Mosquito Control Program Plan

Metropolitan Public Health Department

Nashville/Davidson County

Pest Management Services (PMS) of the Metro Public Health Department (MPHD) in Nashville, Tennessee is the program charged with the responsibility of designing and implementing relevant aspects of Integrated Pest Management (IPM) program to reduce risks of mosquito-borne diseases in Davidson County. MPHD’s mosquito management program follows basic principles of Integrated Pest Management (IPM) and is consistent with the Center for Disease Control and Prevention (CDC) guidelines for surveillance, prevention, and control of WN virus.

The overarching goal of the mosquito management program is to reduce risk of mosquito-borne disease transmission to humans.

Currently, West Nile virus (WNV) is the mosquito-borne disease of primary concern. Since WNV was first reported in the US in 1999, it has caused more than 24,400 cases of illness and more than 1,000 deaths in the entire country. In 2003, 9,862 human cases including 264 deaths were recorded. The numbers for 2006 were 4,269 cases and 177 deaths. In 2007, 3510 cases and 109 deaths were reported nationally in 2007. West Nile virus was first reported in Tennessee in 2001.

While Davidson County does have a significant burden of nuisance mosquitoes, the area has not experienced significant outbreaks of WNV disease.  Only 4 cases of human WNV infection have been reported since 2002, representing an average incidence of 1.1 per million persons per year.  The county has never experienced epidemic transmission of West Nile virus, St. Louis encephalitis, or disease from another arbovirus.

The MPHD IPM program is designed only to control mosquito-borne diseases, not to reduce “nuisance mosquitoes.”  Individual properties will not be treated or sprayed based on complaints about mosquito nuisance. The number of complaints, requests, or perceived “mosquito problems” are not used as a trigger for adult mosquito control. However, service requests and complaints are recorded and mosquito surveillance may be targeted to those areas with high number of complaints in order to determine which species of mosquitoes are causing the nuisance and inform public recommendations for managing the nuisance while minimizing pesticide use.

The primary goal of IPM is to design and implement a comprehensive approach to pest management that is effective in protecting public health, and also practical and cost-effective. As recommended by CDC and practiced widely in urban areas in North America, the approach includes public education, source identification and reduction activities (elimination of standing water that can serve as breeding sites for Culex mosquitoes), surveillance to track mosquito populations and presence of virus, larviciding, and adulticiding (spraying).

This document describes each component of the comprehensive mosquito management program.
 

Public Education

Public education is a critical component of MPHD’s mosquito management program.  MPHD works with other Metro agencies, neighborhood organizations, various media, the State of Tennessee Department of Health, and citizens to heighten awareness and to educate and involve the public in elimination of breeding sites and avoidance of mosquito bites.

The mosquito season in Nashville begins with the highly publicized Backyard Inspection Day (BID). During Backyard Inspection Day, a section of the city is selected for inspection for breeding sites and provision of educational materials to citizens. A public relations/education campaign is used to sensitize the entire community to the beginning of the mosquito season and to remind individuals of the role each person plays in eliminating and reducing mosquito breeding sites. During the Backyard Inspection Day, flyers and other information are handed out to the community. Phone calls about the event are made to homes located within the area and a phone line is dedicated for residents’ information and registration to have their yards inspected. Individual homes are inspected for breeding sites during the event.

Information on elimination of mosquito breeding sites, significance of specific surveillance findings, and safe methods to avoid mosquito bites are publicized via broadcast media throughout the season, and links to authoritative information on prevention is provided on the MPHD web site.

The public can access more information by visiting the website http://healthweb.nashville.gov) or by calling 340-5668.
 

Environmental Surveillance & Source Reduction

Earlier MPHD source reduction efforts included an interagency initiative to reduce stagnant water in stormwater treatment devices such as retention/detention ponds and catch basins. The mosquito task force that supported this initiative included experts from Metro Water Services, Metro Public Works, MPHD, representatives from community organizations and Tennessee Department of Health.

MPHD was also  part of the Stormwater regulations review committee in 2004 that instituted better design requirements for stormwater treatment devices and policy changes to improve the maintenance of the over 5,000 such facilities in Davidson County.

Through this process and working with Stormwater Division of Metro Water, we identified many breeding sites in the city.

PMS environmental health staff will conduct year round, county-wide environmental scans to identify possible breeding sites, monitor sites for stagnant water, determine factors underlying identified stagnancy (such as illegal dumping, overgrown vegetation, improperly maintained storm drains, clogged ditches) and explore ways to mitigate the problem by working with appropriate authorities or individuals.

Environmental surveys for mosquito breeding sites will begin by March of each year. For ease of reference, habitat maps may be created containing results of the environmental survey.
 

Larval Surveillance

The goal of larval surveillance in Davidson County is to identify areas of high mosquito breeding activity for larvicide applications.

• Larval surveillance will be initiated in all four zones in late March to early April. Larval sampling will be used to obtain estimates of larvae population density through dipping
   
• 3 – 5 dips per site will be the target. Information collected will include counts of larvae per dip and numbers of larvae in each instar or size category noted. This will help in making an educated guess of when mosquitoes will emerge and guide control efforts
   
• Larval densities, stage of development, proximity to populated areas and size of area are among issues that will be considered in determining the need for larviciding.
   

Larviciding

When standing water that breeds mosquitoes cannot be eliminated it can be treated with larvicides to prevent the emergence of vector mosquitoes.  Along with public education and source identification and reduction, larviciding represents a preferred method of mosquito management at MPHD.  Following environmental inspections of breeding sites, larvicides are applied as needed based on larval presence as determined through dipping. Only sites with significant numbers of mosquito larvae present will have larvicide applied.

Both chemical based and biological larvicides are used to control mosquitoes at this developmental stage. Environmentalists at MPHD treat over 700 sites every year. We currently use Bacillus thuringiensis israelensis (Vectolex CG), methoprene (Altosid XR) and Aliphatic Petroleum Hydrocarbons (GB-1111) to control larval population. Vectolex CG is very effective in controlling Culex larvae. The specific type of larvicide applied in an area and the dosage rate depend on the stage of larval development, larval population density, level of vegetation cover, and extent of suspended particles in the body of water. For example, GB-1111 will be more effective for later stage larval instars.
 

Adult Mosquito Surveillance

The goals of the adult mosquito surveillance for Metro Public Health Department are:

1. To identify distribution and density of Culex mosquito species in Davidson County and monitor these mosquito populations over time.

2. To identify infected Culex mosquitoes.

3. To obtain accurate temporal and spatial baseline data.

4. To obtain timely information necessary to guide control and intervention activities against WN virus and other mosquito-borne diseases, including spatial and temporal evidence of WN virus amplification.

5. To serve as an indicator of the effectiveness of employed local control strategies.
    

 Approach

• Adult mosquito surveillance will utilize traps in a representative number of permanent locations in the four mosquito zones. The permanent locations will be selected based primarily on suitability for attracting Culex mosquitoes, past history of WNV activity, and past history of high mosquito population.

• Ad hoc sites may be used occasionally to fill in temporal or spatial gaps in surveillance if needed.

• Trap locations will be selected by Pest Management staff.

• Each permanent location will be described and mapped sufficiently using addresses, GPS, and other relevant information.

• Traps will be set twice a week at permanent locations: Monday night to be collected on Tuesday mornings and Wednesday night to be collected on Thursday morning. When a holiday or weather interferes, these dates may be altered.

• Mosquitoes will be counted, identified by species, and tested for WN virus activity beginning May 1 or at a start date determined by the Tennessee Department of Health Laboratory.

• Adult mosquito surveillance will be initiated in late April or early May, and continued until the end of mosquito season.
   

Bird Surveillance

Bird surveillance will include keeping records of the number of reported dead birds and testing of selected birds for WN virus. Along with mosquito and larval surveillance data, bird mortality associated with WN virus infection will be used to provide additional information about local WN virus transmission in an area.

Bird surveillance will begin in late April to early May and will coincide with the initiation of adult mosquito surveillance. Citizens are encouraged to report sightings of dead birds.

Crows, Blue Jays and Robins that are in good condition (without obvious decomposition and picked up within 24 hours of death) will be accepted for laboratory testing.
 

Adulticiding

MPHD recognizes that infected adult mosquitoes can transmit diseases of public health importance. The preferred mosquito control strategies of the Department involve the use of non-chemical mosquito control methods including public education, breeding site reduction and application of larvicides as previously described.  However, despite intensive efforts to eliminate breeding sites and apply larvicides, there may be times when a significant and increasing number of infectious adult mosquitoes are present in an area.  If this situation is present, the risk to humans is high.  Public communication about risk and avoidance of mosquito bites is essential, but at that stage adult mosquito control is the only way to reduce the number of infected adult mosquitoes. Thus, while it is considered a “last resort”, using adulticide to quickly reduce the population of infected adult mosquitoes may be necessary on some occasions and is an accepted and recommended way to reduce risk of disease.

When predetermined action thresholds (discussed below) are met or exceeded, adulticide application may be implemented to reduce risks of WN virus transmission by decreasing the density of infected adult mosquitoes.

Metropolitan Code of Laws (MCL) §10.32.180 governs public communication and opt-out provisions for mosquito spraying activities.  Provision D of the ordinance states: “The department of health shall be relieved from any of the foregoing requirements of this section if the director of health determines that a documented threat to public health exists.”

The revised policy of MPHD shall be to spray for adult mosquitoes ONLY when a documented threat to public health exists. Adulticide application will be considered at the level of CDC risk category 3 (moderate probability of human outbreak). In prior years, the threshold for spraying was in CDC risk category 2 (low probability of human outbreak).  Based on prior documented risk, the department expects conditions described as thresholds for spraying to be rarely if ever met, and for spraying to be a rare event.

Because MPHD will spray for adult mosquitoes ONLY when a documented threat to public health exists, MPHD is exempt from the application of MCL § 10.32.180.  However, MPHD’s policy will be to comply with the public notification expectations of MCL § 10.32.180.

If spraying occurs, MPHD will utilize ultra low volume (ULV) application of insecticides from equipment mounted on trucks for adult mosquito abatement once predetermined action thresholds are met or exceeded. The department currently applies sumithrin/piperonyl butoxide (Anvil 2+2) to control adult mosquitoes. This compound is registered by the Environmental Protection Agency for use in residential and recreational areas.

All application of pesticides will be conducted in compliance with applicable federal and state regulations.

The department will broadcast spray an area from the street. In rare occurrences, spray may be conducted from alleys or trucks might enter larger properties when the size of the property calls for such action.

Indicators of WN virus activity will include identification of pools of mosquitoes positive for WN virus, positive birds or human cases.

Finding two or more positive pools of mosquitoes in the same area, collected 2-14 days apart will be considered evidence of focal amplification and sustained viral activity.  This will be the primary action threshold considered in making a determination to apply adulticides.

The action threshold for application of Anvil by MPHD corresponds to recommendations at risk category 3 (moderate probability of human outbreak) by the Centers for Disease Control and Prevention’s (CDC) for phased response to WN virus surveillance data (see CDC chart as adapted below).  Spraying for adult mosquito control will be considered when surveillance demonstrates evidence of amplification of WNV.

Once surveillance data indicate that this threshold is met and a decision to spray is made by the Director of Health, spray locations, boundaries, and frequency of adulticide application will be decided based on the spatial distribution and density of vectors shown by surveillance, infected birds, distribution of human cases, local ecological considerations, population at risk, and other relevant factors.
 

Public Notification

If a decision is made to spray, MPHD will fulfill the public notification expectations described in the ordinance, but opt-out provisions are not appropriate when adulticide application is conducted to reduce a documented threat to public health.

Steps will be taken to inform the public through local television, radio, print media, MPHD’s website, emails to individuals who have requested individual notification and phone calls to homes in the area to be sprayed. MPHD will keep a database of phone numbers for individuals who want to be notified when a spraying event is scheduled. We will attempt to notify the individuals of spraying events using an automated phone system.

Metro Public Health Department will post signs at least 48 hours before spraying is conducted. MPHD will use its best efforts to post the signs within 100 feet of all entry intersections around the perimeter of the affected spray area and all major artery intersections within the affected spray area. The signs will read ”Mosquito virus risk. Spray zone” and will include the date and time of spraying, as well as a phone number and web site where additional information can be obtained.

Local emergency rooms, poison control center, and physicians and veterinarians who are participants in the Tennessee Health Alert Network or a similar listserv will be advised by health alert of the plans for ULV, the profile of the agents used and the anticipated levels of exposure.
 

Additional Public Relations Steps

For persons who choose to take steps to avoid exposure to the spray, several warning opportunities are in place.  First, public communication outlined above will indicate the areas to be sprayed and the time of day when spraying is planned.  Lead trucks will drive ahead of the spray trucks as an additional notification of spraying.  Horns (before 8:00 PM), flashing lights, and noise from the spray equipment will also help to alert pedestrians.  These measures should be sufficient to forewarn pedestrians who wish to reduce potential for exposure to pesticides by moving away from the path of the truck.

ULV application applies <3 ounces of formula (2% anvil) per acre of land. The dose of pesticides for individuals who are outdoors in the swath of the ULV pesticide is low. This situation is not regulated by EPA and exposures would be below thresholds for significant documented public health concern.

However, because of potential for public concern, reasonable efforts to avoid spraying groups of people will be taken. For example, spraying in public parks where large numbers of people have congregated will be delayed and the area will be sprayed at a later hour.

The effectiveness of spraying efforts will be monitored by counting the numbers of mosquitoes in traps before and after spray events and calculating changes in the density of infected mosquitoes.

We will also create a database to record complaints related to mosquito management.
 

Human Case Surveillance

The goals of human case surveillance are to assess local impacts of WN virus, monitor trends and identify geographic areas in need of targeted mosquito control interventions. A positive case by itself will not be used as an independent trigger for application of adulticides.

MPHD works with Tennessee State Department of Health to conduct follow-up of potential human cases of West Nile disease in Davidson County. The Tennessee State Department of Health also attempts to keep health care providers informed of the procedure for definitive diagnosis via laboratory testing of potential arboviral infections.

When a laboratory confirmation of a human case is made, MPHD staff will provide educational materials to residents in the area, and provide information on mosquito breeding sites reduction and ways of avoiding mosquito bites to residents of Davidson County through local media. We may also deploy ad hoc traps to the area to obtain information on WN virus activity.

For more information on mosquito management, contact the Health Department at 340-5668 or through the Department’s web site (http://healthweb.nashville.gov).