Migrating mosquitoes that carry diseases from the tropics to cooler climes might be better at spreading disease in their new, cooler home, according to a recent study in the Proceedings of the Royal Society B. Researchers from Yale University in New Haven, Connecticut, found that mosquitoes from warm places, when exposed to a cooler environment, were more likely to pick up dengue virus than mosquitoes adapted to the cold.
The findings could have real world implications, as mosquitoes do get around; for example, scientists found that they traveled in tire shipments from Florida to the Netherlands in 2010. Plus, the Earth’s changing climate means the pests could expand into new regions. The work highlights the potential importance of testing mosquito disease-transmission capabilities under relevant temperatures, and implies that even in non-tropical locations, the arrival of traveling mosquitoes by boat or plane could herald an outbreak.
Dengue affects nearly 400 million people each year, and rates are rising. In the recent paper, Andrea Gloria-Soria and colleagues analyzed how three factors affect the likelihood that mosquitoes will pick up dengue virus: the strain of mosquito, the strain of virus, and the temperature.
She acquired genetically distinct sets of Aedes aegypti mosquitoes and dengue strains from two parts of Vietnam: Hanoi and an area near Ho Chi Minh City. Hanoi averages 23 degrees Celsius; Ho Chi Minh, 28 degrees. She fed each mosquito type with either of the two viral strains, then incubated them for 10 days at either 25, 27, or 32 degrees. Then she checked for the presence of dengue RNA to calculate the infection rate.
The difference between the two types of mosquito was most noticeable when infected with the warmer weather Ho Chi Minh virus and raised at 25 degrees—about 25% of the Ho Chi Minh mosquitoes acquired the virus. Among the Hanoi strain—the one acclimated to temperatures around 23 degrees—fewer than 10% of mosquitoes were infected.
“The ability of a mosquito introduction or a mosquito population to start an epidemic doesn’t only depend on which mosquito is there, and which virus you have, but will also depend on the current environmental temperature,” concludes Gloria-Soria. If Ho Chi Minh mosquitoes were, for example, transported to Hanoi, they might be more likely to transmit dengue fever than they would at home, and more susceptible than the local biters. Gloria-Soria cautions other mosquito researchers, who often test disease transmission at a standard 28 degrees, that they might want to match the temperature to the relevant environment.
Louis Lambrechts, a specialist in insect-virus interactions at the Pasteur Institute in Paris, praised the study for being one of few to analyze the interactions between strain types and environment underlying infection. He did caution, though, as did Gloria-Soria, that the results were limited to only two strains of dengue and two types of mosquito. “The results cannot be readily generalized,” says Lambrechts.
Gloria-Soria hopes to expand her work to more populations and temperatures. She’d also like to research the underlying biology. As Lambrechts points out, susceptibility fluctuations could result from something in the viral replication cycle, the mosquitoes’ response, or both.
One possibility, Gloria-Soria hypothesizes, is that the temperature response stems from the insects’ RNA interference systems. These serve as a primary line of defense against pathogens, by chopping up invaders’ RNA genomes. Cold temperature is known to destabilize the system. Perhaps, she speculates, the RNA interference pathways of the cool-weather, Hanoi mosquitoes are already adapted to work in the temperatures in that region. But the RNA interference molecules of Ho Chi Minh mosquitoes, adapted to warmth, might suffer when transplanted to cooler areas, making them more susceptible to the virus.
For now, Gloria-Soria cautions that even those who live in temperate climates should take care to protect themselves from mosquito bites—it’s always possible some tropical insect has made a journey, and it might be more sensitive to infection in its new home.