Researchers developed a mathematical model to study the impact of heavy rainfall events (HREs) such as hurricanes on the transmission of vector-borne infectious diseases in temperate areas of the world, including the southern coastal United States.

In the aftermath of this type of extreme weather event, the mosquito population often booms in the presence of stagnant water. At the same time, the breakdown of public and private health infrastructure can put people at increased risk of infection.

The study, which appears in Philosophical Transactions of the Royal Society B, finds that the risk of a disease outbreak is highest if the HRE occurs early in the transmission season, or the period of time when mosquitoes are able to pass on the virus to humans.

According to the study, an HRE that occurs on July 1 results in 70 percent fewer disease cases compared to an HRE that occurs on June 1.

“Mosquitoes are very sensitive to temperature not only in terms of their ability to survive and reproduce, but also in their ability to infect individuals,” says lead author Gerardo Chowell, professor of mathematical epidemiology in the School of Public Health at Georgia State University.

Image: Royal Society

“The warmer it is, the faster an infected mosquito will be able to transmit the virus. Considering that mosquitoes have an average lifespan of less than two weeks, that temperature difference can have a dramatic effect on disease outbreaks.”

Population displacement can also affect the spread of vector-borne disease in a few ways, researchers say. When people opt to leave the area, it reduces the number of local infections, while potentially increasing the number of infections elsewhere.

However, those individuals who don’t leave during an HRE may have a higher risk because officials may neglect standard measures to combat mosquito breeding (such as removing pools of stagnant water) when fewer people remain in the area.

Further, as people move into a disaster area to offer emergency relief—or when they return after the event—the number of local infections rises.

“Since mosquito-borne diseases tend to be spread by the movement of people rather than the movement of mosquitoes, disaster-induced movements of people can shift where and when outbreaks occur,” says coauthor Charles Perrings, professor in the School of Life Sciences at Arizona State University.

As HREs become more frequent in the southern US and other tropical areas there’s a need to develop further quantitative tools to assess how these disasters can affect the risk of disease transmission, Chowell says.

“Our team will now focus on improving methods to quantify the number of people that actually leave during a hurricane, how quickly they leave, and when they return,” he says. “We are also looking at additional hurricanes to study the impact of different displacement patterns.”

Additional coauthors are from Georgia State, Arizona State, and Kyoto University. The National Science Foundation funded the work.