MANILA, Philippines?According to World Health Organization estimates, 40 percent of the world population or roughly 2.5-billion people, are at risk of being infected with any of the four different dengue viruses or serotypes. The WHO also notes that around half a million people are hospitalized due to dengue each year; more than 10,000 of those infected die each year.

Where there?s smoke, there?s fire, the saying goes and in this case, high numbers of dengue cases usually send public health officials searching for large populations of the female Aedes mosquito that carries the disease and transmits it to humans.

Because the mosquito needs water to breed, and because there is no cure for dengue, public health departments typically rely on preventative measures such as insecticides and making sure water containers around households are lidded and that discarded bottles, cans or tires aren?t collecting water to reduce the incidence of dengue.

Not quite correct

But now several doctors and researchers from Thailand, Japan and the United Kingdom think the ?smoke and fire? theory isn?t quite correct. In a paper published July 16 in the online journal PLoS Neglected Tropical Diseases, the team led by Dr. Suwich Thammapalo of Thailand?s Ministry of Public Health tested an alternative theory that suggests controlling the Aedes mosquito population isn?t the way to reduce dengue cases.

According to this other theory, reducing the mosquito population actually increases the incidence of dengue hemorrhagic fever, a more severe case of dengue resulting from a second infection due to a different dengue virus. The reasoning involves something called ?transient cross-serotype immunity.?

Carriers

In places such as Thailand where all four serotypes of dengue coexist, for example, a person exposed to dengue would be immune from the more severe symptoms of the disease but would be a carrier. Due to these carriers, the mosquito then wouldn?t have to be present to spread dengue in the area.

To test the plausibility of this theory, Dr. Thammapalo and his colleagues surveyed nearly a thousand districts and just over a million households in Thailand over a period of three years. From 2002 to 2004, volunteers visited thousands of towns and villages, randomly choosing 40 households in each district to see how many of them had Aedes mosquitoes or larvae present. The percentage of households per district with Aedes mosquito populations was then compared to the number of cases of dengue hemorrhagic fever in the same area.

One-to-one relationship

Dr. Thammapalo and his colleagues reasoned that if the transient theory was involved, then they wouldn?t see something like a one-to-one relationship between the households with mosquitoes and the number of dengue hemorrhagic fever cases. In fact, their results showed that there were significantly more cases of dengue hemorrhagic fever in districts that were considered to have ?moderate? mosquito populations, as opposed to districts with ?high? mosquito populations.

Since dengue hemorrhagic fever results from a second infection, the researchers think the people themselves are transmitting the disease to each other, having become infectious carriers via transient cross-serotype immunity. This is likely why there are still high incidences of dengue hemorrhagic fever in those areas even when there aren?t as many mosquitoes.

While their study lends credence to the transient theory, the researchers cautioned that to fully prove the connection between increased cases of dengue hemorrhagic fever and low mosquito populations, such epidemiological studies need to be done over at least a decade, if not longer, for the results to be further confirmed. For now, all they have are computer simulations stretching out 40 years. Hopefully the next study on this theory will come out sooner than 2050.

E-mail the author at massie@massie.com.