The timing of a hurricane has a significant influence on the storm’s effect on the spread of mosquito-borne infections, such as West Nile virus, Zika, chikungunya and dengue, according to the result of new study published in Philosophical Transactions of the Royal Society B: Biological Sciences.

Average temperatures and the frequency of heavy rainfall events have increased in the United States in the past several decades, particularly in the Northeast, Midwest and Great Plains. Several factors, particularly the growing population in coastal cities, make the nation vulnerable to disasters associated with these events.

Hurricanes Katrina, Sandy, Harvey and Irma all affected densely populated coasts and left many people without access to electricity, water, and other basic services. Flooding led to the displacement and deaths of many people.

Hurricanes and other heavy rainfall events (HREs) can also affect the transmission of vector-borne infectious diseases in the southern coastal United States and other temperate areas of the world. Stagnant water left in the aftermath of a heavy rainfall event provides rich breeding grounds for mosquitoes, which increases the likelihood of human infection.

Significant HREs causing widespread damage can break down public and private infrastructure, which can also put individuals at an increased risk of infection.

Studying the potential effects of vector-borne disease transmission following HREs

Researchers from Georgia State University and Arizona State University teamed up to develop a mathematical model to study the effects HREs have on vector-borne illnesses. They found that the risk of a disease outbreak is highest if the HRE occurs early in transmission season, or if it occurs during a time when mosquitoes are capable of transmitting the virus to humans.

The researchers found, for example, that an HRE that occurs on July 1 results in 70% fewer disease cases than does one occurring on June 1.

“Mosquitos are very sensitive to temperature not only in terms of their ability to survive and reproduce, but also in their ability to infect individuals,” said Gerardo Chowell, professor of mathematical epidemiology in the School of Public Health and lead author of the study. “The warmer it is, the faster an infected mosquito will be able to transmit the virus. Considering that mosquitos have an average lifespan of less than two weeks, that temperature difference can have a dramatic effect on disease outbreaks.”

Population displacement also affects outbreaks

The researchers also found that population displacement resulting from the heavy rainfall event can affect the spread of vector-borne disease in a variety of ways. People leaving the area can reduce the number of local infections but can potentially increase the number of infections elsewhere.

Those who do not leave during the event may be at higher risk, though, because there are fewer people to remove pools of stagnant water or take other measures to combat mosquito breeding after an HRE. As emergency workers move into a disaster area or return after an 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,” said Charles Perrings, co-author of the study.

HREs are becoming more frequent in the southern United States and in other tropical areas, and the increased frequency of these events is creating a need for advanced quantitative tools capable of predicting how these disasters can affect disease transmission risk.

The research team now intends to focus on improving methods to determine how many people evacuate during a hurricane, how quickly they leave and when they return. They also will study additional hurricanes to investigate the effects of various displacement patterns.