3Qs: Why a mosquito is more than just a summertime pest

July 26, 2012

You know that mos­quito bite you’re scratching like crazy on that hot, summer night? According to Rebeca Rosen­gaus, asso­ciate pro­fessor of biology in the Col­lege of Sci­ence, it may be a vector, or car­rier, for dis­eases. And while we may think that spraying pes­ti­cides to pre­vent the spread of some of these dis­eases, such as West Nile Virus, may be a good solu­tion, Rosen­gaus says it’s not sus­tain­able. Northeastern University news office asked the expert to explain why a mos­quito is more than just a sum­mer­time pest.

How do insects, such as mosquitoes, become vectors for disease or parasites and then transmit them to us?

Insects com­prise about 83 per­cent of the known animal life, and 50 per­cent of all living organ­isms. Today at least 1 mil­lion insect species have been iden­ti­fied. While many insects are ben­e­fi­cial to humans, some of them are also detri­mental, par­tic­u­larly those that become agri­cul­tural pests and car­riers of diseases.

The microor­gan­isms that cause dis­eases such as Eastern equine encephalitis, West Nile virus or malaria usu­ally have com­plex life cycles. The microbes require mul­tiple hosts in order to com­plete their mat­u­ra­tion. Insects happen to be excel­lent car­riers of these pathogens because they are trans­ported from one host and injected into the sub­se­quent host. In a way, microbes are “hitch­hiking” inside the unaware insect.

The female mos­quito requires a blood meal prior to repro­duc­tion. She becomes infected during the blood meal, dis­perses the microbe while flying and then re-​​infects a new host once she bites again.

The inter­ac­tions among the micro­bial pathogens, the mul­tiple hosts, and the insect vec­tors have been around for mil­lions of years and do not nec­es­sarily only affect humans. In gen­eral, mos­qui­toes could bite any ter­res­trial warm-​​blooded animal at some stage of its life.

Some have called for increasing the use of pesticide spraying after pools began testing positive for the West Nile virus. Is pesticide spraying a sustainable solution to problem? Which other vector-control options are researchers currently investigating?

The­o­ret­i­cally speaking, you should be able to reduce infec­tion rates by con­trol­ling the pop­u­la­tion size of the insect car­riers. This, how­ever, is easier said than done.

Insects can be killed with toxic chem­ical com­pounds (pes­ti­cides) and bio­log­ical con­trol agents (fungal or bac­te­rial insect pathogens), and by irra­di­a­tion and sub­se­quent release of infer­tile males.

Researchers have recently attempted to genet­i­cally engi­neer the mos­quito itself or its gut bac­teria so that the mos­quito becomes refrac­tive to Plas­modium, the malaria-​​causing microor­ganism. The later strategy of manip­u­lating the bac­teria in the gut of the mos­quito seems to be effec­tive. When mos­qui­toes were fed a blood meal con­taining the genet­i­cally engi­neered bac­teria, the preva­lence of Plas­modium in the gut of the insect was reduced by 85 to 90 per­cent. These dif­ferent tech­niques vary in their effec­tive­ness, and it is impor­tant to keep in mind that both short– and long-​​term cost/​benefit analyses of these strate­gies need to be taken into con­sid­er­a­tion if we are to con­trol insect vector pop­u­la­tions successfully.

In the short term, pes­ti­cides would be effi­cient in reducing mos­quito den­sity. How­ever, mos­quito pop­u­la­tions can evolve resis­tance against pes­ti­cides, ren­dering them non­sus­cep­tible to the pes­ti­cide in the long run.  An iden­tical sce­nario has been respon­sible for the evo­lu­tion of antibi­otic resis­tance in bac­teria. Thus, we should learn from pre­vious expe­ri­ence and avoid fos­tering the evo­lu­tion of pesticide-​​resistant mos­qui­toes by restricting the wide­spread use of pesticides.

Scientists have connected global warming to expanded migratory patterns for insects. How will global warming impact the range of incidence of mosquito-borne diseases?

There is no doubt the Earth is warming. This phe­nom­enon has been accom­pa­nied by an expan­sion north­ward of ani­mals that oth­er­wise would have been restricted by the cold winter months to lower latitudes.

Insects are no excep­tion. The prob­a­bility also exists that infected mos­qui­toes can now not only reach new loca­tions, but also thrive in warmer envi­ron­ments for longer periods of time. This will trans­late into higher risks of infec­tion for humans and other ani­mals as the mos­qui­toes search for new blood meals.

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