Prevention and Control: Integrated Vector Management

Prevention and control of arboviral diseases is accomplished most effectively through a comprehensive, integrated vector management (IVM) program applying the principles of integrated pest management. IVM is based on an understanding of the underlying biology of the arbovirus transmission system and utilizes regular monitoring of vector mosquito populations and arboviral activity levels to determine if, when, and where interventions are needed to keep mosquito numbers below levels which produce risk of human disease, and to respond appropriately to reduce risk when it exceeds acceptable levels.

Operationally, IVM is anchored by a monitoring program providing data that describe:

• Conditions and habitats that produce vector mosquitoes.
• Abundance of those mosquitoes over the course of a season.
• Arboviral transmission activity levels expressed as infection rate in mosquito vectors.
• Parameters that influence local mosquito populations and virus transmission.

These data inform decisions about implementing mosquito control activities appropriate to the situation, such as:

• Source reduction through habitat modification.
• Larval mosquito control using the appropriate methods for the habitat.
• Adult mosquito control using pesticides applied from trucks or aircraft when established thresholds have been exceeded.
• Community education efforts related to risk levels and intervention activities.

Monitoring also provides quality control for the program, allowing evaluation of the effectiveness of larval and adult control efforts, and causes of control failures (e.g., undetected larval sources, pesticide resistance, equipment failure).

Guided by the surveillance elements of the program, integrated efforts to control mosquitoes are implemented to maintain vector populations below thresholds that would facilitate virus amplification and increase human risk (Table 1) (Nasci and Mutebi, 2019).

The objective of the larval mosquito control component of an IVM program is to manage mosquito populations before they emerge as adults. This can be an efficient method if the mosquito breeding sites are accessible. However, larval control may not attain the levels of mosquito population reduction needed to maintain 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 not able to stop outbreaks once virus amplification has reached levels causing human infections.

Numerous methods are available for controlling larval mosquitoes. Source reduction is the elimination or removal of habitats that produce mosquitoes. This can range from draining roadside ditches to properly disposing of discarded tires and other trash containers. Only through a thorough surveillance program will mosquito sources be identified and appropriately removed. In order to effectively control vector mosquito populations through source reduction, all sites capable of producing vector mosquitoes must be identified and routinely inspected for the presence of mosquito larvae or pupae. This is difficult to accomplish with the vector species Cx. quinquefasciatus and Cx. pipiens that readily utilize cryptic sites such as storm drainage systems, grey water storage cisterns, and storm water runoff impoundments. Vacant housing with unmaintained swimming pools, ponds and similar water features are difficult to identify and contribute a significant number of adult mosquitoes to local populations.

To manage mosquitoes produced in habitats that are not conducive to source reduction, pesticides registered by EPA for larval mosquito control are applied when larvae are detected. No single larvicide product will work effectively in every habitat where vectors are found. Information about pesticides used for larval mosquito control is available from the U.S. EPA (https://www.epa.gov/mosquitocontrol/controlling-mosquitoes-larval-stage). Pesticides should always be used according to their label instructions by field staff trained to identify larval production sites and safely implement the appropriate management tools for that site.

Source reduction and larvicide treatments may be inadequate to maintain vector populations at levels sufficiently low to limit virus amplification. The objective of the adult mosquito control component of an IVM program is to complement the larval management program by reducing the abundance of adult mosquitoes in an area, thereby reducing the number of eggs laid in breeding sites. Adult mosquito control is also intended to reduce the abundance of biting, infected adult mosquitoes in order to prevent them from transmitting virus to humans and to break the mosquito-bird transmission cycle.

In situations where vector abundance is increasing above acceptable levels, targeted adulticide applications using pesticides registered by EPA for this purpose can assist in maintaining vector abundance below threshold levels. More detailed information about pesticides used for adult mosquito control is available from the U.S. EPA (https://www.epa.gov/mosquitocontrol/controlling-adult-mosquitoes).

Pesticides for adult mosquito control can be applied from hand-held application devices, from trucks or aircrafts. Hand-held or truck-based applications are useful to manage relatively small areas but are limited in their capacity to treat large areas quickly during an outbreak. Gaps in coverage may occur during truck-based applications due to limitations of the road infrastructure. Aerial application of mosquito control adulticides is used when large areas must be treated quickly. Aerial spraying can be particularly valuable to control Cx. quinquefasciatus or Cx. pipiens which require multiple, closely timed treatments. Both truck and aerially-applied pesticides 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.

Intensive early season adult mosquito control efforts can decrease viral transmission activity and result in reduced human risk (Lothrop et al. 2008). However, depending on local conditions, proactive vector management may not maintain mosquito populations at levels sufficiently low to avoid development of outbreaks. As evidence of sustained or intensified virus transmission in a region increases, emergency vector control efforts to reduce the abundance of infected, biting adult mosquitoes must be implemented. This is particularly important in areas where vector surveillance indicates that infection rates in mosquitoes are continually increasing or being sustained at high levels and evidence of infection found in other species (e.g., human or non-human mammal cases). Delaying adulticide applications until numerous human cases occur negates the value and purpose of the surveillance system. Timely application of adulticides interrupts arboviral transmission and prevents human cases (Carney et al. 2008).

Insecticides to control larval and adult mosquitoes are registered specifically for that use by the U.S. Environmental Protection Agency (EPA). Instructions provided on the product labels prescribe the required application and use parameters and must be carefully followed. Properly applied, these products do not negatively affect human health or the environment. In persons living in treated areas,  ULV application of mosquito control adulticides does not produce any detectable biological changes indicating exposure (Currier et al. 2005, Duprey et al. 2008) or increase asthma or other adverse health events (Karpati et al. 2004). The risks from arboviruses demonstrably exceed the risks from mosquito control practices (Davis and Peterson 2008, Macedo et al. 2010, Peterson et al. 2006).

Individually owned private properties may be major sources of mosquito production. Examples include accumulations of discarded tires or other trash, neglected swimming pools, and similar 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. Executing such legal actions may be a prolonged process during which adult mosquitoes are continuously produced. 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.

Pesticide products and application procedures (for both larval and adult control) must periodically be evaluated to ensure an effective rate of application is being used and that the desired degree of control is obtained. Application procedures should be evaluated regularly (minimally once each season) to assure equipment is functioning properly to deliver the correct dosages and droplet parameters and to determine appropriate label rates to use locally. Finally, mosquito populations should routinely be evaluated to ensure insecticide resistance is not emerging.

Surveillance data describing vector sources, abundance and infection rates, records of control efforts (e.g., source reduction, larvicide applications, adulticide applications), and quality control data must be maintained and used to evaluate IVM needs and performance. Long-term data are essential to track trends and to evaluate levels of risk.

For vector control to be effective, mosquitoes must be susceptible to the insecticide selected for use. In order to delay or prevent the development of insecticide resistance in vector populations, IVM programs should include a resistance management component (Florida Coordinating Council on Mosquito Control 1998). This should include routine monitoring of the status of resistance in the target populations to:

• Provide baseline data for program planning and pesticide selection before the start of control operations.
• Detect resistance at an early stage so that timely management can be implemented.
• Continuously monitor the effect of control strategies on insecticide resistance, and determine potential causes for control failures, should they occur.

Insecticide resistance may be monitored using bioassays in larvae or adult mosquitoes (Brogden and McAllister 1998). The CDC bottle bioassay is a simple, rapid, and economical tool to detect insecticide resistance by determining the time taken for a pesticide active ingredient to kill mosquito vectors. The results can help guide the choice of insecticide used for spraying. The CDC bottle bioassay can be used as part of a broader insecticide resistance monitoring program, which may include field cage tests and biochemical and molecular methods. A practical laboratory manual for the CDC bottle bioassay is available online (https://www.cdc.gov/mosquitoes/mosquito-control/professionals/cdc-bottle-bioassay.html). For additional information contact CDC at USBottleAssayKit@cdc.gov.

The IVM program should include options for managing resistance that are appropriate for the local conditions. The techniques regularly used include the following:

• Management by moderation. Preventing onset of resistance by reducing overall chemical use or persistence:
  • Using dosages no lower than the lowest label rate to avoid genetic selection.
  • Using chemicals of short environmental persistence and avoiding slow-release formulations that increase selection for resistance.
  • Avoiding use of the same class of insecticide to control adult and immature stages.
  • Applying locally: many districts treat only hot spots and use area-wide treatments only during public health alerts or outbreaks.
  • Using less frequent applications; leaving generations, population segments, or areas untreated (when appropriate).
  • Establishing higher thresholds for mosquito mitigation with insecticides, except during public health alerts or outbreaks.
• Management by continued suppression. This strategy is used in regions of high value or persistent high risk (e.g., heavily populated regions or locations with recurring WNV outbreaks) where mosquitoes must be kept at very low densities. It involves the application of dosages within label rates but sufficiently high to be lethal to heterozygous individuals that are partially resistant. If the heterozygous individuals are killed, resistance will be slow to emerge. This method should not be used if any significant portion of the population in question is fully resistant. Another approach more commonly used is the addition of synergists that inhibit existing detoxification enzymes and thus eliminate the competitive advantage of these individuals. Commonly, the synergist of choice in mosquito control is piperonyl butoxide (PBO).
• Management by multiple attack. This strategy involves the use of insecticides with different modes of action in mixtures or in rotations. There are economic limitations associated with this approach (e.g., costs and logistics of switching or storing chemicals), and critical variables in addition to the pesticide mode of action that must be taken into consideration (i.e., mode of resistance inheritance, frequency of mutations, population dynamics of the target species, availability of refuges, and migration). Programs should evaluate resistance patterns routinely and the need for rotating insecticides at annual or longer intervals.

Continuing education for operational vector control workers is required to instill or refresh knowledge related to practical mosquito control. Training focusses on safety, applied technology, and requirements for the regulated certification program mandated by most states. Training should also include information on the identification of mosquito species, their behavior, ecology, and appropriate methods of control.

The objective of a phased response to WNV surveillance data is to implement public health interventions appropriate to the level of WNV risk in a community (Table 1). A surveillance program adequate to monitor WNV activity levels associated with human risk must be in place in order to provide detection of epizootic transmission in advance of human disease outbreaks. The surveillance programs and environmental surveillance indicators described above demonstrate that enzootic/epizootic WNV transmission can be detected several weeks before the onset of human disease, allowing for implementation of effective interventions (Bolling et al. 2009, Jones et al. 2011, Mostashari et al. 2003, Unlu et al. 2009).

All communities should prepare for WNV activity. For reasons that are not well understood, some regions are at risk of higher levels of WNV transmission and epidemics than others (CDC 2010), but there is evidence of WNV presence and the risk of human disease and outbreaks in most counties in the contiguous 48 states. The ability to develop a useful phased response depends upon the existence of some form of WNV monitoring in the community to provide the information needed to gauge risk levels. Measures of the intensity of WNV epizootic transmission in a region, preferably from environmental surveillance indicators, should be considered when determining the level of the public health response. As noted previously, human case reports lag weeks behind human infection events and are poor indicators of current risk levels. Effective public health actions depend on interpreting the best available surveillance data and initiating prompt and aggressive intervention when necessary.

References

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Safety and Quality of Vector Control Pesticides and Practices

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