This article was first published on SciDev by Claudia Mazzeo
[BUENOS AIRES] The conventional method of controlling dengue fever — by targeting the mosquito host of the virus around the homes of infected people — may be misplaced, wasting resources on low-risk areas, while missing important transmission sites, finds a study.
This happens because “human infection is defined by visits to places where contact with infected mosquitoes is likely, independent of distance from home”, according to a study by US and Peruvian researchers published in Proceedings of the National Academy of Sciences this month (15 January).
The finding could help efforts to control this deadly disease, which infects about 50 million people a year globally and which a WHO report this month called a “pandemic threat”.
The main tools to combat the dengue virus target its mosquito vector, but these are often only partially effective, according to the study.
Insecticide spraying campaigns are focused on households and are guided by the observation that Aedes aegypti mosquitoes — the disease’s main vector — travel less than 100 metres from the homes that they live in and around.
“When a case of dengue pops up, we draw a circle around this home — usually 100 to 400 metres [across] — and focus insecticides there,” says Steven Stoddard, the lead author of the research and a medical entomologist at the University of California, Davis.
But the new study suggests that spraying may be insufficient as, through coming into contact with infected mosquitoes, people can spread the disease outside insecticide-sprayed areas.
By tracking people’s movements and dengue transmission in two neighborhoods in Iquitos, Peru, the researchers found that infection risk and transmission rates are substantially elevated among households visited by people infected with dengue.
On average, each infected person would spread the fever to three houses, two of which would be outside the usual insecticide application area of 100 metres, the study says.
This means that “conventional control misses important foci of transmission, resulting in ineffective allocation of resources to locations where risk is low”, it says.
“Our data suggest that the circle [of insecticide spraying] probably should be smaller and that every case should have three or four circles associated with it, drawn around all the houses identified [as being part of that person’s social network],” Stoddard says.
Ricardo Gürtler, an Argentine epidemiologist at the University of Buenos Aires, says the study is “unprecedented in several respects”, such as its focus on how human movement helps the disease to spread.
“The study brings up a new perspective based on social networks, rather than on geographic distance per se,” he says. “It may have important implications for dengue control in non-endemic cities where transmission depends on the introduction of the virus via travelers to endemic areas.”