December 2012

Super-spreading, where one infected person passes on a disease to lots of others, could be an important factor driving dengue transmission in places where the mosquito Aedes aegypti carries the virus, according to research published recently . The World Health Organisation has termed dengue as “the most important mosquito-borne viral disease in the world.” Incidence of the disease has jumped 30-fold in the last five decades. Aedes aegypti , the mosquito that is principally responsible for spreading the virus that causes dengue, has proved adept at making use of human habitation. The female mosquito feeds on human blood and subsequently lays her eggs inside containers holding water that are found in and around homes. The eggs hatch into larvae, which grow and turn into pupae, finally maturing into adults. Studies have found that most of the mosquitoes in each locality typically come from just a few containers and houses, termed ‘super-producers.’

In the course of their research, which has been published in the journal PLOS Neglected Tropical Diseases , Harish Padmanabha of Yale School of Public Health in the U.S. and his colleagues modelled how dengue would spread, given such an aggregation of mosquitoes and variations in the density of people in a place. The simulation was based on the distribution of mosquito pupae and humans in houses in two residential neighbourhoods of Armenia, a city in Colombia in South America where the disease is rife. Since mosquitoes were concentrated in only a couple of homes in each locality, an infected individual in those houses or in their immediate vicinity was likely to get bitten and pass on the virus to a large number of mosquitoes. Those mosquitoes, in turn, would go on to bite other people, thus spreading the disease. The study indicated that dengue transmission “depends heavily on events where an infected person infects many mosquitoes,” Dr. Padmanabha told this correspondent. Such people were the super-spreaders. In contrast, most people with the virus would not infect many mosquitoes. Moreover, “human density amplified the effect of A. aegypti super-production on dengue risk,” the paper noted. Increased human density led to more possibilities for disease spread through both human-to-mosquito and mosquito-to-human transmission.

Also, greater human density in a particular area would increase the frequency of dengue-infected visitors. “We found that even small variations in human density can have a very big effect,” Dr. Padamanabha said. “A mosquito where you have low human density has much less capacity to transmit [the disease] than in an area where there is high human density.” Targeting control measures in areas of high human density could reduce the epidemic potential by decreasing the abundance of mosquitoes in areas were dengue was most likely to be introduced, the paper pointed out.

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