves and their families. As defined in the MSRP, the PCO performs progressively greater roles as Risk Levels increase. If Risk Level 5 is reached and the City Council authorizes ground based adulticiding activities, the PCO coordinates with Environmental Services, Technology Services, Customer Service, and Emergency Management to perform the crucial functions of informing citizens of the spray event, with the goal of a
minimum 24-hour notification. An outline of the PCO’s plan for this mosquito season is provided as an Exhibit. Information about emerging diseases
In recent years, mosquito-borne viruses from other areas of the world have caused outbreaks in the Western Hemisphere. Mosquito-borne viruses that have been restricted to the tropics for the last several decades are also beginning to move northwards, and are concerns for outbreaks in the southern portions of the United States. Chikungunya virus, for example, has become firmly established in the Caribbean, and there have been cases of locally-transmitted Chikungunya virus
recently observed in Florida. Dengue is a serious disease of the Americas, Asia and Africa that has become more common in south Texas and the Florida Keys in recent years. Zika virus has been responsible for outbreaks in tropical areas throughout the world for many years, and was found in the Western Hemisphere in 2015. The progression of this virus has
been extensively covered by the media, and the rapid spread of the disease and possibility of transmission from human to human without mosquito vectors is particularly troubling. Normally, Zika virus infection is mild, and only about 1 in 5 people infected with the virus will feel ill. Symptoms are typically mild fever, joint pain, conjunctivitis, and rash. However, recent analyses suggest a link between Zika virus infections in pregnant women and
microcephaly, and a possible link between Zika infection and Guillain-Barre syndrome. The transmission pathways for Zika virus and WNV are quite different, as is the behavior of the major vectors for each disease. Vector borne Zika virus transmission is predominantly spread by Aedes aegypti mosquitoes, although Aedes albopictus is also a possible vector. West Nile Virus
is a disease that is carried by birds, and is primarily spread between birds and mosquitoes. The main WNV vector, Culex quinquefasciatus, is fairly non-selective for its host, and will feed on both birds and humans. Culex quinquefaciatus also feeds most actively from dusk into the night. Under the right circumstances, the disease can amplify between mosquitoes and birds. Human WNV infections tend to occur when the infection rate is high enough in the mosquito population
to “spill over” into humans through non-selective feeding by the infected mosquitoes. Zika virus, however, is a disease that is spread between mosquitoes and humans, and can also be spread from human to human through sexual contact. Aedes aegypti, the predominant vector for Zika, greatly prefers human hosts. Since Aedes mosquitoes also tend to be active throughout the day and into the evening, the likelihood of getting bitten during the daytime is much higher for
Aedes compared to Culex. Therefore, although both viruses can be spread by mosquitoes, the source and mode of transmission of the virus is distinctly different. If humans in the infective stage are not present in the population, there will be no opportunity for transmission of Zika
Date: May 3, 2019 Report No. 2019-094
virus to mosquitoes. Likewise, because Zika virus can be spread from person to person through sexual contact, the disease could have a high incidence rate in the human population without having a high incidence rate in mosquitoes. WNV, however, tends to amplify greatly between
birds and mosquitoes before it reaches high enough rates of infections in mosquitoes to start spilling over into the human population. Because of this amplification, mosquito surveillance and testing can provide valuable information on WNV risks. However, similar surveillance and testing is currently of limited usefulness for Zika virus risk assessment in the United States due to the differences in mode of transmission and viral amplification outlined above. Although the
Center for Disease Control (CDC) does not recommend mosquito testing as a risk prediction tool for Zika Virus surveillance, Denton County has put together a grant funded program to complete limited monitoring that targets Aedes mosquitoes and is specifically designed for assessing the prevalence of Zika virus. The City of Denton is participating in this program. As more
information is collected and evaluated, it is possible that the City of Denton Mosquito
Surveillance and Response Plan will be modified to address these emerging diseases. CONCLUSION: This update is being provided for informational purposes. Staff will be glad to provide
additional information to the Council upon request. ATTACHMENT(S): 1. 2019 Mosquito Surveillance and Response Plan
STAFF CONTACT:
Deborah Viera Assistant Director of Environmental Services 940-349-7162
Mosquito Surveillance and Response Plan
City of Denton, Texas
2019 SEASON
prepared by Kenneth E. Banks, Ph.D.
Department of Environmental Services and Sustainability
City of Denton
TABLE OF CONTENTS:
1.0 Introduction …………..…………………………………………………… 1
2.0 Objectives ….…………….....……………..………………………………. 2
3.0 West Nile Virus Concerns ....……………..………………………………. 2
4.0 The Role of Residents ....……………..…………………………………….. 4
5.0 Denton's Plan for Mosquito Surveillance ………….………………….…... 4
5.1 Adult mosquito collections ....………...……...………….… 6
6.0 Mosquito Control Strategies ...…………………………........................…. 6
6.1 Larviciding vs. adulticiding ..………...……...………….… 8
6.2 Rationale for different treatment methods .......………….… 9
6.3 Integrated Pest Management ………………...........…….…. 9
7.0 Denton's Plan for Mosquito Control ...…………………………………...... 10
7.1 Adulticiding ......................................................................... 10
7.1.1 Triggers for adulticiding .......................................... 10
7.1.2 When to use adulticides ........................................... 11
7.1.3 Where to use adulticides .......................................... 11
7.1.4 Human Population density considerations ................ 11
7.1.5 Mosquito population considerations ......................... 11
7.1.6 Local perspectives on spraying ................................. 12
7.1.7 Denton's plan for mosquito adulticiding operations . 12
7.2 Plan for Public Education Concerning Mosquitoes .............. 13
7.3 City of Denton Response Plan .............................................. 14
7.3.1 Risk Level 1 - Normal Response .............................. 15
7.3.2 Risk Level 2 - Enhanced Response ........................... 16
7.3.3 Risk Level 3 - Public Health Concern ....................... 17
7.3.4 Risk Level 4 - Public Health Warning ...................... 18
7.3.5 Risk Level 5 - Public Health Alert ......................... 19
8.0 Emerging diseases …………....…………………………………................ 20
References ..............................................………………………………………...... 22
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1.0 INTRODUCTION
Mosquitoes are insects that belong to the order Diptera, or True Flies. Female
mosquitoes have modified mouthparts that form a long piercing-sucking proboscis, while
male mosquitoes have mouthparts that are incapable of piercing skin. There are over
2,500 different species of mosquitoes that have been identified throughout the world,
with approximately 150 species occurring in the United States. The Texas Department of
Health estimates that there are approximately 82-84 mosquito species in the State of
Texas, although only about 12 of these mosquito species have been implicated in the
transmission of serious diseases.
Mosquitoes typically need still, stagnant water that is isolated from fish or other small
predators to complete their metamorphosis from egg to adult. Larval habitats can range
from marshes, freshwater wetlands, and tree holes to human-made structures like
catchments, drains, gutters, and discarded tires. Not all species feed on humans and
other mammals, and many species feed mostly on birds, amphibians, or reptiles. Only a
small percentage of the known mosquito species are considered to be diseases vectors.
Although only a small percentage of mosquito species are capable of transmitting
diseases, mosquitoes are still considered to be a very important vector for disease
transmission. Within the United States, the occurrences of mosquito-borne illnesses
have been relatively rare in recent years. However, epidemics of mosquito-borne
diseases were once common in the United States. Outbreaks of Yellow Fever have been
recorded as far north as Philadelphia during the Colonial Period, and Dengue fever was
prevalent along the Gulf Coast until the mid-1940s. At one time, malaria was well
established in the continental United States, especially in the south. Other mosquito-
borne illnesses like LaCrosse, St. Louis, and Eastern Equine Encephalitis are still threats
in certain areas of the country. Although many of these historical mosquito-borne
diseases have been eliminated or at least controlled, the introduction and subsequent
rapid spread of West Nile Virus within the United States is a topic of current concern.
Mosquitoes may be controlled through a variety of different physical, chemical, and
biological methods. Physical methods usually involve source reduction, which is simply
the physical removal of mosquito breeding habitats. Biological measures mainly center
on the use of bacteria that kill mosquito larvae or the use of natural mosquito predators.
Chemical treatment typically involves the application of pesticides to attempt to control
larva or adult mosquito populations.
Mosquito control pesticides are applied by various means, depending on the type and size
of the area being treated. No matter how pesticides are applied, however, pesticides
have the potential to impact non-target species, including humans. Depending on the
pesticides used, potential impacts may include acute or chronic toxicity, reproductive and
/ or developmental effects, and indirect effects through the food chain. The tests
required for pesticides registration also may not address non-target impacts.
Information regarding the effects of inert ingredients in pesticide formulations, food
chain effects, multi-generational effects, and the interaction of specific pesticides with
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other chemicals in the environment may also be lacking. It is therefore important to
realize that there are risks associated with the use of pesticides just as there are risks
associated with the potential for human or animal infection by a mosquito-borne illness.
The purpose of this response plan is to provide a systematic way to consider and balance
the risks associated with mosquito control measures.
2.0 Objectives
The City of Denton Mosquito Surveillance and Response Plan was developed to meet
several objectives. Specifically, the Plan:
Provides guidelines and information on mosquito populations, prevalence of
diseases, and control strategies;
Provides a systematic approach for using mosquito sampling and human disease
data to establish Risk Levels;
Establishes actions that will be undertaken for each Risk Level;
Provides municipal staff and elected officials with a decision-support system;
Outlines the roles and responsibilities of municipal staff and elected officials.
3.0 West Nile Virus (WNV) Concerns
West Nile Virus was first recorded in North American during August 1999 shortly
following the Center for Disease Control and Prevention (CDC) and the New York City
Department of Health responses to an unusual outbreak of encephalitis in northern
Queens, New York. The cause of the observed illnesses was determined to be West Nile
virus. During 1999, approximately sixty people were diagnosed with West Nile virus
and seven elderly residents died from the infection. During the summer of 2000, WNV
activity was detected again in New York City and appeared to be spreading. Fourteen
people in the City of New York were diagnosed with acute WNV infection, six were
identified in New Jersey, and 1 was reported from Connecticut. Outside of New York,
WNV was detected in 12 states and in the District of Columbia. During 2001, the virus
appeared again in New York City, causing seven cases that required hospitalization. In
2001, a total of 66 human cases with nine fatalities were reported throughout the nation,
mostly spread throughout the eastern United States. During 2002, there were 4,156
laboratory-positive human cases and 284 deaths. The number of cases increased
dramatically in 2003 as the disease spread westward, with 9,862 cases and 264 deaths.
After 2003, annual national case counts have ranged from a low of 712 during 2011 to a
high of 5,674 in 2012. In 2012, the highest incident rates in the nation were observed in
the North Texas region, including Denton County. Fortunately, subsequent seasons have
been relatively mild when compared to 2012.
The relatively rapid spread of West Nile virus and the increase in disease incidence
indicates that WNV is permanently established in the United States. It is likely that the
virus survives the winter either within birds that remain in the area or within mosquitoes
that survive the winter. When spring returns, the virus can amplify within birds and is
readily passed to early season mosquitoes. As mosquito populations increase,
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mosquitoes begin to feed more frequently on birds, causing an increasing number of birds
and mosquitoes that are infected. If environmental conditions are favorable for
transmission, the virus will amplify to a theoretical point of spillover. At spillover, the
virus can bridge out of the bird-mosquito cycle through mosquitoes that feed on birds,
humans, and other animals. At the point of spillover, transmission to humans becomes
more likely. Currently, mosquito bite avoidance and mosquito control are the only
practical method of protecting the human population from WNV infection. There are no
known specific treatments or cures for the disease, and vaccines are currently not
available for public use. The virus also kills a variety of native and non-native birds, and
may have impacts on other wildlife.
Weather plays a major role in the prevalence of WNV, and the relatively warm winter
and spring experienced in North Texas during 2011 to 2012 may have been a significant
factor in the large amount of disease present during the 2012 mosquito season.
Understanding the types of mosquitoes in the area that transmit the disease is also a
crucial consideration for directing control responses and public education activities.
Since not all mosquitoes carry the virus, enacting mosquito control efforts that attempt to
non-selectively reduce all mosquito populations will not necessarily reduce the risk of
WNV. In fact, based on 15 years of mosquito monitoring and disease analyses in the
City of Denton, there appears to be only one mosquito that serves as a vector for this
disease. Approximately 94 percent of the WNV infected mosquitoes captured in
Denton’s monitoring program over the last 15 years have been Culex quinquefasciatus,
commonly known as the southern house mosquito. Approximately 99% of the
mosquitoes that have tested positive for WNV during the last 15 years of monitoring have
been some type of Culex species. Control efforts enacted by the City are therefore
focused on Culex species, and are particularly focused on Culex quinquefasciatus.
The past experience of numerous mosquito control districts suggests that a mosquito
control program should be based on the principals of Integrated Pest Management (IPM).
The principals of IPM are:
knowledge of mosquito biology and the epidemiology of the mosquito-borne
diseases;
surveillance and monitoring efforts for the detection and status assessment of
mosquito populations and / or mosquito-borne diseases;
a multifaceted prevention and control program comprised of a system of control
tactics which are compatible with each other and which are proven effective;
continue program evaluations and updates to ensure that the best methods are
being used to meet the prevention and control objectives of the program; and
continue education of the public to create awareness, understanding, and support.
These general guidelines have been used to develop the threshold-level responses of this
surveillance and response plan.
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4.0 The Role of Residents
Residents can play an important role in reducing the number of adult mosquitoes by
eliminating standing water that may support the development of mosquito larva and pupa.
For example, residents can properly dispose of discarded tires, cans, buckets, maintain
pools correctly, unclog blocked gutters and drains, dump water from bird baths and pet
dishes at least every 2-3 days, ensure that air conditioning condensate is not pooling for
several days, control irrigation so that standing water is not produced, irrigate so that
runoff is not produced, and perform similar activities around homes and businesses.
Water that cannot be eliminated should be treated with a biological mosquito larvicide
such as Bacillus thuringiensis israelensis (Bti) in accordance with manufacturer’s
recommendations.
Residents should be aware that Culex quinquefasciatus, the target mosquito for control,
tends to prefer to lay its eggs in “artificial” sources of water such as those listed above, as
opposed to laying eggs in wetland areas, streams, and ponds. Generally, this mosquito
cannot fly long distances, so emerging adult mosquitoes often remain near the habitat
they lived in as larva and pupa. Adults will rest during the daytime in vegetation,
particularly in areas where humidity is high. Irrigated urban and suburban landscapes
can offer excellent habitat, especially if there is standing water present. Underground
storm drains that receive small amounts of runoff from irrigation are also good habitat for
these mosquitoes. As a result, Culex quinquefasciatus tends to become more associated
with urban and suburban areas during the summer months. Often these same areas will
also have birth baths, bird feeders, trees, and other vegetation that serve to attract birds,
especially during the drier times of the year when food and water become scarce. As a
result, the opportunities for WNV disease amplification in urban and suburban areas can
become more pronounced during summer months. The role of residents and business
owners in controlling larval habitat is therefore a crucial component of reducing local
WNV risk.
5.0 Denton’s Plan for Mosquito Surveillance
The risk of mosquito-borne diseases depends on the size of mosquito populations and the
incidence rate of disease, as well as the likelihood of the mosquito population changing in
response to climate factors. Although mosquito abundance can be estimated through
collection of either immature or adult mosquitoes, adult mosquito abundance is a key
factor contributing to the risk of virus transmission. For this reason, the City of Denton
monitoring program primarily focuses on collecting adult mosquito populations for
targeting control measures and gauging the potential for disease outbreak. The current
mosquito surveillance program is a partnership between the City of Denton and the
University of North Texas. This surveillance program collects adult mosquitoes through
the use of both CDC light traps and gravid traps. Captured mosquitoes are sent to the
Texas Department of State Health Services for testing. Each sample (sometime referred
to as a “pool”) consists of female mosquitoes that are collected at a specific collection
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site, in one of the two types of trap. The information obtained from these surveillance
efforts is used to map mosquito populations, provide public information, and to determine
the incidence of WNV or other arbovirus.
The mosquito surveillance program allows analysts to map potential mosquito breeding
grounds and areas of persistent disease. Using this information, more targeted efforts
towards habitat disruption, source reduction, larviciding operations, and other control
mechanisms are possible. An effective, broadly applied surveillance program can allow
analysts to detect the presence of WNV of other mosquito-borne viruses during the
amplification phase. If targeted mosquito controls are implemented at the amplification
stage, the likelihood of bridging can be minimized, thus reducing the risk of human
transmission. Depending on weather patterns and monitoring results, trapping can be
expanded or contracted and the trapping season may be lengthened or shortened. If
control measures are applied, trapping data may also be of some use for evaluating the
effectiveness of control measures.
The overall goal of the mosquito surveillance program is to use data on mosquito
populations and mosquito virus infections rates to:
assess the threat of human disease;
determine the geographical areas of highest risk;
determine the need for intervention events, and the timing of these events;
identify larval habitats that are in need of targeted control;
monitor the effectiveness of control measures; and
develop a better understanding of transmission cycles and potential vector
species.
Adult surveillance methods should:
Use both fixed and flexible trap locations if possible
o Fixed positions allow for the development of a database so year to year
comparisons are possible;
o Flexible sites allow for responses to epidemiological and natural events.
Use more than one trapping method (CDC light traps, gravid traps, etc...)
o Testing data collected in the City of Denton’s monitoring program
suggests that if resource limitations require a choice between CDC light
traps or gravid traps, gravid traps are preferred. These traps have been
more effective in collecting Culex quinquefasciatus, and more effective
for establishing the prevalence of WNV.
Account for different influencing factors, including:
o Habitat size and diversity;
o Resource availability;
o Proximity to human population centers and / or recreational areas; and
o Flight ranges of target mosquito vector(s).
Advantages of using adult mosquito surveys include:
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The ability to provide early evidence of viral activity in an area;
Helpful for determining if viral activity is local and / or restricted to a few areas;
Providing information on potential mosquito vector species
Providing estimates of vector species abundance and temporal prevalence;
Providing information on virus infection rates for different mosquito species;
Providing information on potential risks to humans and animals;
Providing baseline data that can be used to guide emergency controls.
Disadvantages of using adult mosquito surveys include:
The amount of labor required, and associated expenses;
The need for substantial expertise to ensure proper collection and handling of
mosquitoes;
The delays associated with receiving mosquito arbovirus screening results;
The fact that collectors may be at risk from mosquito bites (although using
personal protection methods can minimize risks).
5.1 Adult mosquito collection
The adult mosquito surveys implemented by the City of Denton in conjunction with the
University of North Texas are designed to determine the relative abundance of various
species present during the sampling period as well as the incidence of arthropod-borne
virus/diseases within the captured specimens. Using this information, City of Denton
personnel determines the need for various control measures, and can conduct more
effective searches for larval breeding places, assess the extent of the problem, and
potentially gauge the effectiveness of control measures. Reports concerning the
incidence rates of arthropod-borne virus/diseases and the relative risks to citizens are also
produced. Currently, collections are made using both CDC light traps and gravid traps
deployed simultaneously.
6.0 Mosquito Control Strategies
Human activities can greatly affect the ecology of mosquito populations. Large
concentrations of people or animals, for example, can increase exposure rates and the
probability of disease outbreak. The use of irrigation, development of drainage networks,
elimination of mosquito predators, prevalence of improperly maintained birdbaths and
other water holding containers can increase the numbers of certain types of mosquitoes.
Expanding international trade and travel has increased the chance of introducing new
mosquito species into our areas, as has happened with the Asian tiger mosquito (Aedes
albopictus). The introduction of new diseases is also a concern, as has been seen recently
with both Chikungunya and Zika viruses.
Mosquito control strategies have changed dramatically over the last few decades. Diesel
oil, inorganic poisons, and source reduction using ditching operations were the basic tools
of early mosquito control programs. Chlorinated hydrocarbons, organophosphates,
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pyrethroids, monomolecular oils, bacteria, and natural predators are more recent
additions to mosquito control efforts. With the growth of ecological consciousness and
environmental science, people began to realize the environmental damage that
accompanied the use of broad-spectrum chemical control agents, particularly those that
did not readily break down in nature. Concerns were also raised because many mosquito
populations also appeared to develop resistance to the more commonly used chemical
control agents.
Over the past few years, major advances have been made in the areas of biological
mosquito control. Biological control strategies may include using natural predators like
Gambusia affinis (Mosquito fish), fungi, protozoans, round worms, flat worms, and
bacterial agents such as Bacillus thuringiensis israelensis (Bti). Each biological control
agent has certain benefits and restrictions. In order to use a biological control agent
successfully, the applicator must have a basic knowledge of biology associated with the
control agent. Some biological control mechanisms, for example, are limited by salinity,
temperature, or organic pollution and some mosquito species are much more susceptible
to specific types of biological control agents. All of these factors must be considered
when choosing and applying biological control agents.
The perfect pesticide is one that is easily applied, reasonably inexpensive, not toxic to
non-target organisms, and that eliminates the pest quickly before it becomes a threat.
Although no single pesticide can combine all of these factors, certain types of Bacillus
bacteria have been developed into pesticides that are very close to the perfect pesticide
model. Bacillus thuringiensis israelensis (Bti), for example, is a naturally occurring soil
bacterium that produces a poison capable of killing mosquito larvae. Bti is considered
ideal for mosquito management because of its specificity for mosquito larvae and because
of the minimal to no toxicity to non-target organisms. These bacteria form reproductive
cells, called endospores, which enable them to survive in adverse conditions. The
endospores of Bti also contain crystals of an insecticidal protein toxin called delta
endotoxin. Once ingested by a mosquito, the alkaline conditions of the stomach dissolve
the crystal and release the delta-endotoxin. The toxin has an affinity for the stomach wall
lining causing the cells to first swell then rupture. When enough stomach cells burst, the
mosquito larvae is unable to effectively digest food. Once affected, larvae stop eating
and rapidly die. Currently, Bti is commercially available in powder, liquid, granular,
capsule, and “briquette” formulations.
Agencies applying pesticides directly to waters of the United States, or where deposition
may enter waters of the United States, are required to obtain a National Pollutant
Discharge Elimination System (NPDES) permit from the Texas Commission on
Environmental Quality (TCEQ). This permit is titled “General Permit to Authorize Point
Source Discharge of Biological Pesticides and Chemical Pesticides That Leave a Residue
in Water”. Agencies must comply with all applicable provisions of this permit (see
TCEQ General Permit TXG87000). The City of Denton complies with all requirements
necessary to maintain the City’s permit under TXG87000.
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6.1 Larviciding vs. Adulticiding
In the past, many mosquito control programs have relied heavily upon adult mosquito
controls using chemical agents. In certain areas, routine mosquito spraying has been an
integral component of control strategies. However, even near-continuous exposures to
pesticides may not kill all mosquitoes. Those mosquitoes genetically able to resist higher
pesticide concentrations may survive and pass on this resistance to future generations.
Eventually, the pesticide becomes less effective as resistance increases in the mosquito
populations.
Resistance can be minimized through the "Management by Moderation" approach.
Management by moderation is an attempt to prevent the onset of resistance by:
using doses that are no lower than the lowest recommended application rate to
avoid genetic selection of resistance genes;
using less frequent applications;
using chemicals of short environmental persistence;
avoiding the use of slow release formulations;
avoiding using the same class of pesticides to control both adults and immature
stages;
applying pesticides to only hot spots. Area-wide treatments should only be
considered during imminent public health threats;
leaving certain generations, populations, or population segments untreated; and
establishing action thresholds that accentuate control mechanisms other than
chemical control of adult mosquitoes.
Although management by moderation is a viable means of minimizing resistance, there
are other environmental and human health concerns associated with the application of
pesticides for adult mosquito control. Routine mosquito spraying, for example, has the
potential to expose the public to pesticides. Depending upon the pesticide used, effects
on humans are possible and may be more likely for people who already suffer from
asthma or other respiratory problems. If proper safety precautions are not followed,
applicator personnel may also be in danger of overexposure. Managers must therefore
decide whether mosquito populations and the related threat of disease transmission or the
chemicals used to control mosquitoes represent the bigger threat to humans.
All mosquitoes begin their lives in water. Prime breeding sites include discarded tires
left outdoors, poorly maintained bird baths, clogged rain gutters, unused swimming pools
and plastic wading pools, pet dishes, or any other container capable of holding water for
more than a few days. Mosquito breeding can therefore be prevented by either
eliminating the source of water (source reduction) or by killing larvae (larviciding).
Larviciding programs use a combination of source reduction, biological, and possibly
chemical measures to control mosquito larvae before they develop into biting adults. If
properly implemented, this strategy can be the most effective, economical and safest
method for mosquito control because mosquito larvae are minimized, thus reducing the
need for adult mosquito control and subsequently reducing the impacts of control
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measures on non-target organisms. Larviciding programs also offer the opportunity to
use biological controls, which minimizes the impacts of the control program on non-
target organisms and lessens the risk of chemical exposures to the public. Using
biological controls also minimizes the chance of pesticide resistance in the mosquito
populations. Experience suggests that the most effective and economical way to reduce
mosquito populations is by larval source reduction through a locally funded abatement
program. The goal of this program should be to monitor mosquito populations and
initiate controls before diseases are transmitted to humans or domestic animals (CDC,
2001).
When larval control is not possible or more immediate control measures are needed, adult
mosquito control may be required to diminish populations of infected mosquitoes and
interrupt viral transmission. Adult mosquito control products may be applied using
ground based equipment, fixed wing aircraft, or helicopters. In all cases, products
should be applied in ultralow volume (ULV) formulations and doses, and should be
comprised of pyrethroids.
6.2 Rationale for different treatment methods
Chemical usage should not be viewed as a long-term control strategy, and should be only
implemented when there are occasional episodes of heavy uncontrolled breeding
concurrent with a high degree of public health risk. This strategy is most appropriate
because prolonged use might lead to the development of resistance in mosquito
populations, thereby limiting overall management options (Tabashnik, 1990). The most
efficient and effective program is one in which mosquito larvae are prevented from
becoming biting adults through the use of biological control agents. The bacterium Bti or
the related bacterium Bacillus sphaericus (Bs.) are considered to be the among the most
environmentally acceptable commercially available biological control agents because of
their relative specificity for mosquitoes and negligible toxicity for vertebrates (Rishikesh
et al., 1983). Larvivorous fish can also be a valuable component of an integrated control
program, either alone or together with chemical control agents (Walton et al., 1990;
Walton and Mulla, 1991; Reed et al., 1995). For these reasons, the City of Denton relies
heavily on source reduction and larviciding for mosquito control.
6.3 Integrated Pest Management
Integrated pest management dictates that control efforts should be dependent on threshold
levels. This means simply that a certain defined risk needs to exist before particular
control measures are recommended. Levels of risk are based on knowledge of mosquito
biology, the epidemiology of the mosquito-borne diseases, and monitoring efforts for the
status assessment of mosquitoes and / or mosquito-borne diseases. Risks levels are then
used to design multi-tactic prevention and control program that are comprised of a system
of tactics which are compatible with each other and which are proven for their
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effectiveness. Continual program evaluations and updates ensure that the best methods
are being used to meet the prevention and control objectives of the program, and
continued public education is used to create awareness, understanding, and support.
Frequent mosquito population assessments allow analysts to map potential mosquito
breeding grounds and determine overall disease transmission risks. Using this
information, more targeted efforts towards habitat disruption, source reduction,
larviciding operations, and other control mechanisms are possible.
7.0 Denton's Plan for Mosquito Control
The primary objective of mosquito control is to decrease the risk of mosquito-borne
human diseases. This objective should be accomplished by:
Stressing source reduction as a viable means of control, both by residents and on
municipal properties, including enforcement actions for stagnant water located
on private property;
Aggressively larviciding where such activities are feasible, practical, and likely
to be effective. This includes providing limited supplies of larvicides to citizens
for use on private property.
Promoting the use of personal mosquito protection measures, especially for the
elderly and those individuals with compromised immune systems, through
public education and outreach.
Providing public information so that citizens are informed about the current Risk
Level, areas of the City where WNV has been located, current municipal control
measures, and what can be done by the public to help reduce risks.
If warranted, implement adult mosquito control measures through targeted ULV
pesticide applications (adulticiding).
7.1 Adulticiding
Adulticiding should and will be considered a supplemental control measure. The
decision to spray shall be based on the conditions listed (in no particular order) below.
7.1.1 Triggers for adulticides: Adulticiding shall be considered only when there is
evidence of WNV activity at a level suggesting a high probability of human infection. In
general, finding an isolated WNV-positive mosquito pool does not by itself constitute
evidence of an imminent threat to human health and does not warrant adulticiding.
Physicians and laboratories are required by Title 25 of the Texas Administrative Code
(Pt. 1, Ch. 97, Subch A, §97.3) to report cases of WNV infection or positive test results in
humans to the State and County Health Departments. Positive human WNV cases are
investigated by Health Department officials. During an investigation, the Health
Department collects demographic and clinical information from the patient and attempts
to determine date of onset, whether the infection was acquired locally or from a region
outside of the patient’s residence, and whether the infection may have been acquired by a
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non-mosquito route of transmission such as a blood transfusion or organ transplant. To
help aid control measures, the Health Department provides some of this information to
municipal officials within the municipality where the patient resides. Human case
information is used by municipal officials as one of the trigger conditions for the risk
levels outlined in this plan
7.1.2 When to use adulticides: The goal of spraying is to reduce the risk of human
diseases by decreasing the number of target adult mosquitoes as much as possible.
However, today’s pesticides tend to only be effective when physically contacting the
mosquito, which is most likely to happen when mosquitoes are actively flying. This
typically will mean that spraying should be conducted between dusk and dawn. Since
the target mosquito, Culex quinquefasciatus, is thought to be most active soon after dusk,
spraying should be timed to strike the best balance between impacting the target
mosquito during its most active time and minimizing impacts on non-target organisms.
It is also important to realize that weather conditions, including air temperature and wind
speed, have a large influence on the effectiveness of adulticides. Spray events should be
cancelled and rescheduled if conditions are not conducive for effective spraying.
7.1.3 Where to use adulticides: The terrain of the proposed spraying area has a major
impact on the pesticide effectiveness. If there is substantial vegetation, dense shrubbery,
trees, or hedges, pesticide applications can be rendered ineffective. The density of
houses and other physical obstructions can also influence the effectiveness of pesticides.
Applications should be made so that the maximum penetration into these types of
obstructions is achieved. In some cases, a mild breeze may be able to help distribute the
pesticide. Application strategies should consider the meteorological conditions at the
time of application, and use these conditions to help improve application effectiveness if
possible.
7.1.4 Human population density considerations: The human population density in an
area where there is evidence of intense epizootic activity should also be considered. If
the area in question is rural and does not contain many people, the cost and potential risk
associated with spraying may not justify its use. If the area in question is densely
populated, adult mosquito control may be more justified as long as adequate
precautionary measures are taken.
7.1.5 Mosquito population considerations: Information from mosquito surveillance
can be helpful in determining when to conduct mosquito control and to assess the
effectiveness of control measures. It is also important to know the numbers and species
of the vector populations in specific localities. The best way to obtain this information is
through mosquito trapping efforts. The City of Denton formed a partnership with the
University of North Texas to provide the specially trained staff needed for systematic
mosquito trapping efforts.
While trapping efforts are crucial for assessing mosquito populations and the prevalence
of disease, it is important to realize that surveillance specimens requires some processing
time after collection. In the time between the collection date and the date of test results,
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circumstances may have occurred which would alter a decision to spray. For example,
weather conditions may have adversely affected mosquito populations, local mosquito
habitats may have been altered, or larviciding efforts may have reduced the number of
newly emerged adults. All of these occurrences may result in a reduced need to spray,
and should be taken into consideration during control efforts.
Surveillance information should also be evaluated through time to determine the progress
of diseases and the relative risk of disease transmission. Surveillance information may
also be of some use in gauging the effectiveness of control measures, although
establishing control effectiveness is difficult using only surveillance information. In all
cases, the available surveillance information should be evaluated as a part of the decision
making process for enacting adulticidal activities.
7.1.6 Local perspectives on spraying: Different communities have different
perspectives on the benefits and risks associated with adulticiding activities. While these
perspectives are valid and should be considered, individuals are likely to have strong
opinions on either side of the issue. The City of Denton has incorporated a rationale
framework within the mosquito control program that is designed to be protective of both
human health and the environment. The decision to spray, however, is a complex issue
that will likely be faced without complete information. Thus, there will be citizens that
do not believe that the City of Denton has done a good job with regards to reducing
public health and environmental risks, regardless of the decision. The mosquito control
program should therefore remain flexible and should attempt to address citizen
complaints through public education and dialogue.
7.1.7 Denton's plan for mosquito adulticiding operations
Once arbovirus activity is detected and the decision is made to implement mosquito
control using adulticides, the size of the area to treat must be determined. Unfortunately,
there is no simple formula for determining how large of an area to treat, nor is there
adequate information to guide decisions about the degree of vector population
suppression that must be attained, or for how long this suppression must be maintained to
reduce the risk of disease. The CDC (2001) suggests considering the following factors
where deciding the scope of the adulticiding effort:
the general ecology of the area;
the flight range of vectors that are known or are believed to be of importance in
the area;
the population density of the vectors;
the length of time since virus-positive mosquito pools were collected;
the potential risk to the human population (including the age demographics of the
area) as well as the community perception of the relative risk of pesticides versus
the risk of arbovirus infection; and
the season of the year - how much time the transmission risk can be expected to
persist until the vectors enter diapause;
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It is very likely that some of these factors will be unknown or poorly known, and
practical experience in conducting a mosquito control program is needed to refine control
recommendations. If adulticiding operations are conducted, the following parameters
should be monitored:
Weather conditions during the application (temperature, wind speed, wind
direction, etc...);
Flow rate of Ultra-Low Volume applications; and
Overall amount of pesticide applied.
If the application of adulticides is deemed necessary, the public must be informed. The
following actions / activities will take place prior to adulticide applications:
Information, including a map of the proposed spray area, will be released 24
hours in advance through the media and through the city of Denton web site.
Under certain conditions in Public Park areas, applications may take place in less
than 24 hours notice. For these cases, the facility will be closed to the public
during, and a few hours after, application.
Press releases should be prepared and shared with the appropriate media
CodeRed, the Customer Service automated calling systems, and available forms
of social media should be used to inform citizens of spray events. CodeRed will
make an automated phone call with a message to all individuals that have signed
up for the service. The Customer Service automated calling system will make an
automated phone call with a message to all individuals within an area designed
by the spray map. For both systems, it is possible that some residents outside of
the spray area will receive one of these calls. However, these systems are the
most rapid and accurate means of notifying the public of upcoming spray events.
If practical, street signs indicating that spraying will occur in the neighborhood
should be placed at major roadways entering and exiting the targeted area.
7.2 Plan for Public Education Concerning Mosquitoes
Public education is a key component of a successful mosquito control program. Since
the appearance of the West Nile Virus in the United States, the City of Denton has
produced and aired many public service announcements, given numerous presentations,
and provides information on the City web page concerning this disease. Depending
somewhat on Risk level, the following key information will be conveyed to the public:
The location of WNV positive mosquito trap locations and current risk level;
Comprehensive prevention strategies and activities used by the City of Denton to
address the threat of West Nile Virus;
The public will be advised to eliminate standing water sites by removing all
materials that can hold water for longer than 2-3 days; and
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The public will be informed about the symptoms of West Nile Virus (headache,
high fever, muscle pain, weakness, and disorientation).
Techniques used to disseminate information may include any or all of the following:
Televised public service announcements using Denton's local cable channel;
Radio announcements;
Brochures for public use placed in municipal buildings;
Postings describing the current risk level placed in municipal buildings;
Brochures and / or fact sheets to be distributed to community-based organizations,
community boards, elected officials, schools, nursing homes, libraries, outdoor
activity sites, etc.;
Presentations to elected officials and / or community groups concerning the
current risk level, mosquito population and disease status, and mosquito control
activities; and
Press releases describing West Nile virus response activities.
7.3 City of Denton Response Plan
The purpose of this response plan is to:
1. Minimize human illness through public education and targeted vector
control.
2. Provide an assessment of virus transmission risk to humans that can be
used to plan and adjust control activities.
3. Map the density of mosquitoes and the incidence of the virus within the
City of Denton.
4. Identify areas where the incidence of disease is high and provide
appropriate warnings to the citizens of Denton.
5. Identify the key vector species that carry diseases within Denton.
The prevention of West Nile virus, as with many diseases, is most efficiently
accomplished by ensuring that prompt, accurate information reaches the public. If the
appropriate information reaches the public in a timely manner, personal protective
measures may be implemented, and control strategies can be enacted efficiently.
The City of Denton will provide continuous information on the city web page concerning
West Nile virus, including a frequently asked questions (FAQs) section, disease
symptoms, personal preventative measures, and sources of additional information. If a
sampled mosquito pool tests positive for arthropod-borne virus/diseases, information
describing the location of the sampling event, the date, and other pertinent information
will be provided.
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This response plan is partially based on Texas Department of Health recommendations.
The plan is divided into 5 levels based on the risk of human disease. Each risk level is
described below, along with specific recommended responses. In accordance with the
principals of Integrated Pest Management Control measures are recommended for each
level.
Generally, there are 4 surveillance / observational factors that are evaluated to determine
if the trigger condition has been met for a particular risk level. These factors are:
1. Environmental / climatic conditions (mainly temperature, rainfall, and season)
2. Adult Culex quinquefasciatus abundance
3. Virus infection rates in Culex quinquefasciatus and other mosquito vectors
4 Confirmed infections in humans, including epidemiological information.
7.3.1 Risk level 1 - Normal Response
Condition: Probability of human outbreak is none or remote
Trigger: Normal mosquito activity with no evidence of arthropod-borne
virus/disease detected during the past 3 years in vectors, humans, or other
hosts.
Recommended Response:
Surveillance:
Surveillance is optional. Under the normal response, mosquitoes are considered to be
only a nuisance without significant influences on public health. Complaint calls and
informal surveys of larva and adults will be used to direct mosquito control efforts.
Public Information / Education:
Publicize methods for mosquito reduction and personal protection prior to the main
season for mosquito activity and outdoor human activities.
Control Measures
Use public information to promote source reduction and personal protection. Conduct
standard larviciding approaches using Bacillus thuringiensis israelensis (Bti) .
Rationale:
Larviciding and source reduction / elimination are considered to be the most effective
long-term solutions for mosquito control. The control measures are designed to
accomplish mosquito control by preventing larval mosquitoes from becoming biting
adults.
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7.3.2 Risk Level 2 - Enhanced Response
Condition: Probability of human outbreak is low
Trigger: Normal mosquito activity with little or no evidence of arthropod-borne
virus/disease. Enhanced response level is due to recent historical presence
of arthropod-borne virus/disease in vectors, humans, or other hosts within
the vicinity of Denton (approximately 100 miles).
Recommended Response:
Surveillance:
Surveillance is optional, but strongly encouraged. If possible, conduct routine
surveillance of adult mosquito populations using light traps baited with CO2 and gravid
traps (at minimum). Collected mosquito species should be identified to species level and
mosquito pools should be screened for the presence of arboviruses. If positive tests are
obtained during arthropod-borne virus/disease screening, additional tests should be
performed to determine what type of arbovirus is present.
Public Information / Education:
Implement all steps for public information established by the Public Communication
Office for this Risk Level. Generally, this will reflect routine public information
concerning methods for mosquito reduction and the importance of personal protection
measures. This information should be disseminated at the beginning of the mosquito
season.
Control Measures
Use public information to promote source reduction and personal protection. Conduct
standard larviciding approaches using Bacillus thuringiensis israelensis (Bti).
Rationale:
Larviciding and source reduction / elimination are considered to be the most effective
long-term solutions for mosquito control. The control measures are designed to
accomplish mosquito control by preventing larval mosquitoes from becoming biting
adults.
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7.3.3 Risk Level 3 - Public Health Concern
Condition: Probability of human outbreak is low to moderate
Trigger: Arthropod-borne virus/disease isolated from mosquitoes collected during
trapping activities at a single monitoring site.
Recommended Response:
Surveillance:
Continue to conduct routine surveillance of mosquito populations. If resources allow,
increase surveillance in the area where the positive sample was collected. Use
geographic information systems to plot the location of the positive sample and provide
this information to the public via the City of Denton web page.
Public Information / Education:
Implement all steps for public information established by the Public Communication
Office for this Risk Level, which includes public notification, press releases, and City of
Denton web page updates with a map of the positive trap location. Notify medical
professionals, veterinarians, and all those registered with the City of Denton’s various
public notification options of the presence of the disease.
Control Measures
Use public information to promote source reduction and personal protection. Conduct
enhanced larviciding using Bacillus thuringiensis israelensis (Bti). Increase efforts in
areas where positive mosquito pools were detected. Control measures will be
implemented based on the following IPM criteria: time of year, the extent of previous
mosquito control activities, the current level of mosquito activity, weather conditions, and
surveillance results.
Prepare for ground level spraying activities using ULV applications of pyrethriods.
Ensure adequate funding and personnel are in place for conducting ULV applications.
Rationale:
Larviciding and source reduction / elimination are considered to be the most effective
long-term solution for mosquito control. Increasing these measures in areas where
positive mosquito pools are detected offers an additional degree of risk reduction.
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7.3.4 Risk Level 4 - Public Health Warning
Condition: Probability of human outbreak is moderate to high
Trigger: Multiple mosquito pools collected at different times and locations test
positive for arthropod-borne virus/diseases.
Single human case confirmed with laboratory testing
Recommended Response:
Surveillance:
Continue to conduct routine surveillance of mosquito populations. Increase surveillance
in areas where WNV positive samples have been collected if resources are available. Use
geographic information systems to plot the location of positive mosquito samples and
provide this information to the public via the City of Denton web page.
Public Information / Education:
Implement all steps for public information established by the Public
Communication Office for this Risk Level, which includes public notification,
press releases, and City of Denton web page updates with a map of the WNV
positive trap location(s).
Update web page map for all subsequent WNV positive trap locations
Notify medical professionals, veterinarians, public health officials, and all those
registered with the City of Denton’s various public notification options of the
presence of the disease.
Control Measures
Use public information to promote source reduction and personal protection. Intensify
enhanced larviciding efforts using Bti in targeted areas. Consider using Spinosad
products in areas where positive mosquito pools were detected, if applicable. Control
measures will be implemented based on the following IPM criteria: time of year, the
extent of previous mosquito control activities, the current level of mosquito activity,
weather conditions, and surveillance results.
Finalize all actions needed for implementing ground level spraying activities using ULV
applications of pyrethroids
Rationale:
Larviciding and source reduction / elimination are considered to be the most effective
long-term solutions for mosquito control. Increasing these measures in areas where
positive mosquito pools are detected offers an additional degree of risk reduction.
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7.3.5 Risk Level 5 - Public Health Alert
Condition: Human outbreak is confirmed
Trigger: Multiple human cases. Cases must be confirmed by laboratory testing.
Recommended Response:
Surveillance:
Continue to conduct routine surveillance of mosquito populations. If resources allow,
increase surveillance in areas were arthropod-borne virus/disease positive samples were
collected. Use geographic information systems to plot the location of WNV positive
mosquito samples, and provide this information to the public.
Public Information/Education:
Implement all steps for public information established by the Public
Communication Office for this Risk Level, which includes public notification,
press releases, and City of Denton web page updates with a map of the WNV
positive trap location(s).
Update web page map for all subsequent WNV positive trap locations
Notify medical professionals, veterinarians, public health officials, and all those
registered with the City of Denton’s various public notification options of the
presence of the disease.
Increase public education, emphasizing source reduction, personal protection, and
disease symptoms.
Control Measures:
Use public information to promote source reduction and personal protection. Intensify
larviciding efforts using Bti in targeted areas. Additional larvicides containing IGRs or
pesticides such as Spinosad may be applied at this time. Intensely treat standing water in
City Parks. If the threat to human health is considered imminent, consider targeted
adulticide applications using ground-based Ultra-Low Volume (ULV) equipment. The
decision to spray will be made by a Resolution of the City Council of Denton prior to the
first application, for each mosquito season. Based on information available at the time
of the Council decision, the Resolution may include specific requirements in addition
those outlined in this plan. ULV applications should be implemented in the vicinity
(approximately 1 mile) of areas where positive human cases were detected. When ULV
applications are initiated, notification will be provided to the public as outlined in Section
7.1.
During 2012, the City of Denton utilized Duet, a synthetic pyrethroid product with an
additional chemical designed to induce “benign agitation” of mosquitoes. The purpose
of benign agitation is to increase the likelihood of mosquitoes flying during the
application, which increases application effectiveness. The Duet product is also mineral
oil based, versus water based, which likely increases the compound’s effectiveness in the
20
higher air temperatures typical during summer applications. Mineral oil based
compounds may also be less likely to affect non-target organisms in aquatic
environments because of the lack of solubility of the oil based compound in water.
Control measures will be implemented based on the following IPM criteria: time of year,
the extent of previous mosquito control activities, the current level of mosquito activity,
weather conditions, the species of mosquitoes that test positive for disease, the number of
local mosquito pools which test positively for disease, the likely time until a killing frost,
the density of roads or other access to mosquito breeding grounds, and the density of
human populations. Epidemiological data associated with reported human cases of
WNV will also be considered. In general, if an area has been sprayed, that area will not
be sprayed again in response to an additional human case unless the date of onset of the
new case indicates exposure occurred after the previous spraying event. Areas will also
not be sprayed in response to a reported human case if epidemiological information
clearly indicates that exposure did not occur in the City of Denton. If public health
emergencies are declared at the county or state level, the recommended responses
associated with the declaration will take precedence over the control plan of the City of
Denton.
Rationale:
Larviciding and source reduction/elimination are considered to be the most effective
long-term solutions for mosquito control. Increasing these measures in areas where
positive mosquito pools are detected offers an additional degree of risk reduction.
However, at this Risk Level, applications of adulticides may be needed to rapidly reduce
mosquito populations and halt disease transmission. The objective is to kill a high enough
proportion of older adult mosquitoes to break the disease transmission cycle. If adult
mosquito controls are implemented, the approaches for adulticiding outlined in this plan
are effective and are designed to minimize adverse effects to the environment and non-
target organisms.
8.0 Emerging Diseases
In recent years, mosquito-borne viruses from other areas of the world have caused
outbreaks in the Western Hemisphere. In addition, mosquito-borne viruses that have
been restricted to the tropics for the last several decades are beginning to move
northwards, and could cause concerns for outbreaks in the southern portions of the United
States. Chikungunya virus, for example, has become firmly established in the
Caribbean, and two cases of locally-transmitted Chikungunya virus were observed in
Florida in 2014. Dengue is a serious arboviral disease of the Americas, Asia and Africa
that has become more common in south Texas and the Florida Keys in recent years.
Zika virus has been responsible for outbreaks in tropical areas throughout the world, and
was found in 2015 for the first time in the Western Hemisphere. The rapid spread of this
disease and the possibility of transmission from human to human without mosquito
vectors is particularly troubling. To date, local transmission of Zika virus in Texas has
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only been documented in Brownsville. However, the City of Denton has experienced
human cases of Zika virus, and has developed a response protocol in coordination with
the Denton County Public Health Department to ensure that appropriate measures are
taken to minimize the likelihood of transmission of the virus from the infected person to
local mosquito populations. The City of Denton also documented two cases of
California Seropositive viruses (a group of viruses, some of which can cause human
disease) in mosquito samples collected in 2016. These samples were later confirmed to
be trivittatus virus. While all mammals including humans are capable of contracting
trivittatus virus from the bite of an infected mosquito, this disease is predominately found
in wildlife, and human cases are extremely rare. It is important to note that both Zika
and trivitattus virus are spread by only a few species of mosquito, and control measures
must be specifically targeted to these species. In response to these emerging concerns,
City of Denton staff continues to work with local and State public health personnel to
assess the risk of disease and coordinate responses. Currently, emerging disease
responses are documented within Standard Operating Procedures (SOPs) that are
reviewed with local health officials and mosquito control experts on a regular basis.
Modifications to the SOPs are possible more research is conducted on control strategies
and risk minimization measures. As more information is collected and evaluated, it is
likely that the City of Denton Mosquito Surveillance and Response Plan will be modified
in the future to add information specific to these emerging diseases.
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References:
Center for disease control (CDC). 2001. Epidemic / Epizootic West Nile Virus in the
United States: Revised Guidelines for Surveillance, Prevention, and Control.
Proceedings from a Workshop Held in Charlotte, North Carolina. January 31-
February 4, 2001.
Reed, S.C., Cites, R.W. and E.J. Middlebrooks. 1995. Natural systems for waste
management and treatment. McGraw-Hill, New York, USA.
Rishikesh, N., Burgess, H.D., and Valdekar, M. 1983. Operational use of Bacillus
thuringiensis serotype H-14 and environmental safety. WHO/VBC/83.871,
World Health Organization, Geneva.
Tabashnik, B.E. 1990. Modeling and evaluation of resistance management tactics. In
Roush, R.T., Tabashnik, B.E. (Eds.), Pesticide Resistance in Arthropods.
Chapman and Hall, New York, USA.
Walton, W.E., Mulla, M.S., Wargo, M.J., and S.L. Durso. 1990. Efficacy of a microbial
insecticide and larvivorous fish against Culex tarsalis in duck club ponds in
southern California. Proceedings of the California Mosquito Vector Control
Association 58:148-156.
Walton, W.E. and M.S. Mulla, 1991. Integrated control of Culex tarsalis larvae using
Bacillus sphaericus and Gambusia affinis: effects on mosquitoes and non-target
organisms in field mesocosms. Bulletin of the Society for Vector Ecology 16:203-
221.