An Integrated Operations Approach to Managing Zika Virus Vectors

The current Zika virus outbreak in the Americas offers a stark reminder of how urbanization and ease of travel can magnify exposure to container mosquitoes and the viruses they transmit. Debilitating and sometimes deadly diseases such as dengue, Zika, chikungunya and yellow fever can proliferate quickly in concentrated urban areas, areas that also serve as natural hubs for international travel.

By 2050, it’s expected that 2/3 of all people on Earth will reside in urban areas. Along with close interpersonal proximity comes proximity to a host of man-made vessels (storm drains, toys, pots, refuse, etc.) that serve as prime container-mosquito breeding sites. As people travel between those urban areas, diseases travel with them. In its 2015 travel forecast, the U.S. Department of Commerce (DOC) predicted that the number of visitors traveling to the U.S. from abroad will increase as much as 20% by the year 2020. Outbound travel originating from the U.S. increased by about 10% in 2014.

DESIGNING A CONTAINER MOSQUITO CONTROL PROGRAM

Despite Zika’s introduction in the Americas and the troubling effects of the disease, public health officials have gotten a clear head start on effective means to battle container mosquitoes in densely populated areas. Since Zika is vectored by the same Aedes aegypti and albopictus species that vector dengue and chikungunya, the success of recent trials aimed at controlling these species across multiple geographies and habitats bodes well for programs challenged with mitigating exposure to these threats.

As with any disease vector control challenge, the starting point for container mosquito control should be an integrated approach that utilizes source reduction, larviciding and adulticiding. At the root of program design is a clear understanding of container mosquito biology and their preferred habitats, and how those factors combine to represent a unique and yet well-defined target for intervention. Habitat and species determine application method, which in turn drives selection of a product solution (when applicable) that addresses those specific needs.

Target Species

Aedes aegypti and Aedes albopictus (container mosquitoes)

Day Biters
Highly aggressive
Extremely adaptable
Success at colonizing both artificial man-made and natural containers
It takes very little water for container mosquito larvae to survive to become blood seeking adults

THE HABITAT CHALLENGE

Container mosquitoes prefer man-made habitats such as roof gutters, water-holding refuse, plastic toys, and flower pots, as well as natural, plant-based phytotelmata such as leaf axls, tree holes, and fallen leaves
These habitats are in abundance in urban/semi-urban areas
In many situations, people need to store water on their property to survive
While many of these sites are obvious and water can either be removed or treated with larvicides, other sites can be small and cryptic

THE PRODUCT/APPLICATION CHALLENGE

Given the unique biology + habitat complex that exists for container species relative to other complexes, custom integrated approaches are needed to combat these disease vectors. For many mosquito species, a traditional, integrated approach to source reduction, larviciding, and adulticiding provides a robust, sustainable mosquito control program. For container mosquitoes such as Aedes aegypti which prefers to thrive close to human proximity, the traditional approach presents some significant challenges.

Source Reduction Intervention
Source Reduction is considered the most permanent form of mosquito control because it eliminates or reduces habitats where mosquitoes develop. Since many mosquito species lay their eggs in standing water, source reduction often includes the removal of debris or other vessels in which water collects — items such as buckets or discarded tires.

The Challenge: For this strategy, “reduction” is the operative word since it is often impossible to eliminate all sources of standing water in an affected area. And since container mosquitoes often lay their eggs in sites that are extremely small and difficult to find, complete source reduction is impossible on a large-scale. The cost of source reduction, in person-hours, can also be substantial.

Larviciding Intervention
Larviciding is widely considered the most effective form of mosquito control. Applied using ground or aerial treatments, larvicides kill mosquito larvae before they become adults and become airborne. Larviciding as a base-line intervention is gaining widespread acceptance because itefficiently suppress mosquito populations in their most vulnerable stages of development, and improves the efficiency of adulticides when needed.

The Challenge: Similar to the challenges highlighted for source reduction, traditional larviciding application methods require full knowledge of where the water source is. Since container mosquito larval habitats are often cryptic, historically it has been difficult to find and address all habitats within a given area.

Adulticiding Intervention
Ultimately, it is adult mosquitoes that are responsible for disease transmission. It follows that any program actively combating vector-borne disease will include an important adulticiding component. While the mobility of flying adults renders adulticiding less efficient than other forms of control, adulticides serve as a necessary last line of defense – often the go-to approach when disease outbreaks occur via container mosquito vectors. Ground applications are usually employed to control container mosquito species.

The Challenge: While many common mosquitoes are active at dusk, aggressive, container breeding mosquito species such as Aedes aegypti and Aedes albopictus look for blood sources during the day. Limiting container breeding interventions to adulticides becomes complicated since the atmospheric conditions for spraying adulticides outdoors are not optimal during daylight hours – the time when these species are most active.

Fortunately, these challenges have recently spurred health officials to conduct groundbreaking work — work that shows that if we have a clear understanding of the vector biology and the right, targeted solutions, we can control these mosquitoes and limit the transmission of diseases.

Solution Innovations: Advancements in Product + Application Technology
Containers susceptible to mosquito colonization can be treated effectively with environmentally sustainable larvicides to prevent adult emergence and reduce the incidence of disease. The method of larvicide application – either directly by hand or indirectly using a motorized sprayer into the container – depends on abundance and size of containers, as well asthe operational capabilities and goals of the government health district. A mix of larvicide formulations and application methods based on habitat heterogeneity may be necessary to achieve program goals.

Recent advances in larvicide formulations, application technology, and GIS-based management combine to provide for the economical and effective treatment of infested containers necessary to break the disease cycle. Current operational Public Health programs that successfully control dengue vector populations serve as models for Zika programs since the vector species and habitats are identical. The following are case studies of effective container breeding vector control that utilize these advancements.

The diversity of habitats conducive to container mosquito oviposition can result in larval habitats that are too large for a spray program to provide long-term efficacy. For example drinking water is stored in large containers in every house in many countries. To manage these containers, it has been shown that manual application of target specific safe biological larvicides directly to these containers can be accomplished to reduce disease within an operational program.

WIDE-AREA SPRAYING (WAS) OF BIOLOGICAL LARVICIDES – AERIAL APPLICATION

One of the challenges previously addressed was the treatment of cryptic larval habitats on a large scale. Where infrastructure is available, it has been shown that adjusting aerial application systems to accommodate, low-risk, environmentally compatible biological larvicides (e.g. VectoBac Water Dispersible Granule) to be applied over large areas aerially can greatly impact container mosquito populations and disease. For example, in 2009, the Florida Keys saw the first reported case of locally-acquired dengue in 75 years. Using an innovative application approach, health officials redesigned their integrated container-breeding mosquito control program to include the world’s first wide-area, aerial larviciding application against Aedes aegypti. The idea is to let droplets of mosquito-specific biological larvicides find containers that are breeding mosquitoes which is more efficient and cost effective than physically finding these containers. Since the implementation of the redesigned program, no locally acquired dengue cases have been reported (manuscript submitted for publication).

WIDE-AREA SPRAYING (WAS) OF BIOLOGICAL LARVICIDES – BACKPACK APPLICATION

To treat challenge areas where aerial applications are not feasible, wide-area spraying of biological larvicides using backpack sprayers has been shown to be very effective. Starting as far back as the 1980s, select areas in Southeast Asia have utilized this approach with tremendous success. In Malaysia [2007-2008], health officials achieved effective results with large-scale, wide-area, ground-based larviciding for Aedes aegypti control. Eight hectares were treated with both larvicide and adulticide, and 10 hectares with adulticide alone. The result? Only one new dengue case emerged in the area treated with biological larvicide and adulticide while 15 new cases were reported in areas treated with adulticide alone.

While each of these case studies highlights very specific product + application advancements and opportunities for Zika virus vector control, it is important to remember that container mosquito complexes across the globe are quite different. As such, each will require its own unique, prescriptive approach that incorporates not only these proven advancements, but also other traditional approaches where applicable (e.g. adulticide sprays, indoor residual sprays, treated textiles, etc.). A successful integrated operations approach for container mosquito control begins with an understanding that each pest/habitat complex is unique. Once a science based, data-driven evaluation takes place, the proper mix of interventions can be developed and deployed.