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The Mosquito Paradox

17 July 2008 (All day)
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CDC

Measuring mosquitoes. A researcher aspirates a water sample to test for the presence of Aedes mosquito eggs.

It seems like common sense: Reduce insect populations, and insect-borne diseases will decline as well. But a new study of dengue, a viral disease transmitted by Aedes mosquitoes, suggests the opposite. Controlling mosquitoes may result in more cases of dengue hemorrhagic fever (DHF), a rare and sometimes fatal disease caused by the virus.

Researchers think that tens of millions of people in the tropics become infected with the dengue virus each year. The pathogen can spur dengue fever, which is marked by agonizing muscle and joint pains but is rarely fatal. Some patients, however, develop the more severe DHF, which can cause bleeding and is deadlier. There are still many questions around the epidemiology of both diseases--including the importance of mosquito abundance.

An international team gleaned data from a massive national survey of Aedes mosquitoes performed between 2002 and 2004 in Thailand, where dengue is rife. The researchers compared this information with data on DHF incidence collected by the Thai Ministry of Health. Overall, 83 out of every 100,000 Thai contracted dengue every year. As expected, the team found that DHF incidence went up with the percentage of households per district containing Aedes larvae or pupae (the House Index). But the trend held only to a point. When the House Index climbed beyond 30, DHF incidence gradually declined, the team reports this week in PLoS Neglected Tropical Diseases.

Co-author Yoshiro Nagao of Osaka University Graduate School of Medicine in Japan says the probable explanation--which was supported by computer models--is that DHF usually develops the second time a person is infected. When there are lots of mosquitoes, the second infection is more likely to come shortly after the first one, so there's a better chance that the person still has a strong immune response. When mosquitoes are less abundant, more time passes between infections, antibodies decline, and protection wanes.

The data show that mosquito control can have unintended consequences, says Nagao. For instance, lowering the House Index to 30 across Thailand might lead to a 40% increase in DHF. In an unpublished study, Nagao and others plan to show that the only way out of the problem is a vaccine, several of which are now in clinical trials.

But dengue researcher Duane Gubler of the University of Hawaii, Honolulu, says the story is more complicated. For one thing, the study's models assume that all dengue strains and mosquito populations are similar. In reality, there's "tremendous variation" between virus strains' infectivity and potential to spread, he says, and some mosquito populations are much better vectors than others. Moreover, the House Index, which gauges mosquito breeding sites, may not be a good indicator of the population's exposure to a virus, says entomologist Paul Reiter of the Pasteur Institute in Paris. What's important is not the number of sites but the number of adult mosquitoes they produce, he says.

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