Controlling Dengue Outbreaks

Because there is currently no effective vaccine against dengue and no specific treatment for the disease, controlling and preventing dengue fever outbreaks are essential steps for keeping people healthy. What methods are presently used or being developed to control dengue outbreaks in regions where the threat of dengue is high?

The primary preventative measure to reduce dengue infections is the control of mosquito populations. Because the transmission of dengue requires mosquitoes as vectors, the spread of dengue can be limited by reducing mosquito populations. What can people at risk of dengue infections do to reduce the size of mosquito populations? One practical and recommended environmental management strategy is to eliminate unnecessary container habitats that collect water (such as plastic jars, bottles, cans, tires, and buckets) in which Aedes aegypti can lay their eggs (Figure 1). This strategy is called source reduction. When container habitats are removed and water storage containers are covered with a fine mesh to prevent mosquitoes from getting inside them, mosquitoes have fewer opportunities to lay eggs and cannot develop through their aquatic life stages. Source reduction can be effective when performed regularly, especially when members of a community are mobilized and educated about vector control.

Environmental management initiatives can also include major changes in a community, such as installing water systems with direct connections to residences and replacing wells and other water-storage containers, which can be mosquito-breeding habitats. Smaller-scale environmental changes can also be effective. For example, mosquito populations can be reduced when all members of a community clear blocked gutters and street drains and keep their yards free of containers with standing water. Any open containers should be emptied and cleaned each week to eliminate mosquito eggs and larvae. These efforts can reduce the number of mosquitoes living in an area.

What other measures can members of a community take to reduce mosquito populations? Community-based approaches must go hand in hand with educational initiatives that teach people about mosquito vectors and the risks of having mosquito-breeding habitats near their homes. Educational initiatives can encourage people to take an active role in participating in source reduction. Communities that understand the need to make behavioral changes are the most effective in controlling dengue. In addition to steps that communities can take, what can individuals do to protect themselves from dengue?

People can reduce the risk of mosquitoes entering their homes by using window and door screens or by keeping their doors and windows closed and using air conditioning to keep their homes cool. Aedes aegypti typically bite people during the day, so wearing long pants and long-sleeved shirts can reduce mosquito bites when spending time outdoors. In addition, mosquito repellents can be applied to exposed skin and clothing to lower the risk of mosquito bites. The Centers for Disease Control recommends mosquito repellents that contain DEET, picaridin, lemon eucalyptus oil, or IR3535 as the active ingredient. Sleeping under a mosquito net can also provide protection from being bitten, particularly in areas where people rest in the afternoon or in houses with infants. What about other methods of reducing mosquito populations?

Another method of reducing Aedes aegypti is to use ovitraps (Figure 2). What are ovitraps, and how do they work? These devices are black, cylindrical containers filled with water. The top of the ovitrap is fitted with a circular wire mesh and a floatation ring that floats on the surface of the water in the container. Above the mesh, two paddles are mounted. To Aedes aegypti, ovitraps appear to be ideal locations to lay their eggs. The female lays its eggs on the paddles. The eggs then fall beneath the mesh, where the mosquitoes develop through the immature larval and pupal stages. When the adult mosquitoes emerge, however, they are trapped beneath the mesh and are unable to escape from the ovitrap. Some ovitraps have been adapted to include sticky surfaces that entrap adult mosquitoes, and others include the use of a pesticide on the device. Ovitraps can be also used for mosquito surveillance. When sufficient numbers of ovitraps are used and frequently maintained, the vector population can be diminished. One successful example is in Singapore, where ovitraps were used to eliminate mosquitoes at the international airport. Traps have limitations — they require constant supervision and monitoring to prevent them from becoming productive breeding habitats.

Chemical control can be effective in controlling mosquito populations. For instance, insecticides can be used to kill mosquito larvae or adult mosquitoes. Can insecticides be widely and routinely used? The use of insecticides is recommended in emergency situations during dengue epidemics or when there is evidence that an epidemic is emerging. On a regular basis, however, sustainable, coordinated, community-based environmental approaches are favored over chemical methods for controlling mosquitoes, and limited reliance on these chemicals is preferred. Why are environmental management approaches favored? One reason is that mosquitoes can develop resistance to insecticides. In addition, insecticides are expensive, and high doses can be toxic to humans and other species. Therefore, it is best to be cautious about applying these chemicals.

Could safer insecticides be used to kill mosquitoes? Bioinsecticides are a combination of biological controls and insecticides. One example of a bioinsecticide is Bacillus thuringiensis israelensis (Bti), which is a naturally occurring soil bacterium that can effectively kill mosquito larvae present in water. There are many strains of Bacillus thuringiensis, each having unique toxicity characteristics, and Bti is very specific for mosquitoes. Bti is available in small, slow-release bricks called "mosquito dunks" that float on the water surface and are effective in treating deep water. Other bioinsecticides, such as pyriproxyfen and methoprene, act as juvenile hormone analogues that prevent mosquito larvae from metamorphosizing into adults.

Recently, researchers used mosquitoes to transfer insecticides to larval habitats. They noticed that after taking a blood meal, female Aedes aegypti enjoy resting in damp and dark areas. To take advantage of this behavior, the researchers set up dark, damp stations dusted with a bioinsecticide that targets larvae. When the mosquitoes came to rest on the stations, their legs picked up the bioinsecticide and transferred it to the aquatic mosquito habitats where they laid their eggs. This method was effective in killing the mosquito larvae and reducing the number of adult mosquitoes. Are there any nonchemical approaches to reducing mosquito populations?

Biological approaches are also being considered as alternatives to control mosquito populations. For example, predatory crustaceans called copepods and many varieties of fish, including mosquitofish and goldfish, eat mosquito larvae (Figure 3). When these organisms are placed in container habitats, decorative ponds, and pools, they prey on mosquito larvae, effectively preventing mosquito development. The addition of copepods into large water-storage tanks was successful in limiting dengue transmission in Vietnam. Other live predators — such as dragonflies, small aquatic turtles, and beetle larvae — have also been shown to be effective in killing Aedes aegypti.

New genetic approaches are also being considered as ways to control mosquito populations. Researchers at the University of Oxford and Oxitec genetically engineered female mosquitoes that cannot fly. Being flightless is a huge genetic disadvantage. The flightless female mosquitoes are unable to "sing" and court with male mosquitoes using their wing oscillation "song." Predators can more easily prey on flightless female mosquitoes. The researchers theorize that these genetically engineered mosquitoes could be used to control mosquito populations and reduce dengue transmission. The same group of researchers recently genetically modified male mosquitoes to be sterile, and they released these mosquitoes in a trial in Grand Cayman, a Caribbean island, to wipe out dengue fever.

Dr. Scott O'Neill and his colleagues at the University of Queensland, Australia, came up with another innovative approach to control dengue transmission. These scientists infected mosquitoes with bacteria called Wolbachia pipientis, a parasite that shortens the lives of fruit flies. They hypothesized that Wolbachia could also shorten the lives of dengue-infected mosquitoes. Instead of eradicating mosquitoes, the scientists aimed to use the bacteria to shift the age of the mosquito population. Although mosquitoes would still be around, they would have shorter life spans than they do now. How would a shorter mosquito life span affect dengue transmission? When a mosquito is infected with dengue, eight to twelve days must pass before the mosquito can infect another healthy person, and after that period, the mosquito can continue to infect people for the rest of its life, generally three to four weeks. If its life span is shorter, an infected mosquito would have fewer opportunities to transmit dengue. O'Neill and his colleagues are currently testing another Wolbachia strain that seems to kill the dengue virus inside Aedes aegypti, which could serve as a dengue vaccine for mosquitoes.

In the absence of an effective vaccine that protects humans from dengue, limiting contact between people and vectors is the most effective way to prevent dengue infections. Environmental management approaches involve eliminating the container habitats in which Aedes aegypti lay their eggs. Chemical control involves the use of insecticides to kill immature or adult mosquitoes. New chemical, biological, and genetic approaches are also being developed and may provide promising alternatives to control mosquito populations and prevent dengue infections.

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