Mosquitoes carrying a certain sort of bacterium could seriously cut, even eliminate, dengue transmission in places where the disease is rife, including many parts of India, according to a new study.
Millions of people in tropical areas across the globe get infected by the dengue virus annually. Some half a million, many of them of children, develop severe forms of the disease, requiring hospitalisation, and about 2.5 per cent of them die, according to the World Health Organisation.
However, efforts at reducing infections by controlling mosquitoes that spread the virus — Aedes aegypti and, to a lesser extent, Aedes albopictus — have not been successful in endemic countries.
A team of Australian scientists, led by Scott O’Neill at Monash University, hit upon the idea of using strains of Wolbachia , a naturally occurring bacterium found in many insects, to limit the ability of mosquitoes to pass on the virus to humans it bites (see “From ‘popcorn’ to blocking dengue transmission,” The Hindu , March 13, 2014).
When lab-reared mosquitoes carrying Wolbachia are released, the bacterium goes on to establish itself in the local mosquito population and limits their ability to pass on the virus.
The ‘Eliminate Dengue’ programme is carrying out field trials using this technique in Australia, Vietnam, Indonesia, and Brazil. Preparations are under way for a trial in Colombia as well.
In a paper just published in Science Translational Medicine , a group of researchers have tried to quantify the impact of two Wolbachia strains on dengue transmission by A. aegypti mosquitoes.
When mosquitoes with the w Mel strain were fed blood taken from hospitalised dengue patients in Vietnam, the bacterium provided “significant but imperfect blocking” of the dengue virus, noted Neil M. Ferguson of Imperial College London in the U.K. and the other scientists in their paper. (Dr. O'Neill and two of his colleagues are co-authors of the paper.)
The virus was less able to establish itself in the gut of mosquitoes with w Mel and thereafter spread to their salivary glands. As a result, fewer mosquitoes with the bacterium had the virus in their saliva. (It is through a mosquito’s virus-laden saliva that humans catch the infection.)
The scientists then used a mathematical model to examine how dengue virus transmission could change when mosquitoes had this bacterial strain in them.
Establishment of w Mel-infected A. aegypti “would be sufficient to eliminate dengue in low or moderate transmission settings,” the scientists observed in the paper. But it “may be insufficient to achieve complete control” when the transmission levels were high.
w Mel-bearing A. aegypti mosquitoes were likely to have “a major impact overall” in India, perhaps eliminating dengue from one-third to one-half of the country, and substantially reducing transmission elsewhere, said Prof. Ferguson in an email. He is founding director of the MRC Centre for Disease Analysis and Modelling, and also heads the Department of Infectious Disease Epidemiology at Imperial College London.
“The intensity of dengue transmission in India is likely to vary a lot from place to place, as a result of differences in climate, population density, environmental suitability for Aedes aegypti ,” he observed in the email. “We don’t have good data on dengue epidemiology for all of India as yet, so it is hard to say precisely where w Mel would be sufficient to eliminate dengue and where it would only be able to reduce case numbers.”
The w MelPop strain of Wolbachia, on the hand, had a much stronger blocking effect on the dengue virus. Consequently, this strain was estimated to “give at least 90 per cent blocking of transmission,” according to the paper.
“Our results suggest that establishment of w MelPop-infected A. aegypti at a high frequency in a dengue-endemic setting would result in the complete abatement of DENV [dengue virus] transmission,” the paper noted.
But as this Wolbachia strain greatly reduced the fitness of the mosquitoes that carried it, ensuring that a sufficiently high proportion of the mosquito population harboured the bacterium was a challenge, the paper added.
