July 19, 2008 A study by researchers in Thailand, Japan, and the UK has shown a negative correlation between dengue hemorrhagic fever (DHF) and the density of the Aedes mosquitoes that transmit the virus. The study explains how current efforts to reduce the mosquitoes may actually increase the incidence of the potentially fatal viral disease.
At least tens of millions of people are infected by dengue virus every year. Therefore, health authorities in tropical countries and the World Health Organization, have organized tremendous efforts towards the reduction of Aedes mosquitoes. However, the number of DHF patients continues to increase. To find an answer to this paradox, the researchers tested the hypothesis that more people with a mosquito-borne illness exist where there are more mosquitoes.
The researchers organized community volunteers who visited one million houses in Thailand and surveyed local densities of Aedes mosquitoes. They then compared this data to the number of people with DHF from each of 1,000 districts visited. The researchers found the incidence of DHF in areas of moderate mosquito density to be 40% higher than in areas of the highest mosquito density.
The researchers hypothesized that this paradoxical relationship is due to the peculiar fact that DHF develops most frequently when the patient is infected with dengue a second time. The results of this study imply that the current strategy of dengue control -- reducing Aedes mosquitoes -- may not necessarily be beneficial.
Additionally, the researchers developed computer simulation software based upon their hypothesis. The simulation predicted that epidemiological studies should be continued for a very long duration, preferably over a decade, to clearly detect such a paradoxical relationship between Aedes abundance and incidence of DHF. Such long-term studies are necessary, especially because tremendous efforts and resources have been (and perhaps will be) spent on combating Aedes.
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- Thammapalo et al. Relationship between Transmission Intensity and Incidence of Dengue Hemorrhagic Fever in Thailand. PLoS Neglected Tropical Diseases, 2008; 2 (7): e263 DOI: 10.1371/journal.pntd.0000263
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