West Nile virus, which mosquitoes — as principal vectors — transmit to humans, first arrived in North America in 1999. Since then, it has become the most common mosquito-borne infection in the U.S., Canada, and Europe.
Scientific models at Stanford University, CA, now predict the virus will spread more easily in cooler parts of the country as average summer temperatures rise due to climate change.
Rising temperatures will also likely mean seasonal transmission of the virus starts earlier in spring and ends later in fall.
However, at the same time, the virus may spread less easily in hotter areas.
The model suggests that the optimum temperature for transmission is 24–25°C (75.2–77°F). Around 70% of the U.S. population live in regions where summer temperatures are currently below this range, whereas 30% live in areas above the range.
The authors write in the journal eLife, “we might expect a net increase in transmission of West Nile virus in response to the warming climate, even as hot temperatures suppress transmission in some places.”
“As the climate warms, it is critical to understand how temperature changes will affect the transmission of mosquito-borne diseases,” says lead author Marta Shocket, who was a postdoctoral fellow at Stanford when the models were developed and is now a postdoctoral researcher at the University of California, Los Angeles.
Apart from temperature, many factors may contribute to the transmission rates of mosquito-borne viruses, including land use, control measures, and the evolution of the viruses and their insect vectors.
“Climate change is poised to increase the transmission of West Nile and other mosquito-borne viruses in much of the U.S.,” says senior author Erin Mordecai, Assistant Professor of Biology at Stanford.
“But these diseases also depend on human contact with mosquitoes that also contact wildlife. So factors like human land use, mosquito control, mosquito and virus adaptations, and the emergence of new viruses make predicting the future of mosquito-borne disease a challenge.”
The main hosts of all the viruses they studied are wild birds — which act as “reservoirs” of infection — so their shifting geographical range will also contribute to changes in transmission rates, say the researchers.
Each mosquito species is a vector for several viruses, and each virus can be carried by several different species.
Therefore, for simplicity, the scientists focused on six viruses transmitted by a limited number of extensively studied vectors. In total, they modeled 10 vector-virus pairs.
The viruses were:
- West Nile
- St. Louis Encephalitis
- Eastern and Western Equine Encephalitis
- Rift Valley fever
Their vectors are mosquito species from the genera Culex, Aedes, Coquillettidia, and Culiseta.
To develop their models, the scientists used data from previous research that measured how temperature affected these insects in the lab. The temperature-dependent factors driving transmission by the insects were:
- biting rate
- number of offspring
- development rate
- ease of acquiring and transmitting the virus
- virus incubation rate
The models predict that transmission of the viruses will peak at intermediate temperatures and decline in extremes of cold and heat.
The scientists validated their West Nile virus model by comparing its predictions with actual transmission patterns by county in the U.S. This confirmed that the virus spreads most rapidly at moderate temperatures, but is checked when temperatures are higher or lower than its insect vectors’ optimal range.
They write that their models are “the most comprehensive synthesis to date” of how temperature affects the transmission of these diseases.
However, they note that some gaps remain in the data used to build the models. Filling these holes will improve the accuracy and precision of their predictions.
“As carbon emissions continue to increase and severe climate change becomes increasingly inevitable, it is critical that we understand how temperature change will affect the transmission of mosquito-borne diseases in a warmer future world.”