West Nile virus (WNV) arrived in North America in 1999 and causes outbreaks each year. Some years, there are only a few hundred severe human cases in the U.S. However, in other years, thousands developed meningitis or encephalitis and nearly 300 died. Furthermore, variation at the state level is even higher, varying 50-fold from year to year. The Centers for Disease Control and Prevention has thought that it would be extremely challenging to predict the size of future epidemics.
However, a recent study published in Proceedings of the Royal Society B has found that droughts are associated with larger outbreaks. The researchers analyzed 15 years of data on WNV infections in the U.S. They examined a number of weather variables, including summer temperature, precipitation, and winter severity, thinking that epidemics would be associated with ideal temperatures for transmission. Instead, only drought was associated with the size of outbreaks. The reason for this association is unknown. Data from Colorado suggests that drought increases the percentage of mosquitoes infected with WNV but not the abundance of mosquitoes. One hypothesis is that droughts affect transmission from mosquitoes to birds by stressing birds or changing where they aggregate.
Over the next three decades, many regions in the U.S. are projected to experience increased drought. Models suggest that this could double the size of WNV outbreaks in the future. However, these epidemics are thought to be limited to regions that have not seen many cases, since another result of the study was that the incidence of WNV is wave-like, with large outbreaks followed by fewer cases as the number of susceptible people decreases. This is a contrast to the long-standing hypothesis that WNV has a wave-like pattern of incidence driven by bird immunity rather than human immunity.
Paul SH, Horton DE, Ashfaq M, et al. "Drought and immunity determine the intensity of West Nile virus epidemics and climate change impacts." Proceedings of the Royal Society B. 2017; 284(1848): doi: 10.1098/rspb.2016.2078
Stephen T. "Drought identified as key to severity of West Nile virus epidemics." 2017. earth.stanford.edu/news/drought-identified-key-severity-west-nile-virus-epidemics