Drought Increases Severity Of West Nile Epidemics

Researchers analyzed 15 years of data on human West Nile virus infections across the U.S. They found that in drought years, outbreaks of the mosquito-borne disease were much more severe, particularly in regions where large epidemics have not occurred in the past. 

“We were totally shocked that drought was such an important driver," says Sara Paull, a post-doctoral researcher at UC Santa Cruz and the study's lead author. She says drought doesn’t influence the abundance of mosquitos, but the percentage infected.

“The higher the level of drought, the higher the percentage of mosquitos that were infected with West Nile Virus,” says Paull.

Researchers say it’s not yet clear how drought increases transmission. Disease-carrying birds and mosquitos may aggregate when water supplies are limited, increasing the potential for transmission. Birds already stressed from drought could also have a lower immune response, which could increase the length of time they're infected or the amount of virus in their blood.

Humans develop immunity to West Nile once they’ve been infected. So states, like California, that haven’t had many human West Nile infections could be more at risk. 

"California is such a large state. If you have a huge outbreak in one region there might be a little build up of immunity in that particular spot, but there might be other regions of California that are still in a drought that are still highly susceptible to a large outbreak," says Paull. 

In the study, she and other researchers looked at patterns in the number of severe West Nile virus infections each year in each state and nationally. They looked at a number of weather variables, including winter and summer temperatures and precipitation.

Over the next three decades, drought is projected to increase in many regions due to rising temperatures.

The research suggests that increased drought from climate change could double the size of future West Nile Virus epidemics in some parts of the nation.  

The study was published Feb. 8 in the journal Proceedings of the Royal Society B.