Mosquito Reduction May Not Reduce Incidence Of Dengue

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Main Category: Tropical Diseases
Also Included In: Infectious Diseases / Bacteria / Viruses;  Public Health
Article Date: 16 Jul 2008 - 0:00 PDT

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A new study published in the open-access journal PLoS Neglected Tropical Diseases finds a paradoxical relationship between dengue hemorrhagic fever and the mosquitoes that carry the virus. The surprising result, wrote the researchers from Thailand, Japan, and the UK, was that as the density of the Aedes mosquitoes decreased, there was an increase in the incidence of dengue hemorrhagic fever (DHF). This negative correlation between mosquito density and DHF incidence suggests that the recent drive to reduce the number of these viral mosquitoes actually may be leading to an increase in the occurrence of a potentially fatal disease.

As millions of people suffer from the dengue virus every year, the World Health Organization and health authorities in the tropical world have placed emphasis on reducing the number of Aedes mosquitoes. Their efforts have proved relatively unsuccessful, however, as the number of DHF patients continues to rise. This observation led researchers to test the hypothesis that a higher incidence of a mosquito-borne illness occurs in a location with more mosquitoes.

The research method required community volunteers in Thailand to visit one million houses to gather data on the local densities of Aedes mosquitoes. This survey information was then linked to data on the number of people with DHF who lived in each of the 1,000 districts surveyed. They found a 40% higher incidence of DHF in areas of moderate mosquito density compared to areas of the highest mosquito density.

What is the reason for this counterintuitive finding? One possibility concerns the fact that DHF develops most frequently after the second time the patient has a dengue infection. "Furthermore, even substantial vector reduction... would not necessarily decrease the final incidence [of DHF]...but would result most likely in a greater number of DHF cases accumulated over the course of time. This calculation suggests that it is extremely difficult for vector control [of Aedes mosquitoes] alone to achieve the ultimate goal of control program - reduction of incidence," write the authors.

Relationship between Transmission Intensity and Incidence of Dengue Hemorrhagic Fever in Thailand
Thammapalo S, Nagao Y, Sakamoto W, Saengtharatip S, Tsujitani M, et al.
PLoS Neglected Tropical Disease (2008). 2(7): e263.
doi:10.1371/journal.pntd.0000263
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PLoS Neglected Tropical Diseases (http://www.plosntds.org/) is a peer-reviewed, open-access journal devoted to the pathology, epidemiology, prevention, treatment, and control of the neglected tropical diseases, as well as public policy relevant to this group of diseases. All works published in PLoS Neglected Tropical Diseases are open access, which means that everything is immediately and freely available subject only to the condition that the original authorship and source are properly attributed. The Public Library of Science uses the Creative Commons Attribution License, and copyright is retained by the authors.

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Written by: Peter M Crosta
Copyright: Medical News Today
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