Interim CDC Recommendations for Zika Vector Control in the Continental United States
Accompanying guidance to CDC Guidelines for Development of State and Local Risk-based Zika Action Plans *Does not include guidance specific to US territories
Travel-associated Zika case: Zika virus infection in a person who has traveled from an area with Zika virus transmission.
Suspected Zika case of local mosquito-borne transmission: Symptoms or preliminary test results compatible with Zika virus infection in a person who does not have risk factors for Zika virus acquisition through travel, sexual contact, or other known exposure to body fluids and for whom Zika virus test results are pending.
Initial blood donation screening positive for Zika virus and confirmatory test pending, from a donor who does not have risk factors for Zika virus acquisition through travel, sexual contact, or other known exposure to body fluids.
Confirmed Zika local mosquito-borne transmission: Positive test for Zika virus infection per CDC laboratory guidance in a person who does not have risk factors for Zika virus acquisition through travel, sexual contact, or other known exposure to body fluids.
Positive Zika virus nucleic acid test (NAT) on screening AND confirmation through an approved confirmatory test algorithm in donated blood from a donor who does not have risk factors for Zika acquisition through travel, sexual contact, or other body fluid exposure.
Confirmed, multiperson, local mosquito-borne Zika transmission: Three or more Zika cases of confirmed local transmission in non-household members with onsets greater than 2 weeks apart (the approximate lifespan of an infected mosquito) and less than 45 days in an area of approximately 1-mile in diameter. Identification of overlapping movement within a 1-mile diameter of multiple people with locally acquired Zika virus infection suggests a common location (e.g., residential neighborhood, workplace, or other location) for infected mosquito exposure, because the lifetime flight range of the Aedes aegypti mosquito vector is approximately 150 meters (approximately 500 ft).
Early season mosquito control efforts can decrease the risk of eventual Zika transmission and effective control of Zika virus vectors will depend on prompt and aggressive intervention when human cases are first identified. All at-risk communities should prepare for Zika virus activity, and should prepare and evaluate control plans for mosquito populations in their state as part of Zika Action Plan preparedness efforts. A comprehensive review of health code, enforcement practices, and property access will aid the implementation of a vector control plan.
Many states have existing vector control programs. With the exception of states who have responded to past outbreaks of dengue and chikungunya, most state plans focus on control of mosquitoes for the prevention of West Nile virus (WNV). The biology and behavior of Ae. aegypti and Aedes albopictus are different from the mosquitoes that transmit WNV. Therefore, the tools used for surveillance of these species as well as strategies for control will be different from WNV mosquitoes.
Before mosquito season
- State, tribes, and local governments should consider using an Integrated Mosquito Management (IMM) strategy as they develop their mosquito control plans (See Appendix).
- Public health officials and vector control officials should develop a communications network to ensure timely exchange of information, and collaboratively share information to guide optimum vector control efforts. This network should be part of the state’s Incident Management structure, and should report efforts and plans to the state Incident Manager (IM).
- To prepare for the possible introduction of Zika virus, states, tribes, and local governments should review historical data and maps regarding the presence of Ae. aegypti and Ae. albopictus. If maps are outdated, plan new surveys and assessments to be conducted during mosquito season.
- Responding officials should review existing staffing capacity, resource allocation, and technical expertise at the local level for vector control and consider use of intergovernmental agreements for vector control to help adjacent counties outside their jurisdiction, as well as pre-positioning contracts with vendors to supply additional vector control capacity.
- Responding officials should link vector control efforts with communication efforts. This includes ensuring public education campaigns include information not just on personal protection measures, but also how citizens may reduce or eliminate sites where Ae. aegypti and Ae. albopictus lay eggs, motivate the community to remove and dispose of any water holding containers, and educate the community about what IMM is as practiced by a mosquito control program.
During Mosquito Season
- Using the plan previously developed, survey and map presence of Ae. aegypti and Ae. albopictus within the state and jurisdiction.
- Actively engage community to encourage removal of larval habitat and sites where Ae. aegypti and Ae. albopictus lay eggs, including community cleanup campaigns (tire removal, trash pickup, removal and cleaning of small and large containers). Leverage partnerships with local governments, non-profits, and private sector for support.
- Use larvicides in containers and bodies of water that cannot be removed or dumped.
- Use adulticides when adult mosquitoes are abundant and populations need to be suppressed.
- Conduct rapid insecticide resistance testing for local mosquito populations to identify the insecticides most likely to be effective in the event of Zika virus disease cases.
Implementing Control Efforts
To prevent local mosquito-borne transmission, mosquito control should be implemented for all suspected and confirmed travel-associated and locally acquired Zika virus disease cases. The scope of mosquito control efforts may intensify as more cases are identified. Vector control efforts should align with state, tribal, and local government decisions regarding boundaries for declaring an area as a site of “active Zika virus transmission”. This may model county lines or be a zip code designation. In the case of confirmed, multiperson local mosquito-borne transmission, officials should plan to intensify and expand vector control efforts within the areas of active transmission.
- Implement targeted control efforts around the case-patient’s home or building, including intensified source reduction, along with larval and adult mosquito control in a 150-yard radius (or other boundary as deemed appropriate) around the home or building.
- Consider targeted indoor residual spraying in areas where air conditioning and screens are not widely available.
- Targeted control activities involving home visits should be closely coordinated with concurrent educational efforts and messaging.
- In addition to targeting case-patient homes and the surrounding vicinity, area-wide treatments with larvicides and adulticides using application methods appropriate for the scale of the treatment area should be considered. Control plans should be tailored to the local needs and might require truck or aerial spraying (aerial for areas >2000 acres) or a combination of both. In the case of confirmed, multiperson local mosquito-borne transmission that extends across multiple counties or a state, expand vector control efforts to achieve regional or state coverage.
- Monitor for effectiveness of treatments through trapping and retreat if mosquito numbers begin to increase again.
Effective mosquito management programs based on IMM principles may help prevent the introduction of Zika virus to an area. IMM principles include approaching mosquito control through careful planning, using a variety of interventions targeting both larval and adult mosquito control, and including both chemical and non-chemical methods. When properly planned and executed, IMM ensures a more effective level of control than can be achieved by one single approach. States, tribes, and local governments should develop plans tailored to their individual needs and consider basing those plans on the principles of IMM.
IMM is ideally anchored by a mosquito monitoring program providing data that describe local conditions and habitats that produce Aedes mosquitoes, as well as the abundance of those mosquitoes over the course of a season. These data can help inform decisions about implementing mosquito control activities appropriate to the situation. The implementation of an effective IMM program for Aedes mosquitoes requires trained staff with a knowledge of the mosquito life cycle and expertise in monitoring methods. Details for how to conduct assessment and control activities for both larval and adult Aedes mosquitoes may be found on CDC’s chikungunya website.
Immature Mosquito Monitoring
Larval monitoring can help state, tribal, and local governments monitor Aedes mosquito activity and make early decisions for control, even in advance of suspected Zika cases in humans. This involves sampling a wide range of aquatic habitats, and requires trained inspectors to identify larval production sites, collect larval specimens on a regular basis from known larval habitats, and to regularly look for new sources. This information can be used to determine where and when source reduction or larval control efforts should be implemented. Common methods for collecting information on the number and locations of larval Ae. aegypti and Ae. albopictus are ovitraps and larval/pupal surveys.
Adult Mosquito Monitoring
Adult mosquito monitoring is used to determine the abundance of adult vector mosquitoes and identify areas where control measures are needed. It is also useful to assess the effectiveness of intervention methods. Currently, testing mosquitoes for Zika virus is not recommended, as this virus does not have a known animal reservoir outside of humans in the United States and there is no expected advantage to be obtained over good human surveillance programs.
Various methods are available for monitoring adult mosquitoes. Traps targeting adult Culex species are not effective at capturing Ae. aegypti and Ae. albopictus. The most frequently used trap for adult Aedes mosquito surveillance is the BG Sentinel trap, but other trap types are available. Adult mosquito surveillance should consist of a series of collecting sites at which mosquitoes are sampled on a regular schedule. Fixed trap sites allow monitoring of trends in mosquito abundance over time and are essential for obtaining information to guide control efforts. Additional trap sites can be utilized on an ad hoc basis to provide additional information about mosquito activity and effectiveness of control efforts.
Mosquito Control Activities
Mosquito control should ideally be conducted during mosquito season, even before cases of Zika are recognized, and be based on the results of larval and adult mosquito monitoring programs that have identified areas in need of control. Additionally, at the point of the first human case of Zika, more routine mosquito control efforts must be quickly and aggressively amplified to prevent risk levels from increasing to the point of a widespread human disease outbreak.
Larval Mosquito Control
The objective of the larval mosquito control is to manage mosquito populations before they emerge as adults. This can be an efficient method of managing mosquito populations if the mosquito breeding sites are accessible. However, larval control alone may not attain the levels of mosquito population reduction needed to maintain Zika risk at low levels, and must be accompanied by measures to control the adult mosquito populations as well. In outbreak situations, larval control complements adult mosquito control measures by preventing new vector mosquitoes from being produced. However, larval control alone is unlikely to be able to stop Zika outbreaks once virus amplification has reached levels causing human infections.
Numerous methods are available for controlling larval mosquitoes.
- Source Reduction: Source reduction is the elimination or removal of habitats that produce mosquitoes. This can range from draining and scrubbing water holding containers on a weekly basis to properly disposing of discarded tires, rain barrels, and trash containers that may harbor rain water. This can be difficult to accomplish with the Zika virus vector Ae. aegypti, which readily uses very small water containers. Active community engagement, as well as ensuring community access to trash services for removal of debris, are critical to the success of a source reduction campaign. Source reduction may be improved through home visits to examine possible mosquito breeding sites and educating homeowners.
- Larvicide Application: For situations not conducive to source reduction, pesticides registered by Environmental Protection Agency (EPA) for larval mosquito control may be applied when larvae are detected or added to containers that could potentially serve as breeding sites. Several larval mosquito control pesticides are available. (See Table 1) Methods for delivering larvicides include the use of hand-held application devices, from truck-mounted sprayers, from aircraft, or from a combination of methods. More details are provided below.
- Combined Approach: A combination approach utilizing source reduction and larviciding that istailored to local contexts and the provision of adequate field staff with proper training is required to properly identify larval production sites and implement the appropriate management tools for that site.
Adult Mosquito Control
Source reduction and larvicide treatments alone are unlikely to be adequate to maintain adult mosquito populations at levels sufficiently low enough to limit virus amplification. The objective of the adult mosquito control component of an IMM program is to complement the larval management program by reducing the abundance of adult, actively biting mosquitoes in an area, thereby reducing the number of eggs laid in breeding sites. In addition, during an outbreak, adult mosquito control is crucial to immediately reduce the abundance of biting, infected adult mosquitoes. A list of EPA-registered chemicals available for controlling adult mosquitoes is in Table 2. Numerous methods are available for controlling adult Aedes mosquitoes.
- Targeted Outdoor Residual Spraying: In situations where long-lasting control is desired (at the case-patient household or building level, for example) an approach aimed at outdoor spraying of surfaces likely to serve as adult mosquito resting sites may be achieved with hand-held application devices (Trout et al. 2010) at the target and in a 150-yard radius around the target. Ideally, pesticide decisions should be preceded by an assessment of possible resistance to the chemicals. Targeted adulticide treatments should be accompanied by larval reduction methods as described above. In these situations (i.e., providing a barrier around a pregnant woman’s or case-patient’s home), to arrest viral spread, this method requires aggressive attention and rapid action. (Vasquez-Prokopec 2010)
- Indoor Residual Spraying: Indoor residual spraying should be considered for homes that do not have adequate screening or air conditioning. Two chemicals with EPA registration allowing indoor use for mosquitoes are deltamethrin and bifenthrin. Spraying should target sites within the home where mosquitoes rest. They include the back of closets, under furniture and other dark undisturbed sites behind furniture, and in corners. (Vazques-Prokopec 2010)
Widespread Outdoor Application: In situations where adult Aedes mosquito populations are very high or more widespread local transmission of Zika is recognized, in addition to the household or building targeted approach, more widespread adulticide applications using pesticides registered by EPA can be used. Pesticides for adult mosquito control can be applied to wider areas using hand-held application devices, from truck-mounted sprayers, from aircraft, or from a combination of methods. This type of spraying is known as space spraying, as opposed to residual spraying described above, and its effect is transient when used without concurrent larval control (i.e., the insecticide must come into contact with a mosquito at the time of being sprayed in order to have an impact).
- Hand-held devices are useful to manage relatively small areas, but are limited in their capacity to treat large areas quickly during an outbreak.
- Truck-based applications may reach larger areas, but may have gaps in coverage due to limitations of the road infrastructure.
- Aerial application of mosquito control adulticides is required when large areas must be treated quickly. Applications using trucks and aircraft should be timed around dusk and/or dawn.
- Both truck and aerially-applied pesticides for adult mosquito control are applied using ultra-low-volume (ULV) technology in which a very small volume of pesticide is applied per acre in an aerosol of minute droplets designed to contain sufficient pesticide to kill mosquitoes that are contacted by the droplets. Information describing ULV spray technology and the factors affecting effectiveness of ground and aerially-applied ULV pesticides is reviewed in Mount et al. 1996, Mount 1998, and Bonds 2012.
Risk and Safety of Vector Control Pesticides and Practices
Insecticides to control larval and adult mosquitoes are registered specifically for that use by the US EPA. Instructions provided on the product labels prescribe the required application and use parameters and must be strictly followed. Pesticide use should be restricted to trained and licensed technicians, according to state, tribal, or local legal requirements. Research has demonstrated that ULV application of mosquito control adulticides did not produce detectable exposure or increases in asthma events in people living in treated areas (Karpati et al. 2004, Currier et al. 2005, Duprey et al. 2008).
Legal Action to Achieve Access or Control
Individually-owned private properties may be major sources of mosquito production. Examples include accumulations of discarded tires or other trash, neglected water features that become stagnant and produce mosquitoes. Local public health statutes or public nuisance regulations may be employed to gain access for surveillance and control, or to require the property owner to mitigate the problem. Proactive communication with residents and public education programs may alleviate the need to use legal actions. However, legal efforts may be required to eliminate persistent mosquito production sites.
Insecticide Resistance Management
To delay or prevent the development of insecticide resistance in vector populations, IMM programs should include a resistance management component (Florida Coordinating Council on Mosquito Control 1998). Ideally, this should include annual monitoring of the status of resistance in the target populations or resistance assessments in local areas ahead of decisions for chemical applications.
CDC has developed an assay to determine if a particular active ingredient is able to kill mosquito vectors. The technique, referred to as the CDC bottle bioassay, is simple, rapid, and economical assay compared with alternative methods. The results can help guide the choice of insecticide used for spraying.
Programs in the continental United States and its territories can order free Insecticide Resistance Kits by sending an email to USBottleAssayKit@cdc.gov and requesting an order form. Kits include bottles, insecticide, and manual.
Table 1 Insecticides targeting larval mosquitoes
|Active Ingredient||Chemical Type|
|Bacillus thruengensis isralensis||Microbial|
|Novaluron||Insect growth regulator|
|Methoprene||Insect growth regulator|
Table 2 Insecticides targeting adult mosquitoes
|Active Ingredient||Use||Chemical Type|
|Deltamethrin||Space spray/residual spray||Pyrethroid|
|d-Phenothrin (Sumethrin)||Space spray||Pyrethroid|
|Imidacloprid/beta-cyfluthrin||Residual spray||Neonicotinoid/Pyrethroid mix|
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