Could drought increase the severity of West Nile virus outbreaks?

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Researchers have discovered that drought appears to increase the severity of West Nile virus (WNV) epidemics in the USA; however, acquired immunity in the human population following outbreaks may limit subsequent epidemics. This research could help to predict future WNV threats and guide public health efforts.

The study, published recently in the Proceedings of the Royal Society, analyzed 15 years of data on human infections from across the USA and presented findings reporting that WNV epidemics were larger in drought years.

Since the introduction of WNV to North America in 1999 the intensity of yearly epidemics has varied dramatically, from a few hundred severe human cases nationally to a few thousand. In 2002, 2003 and 2012 it is estimated that approximately 3000 individuals suffered meningitis or encephalitis, resulting in nearly 300 deaths.

Variation at a state level is even higher, with yearly case numbers fluctuating by an average of 50-fold year on year. The cause of this variation has previously been unknown, and has led to speculation by some researchers that predicting the size of future epidemics might not be possible.

In this study, the team analyzed the number of severe WNV infections over time and looked for links with a number of weather variables including drought, winter severity and precipitation. In addition, the team tested the hypothesis that the size of disease outbreaks follows a wave-like pattern, with outbreaks decreasing in size after initial occurrence.

First Author Sara Paull (University of California Santa Cruz; CA, USA) commented: “We found that drought was the dominant weather variable correlated with the size of West Nile virus epidemics,”

“We found strong evidence that in some regions the spread of West Nile virus was indeed wave-like, with large outbreaks followed by fewer cases. However, our analyses indicated that human immunity – not just bird immunity – played a large part in the decrease in human cases by reducing the number of people susceptible to the disease.”

The researchers developed a method to map the influence of temperature on both the biology of the virus and the three mosquito species most important in transmitting it; however, the link to drought was an unexpected finding.

Author Marm Kilpatrick (University of California Santa Cruz) explained: “We thought epidemics would coincide with the most ideal temperatures for transmission. Instead, we found that the severity of drought was far more important nationally, and drought appeared to be a key driver in the majority of individual states as well.”

It is not yet clear how drought might increase viral transmission. However, the team discovered in local-scale data from Colorado (USA), that drought affects the proportion of mosquitoes infected with WNV but not mosquito abundance. The group hypothesized increased mosquito infection might be a result of changes in viral transmission between birds and mosquitoes during periods of drought.

Moreover, the team utilized their findings to model the impacts of climate change on future epidemics, projecting that in the next 30 years increased drought severity could triple WNV cases, but only in regions with low human immunity. These predictions may be a helpful tool to direct public health measures to regions that are most likely to be affected in the future.

Sources: Paull SH, Horton DE, Ashfaq M et al. Drought and immunity determine the intensity of West Nile virus epidemics and climate change impacts, Proc R Soc Lond [Biol]. doi:10.1098/rspb.2016.2078 (2017) (Epub ahead of print) http://news.ucsc.edu/2017/02/west-nile-virus.html

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