North State West Nile Virus Cases Soared In 2014. The Culprit? California's Drought

Feb 27, 2015

West Nile virus cases more than doubled in California last year, with nearly 800 cases being reported. That’s the highest number since 2005. While there could be many reasons for the spike in cases, the one experts are pointing to most might surprise you — it’s California’s drought. 


Credit Jim Hutchison / Flickr, Creative Commons

That may seem a little counterintuitive considering West Nile virus is spread by mosquitos, and mosquitos need water to breed, but if you think about it, says Chico State professor of virology Troy Cline, it actually makes sense.  

“During drought there is less flowing water and more stagnating, standing water, which creates a great breeding ground for mosquitos,” he says. “Also during a drought, mosquitoes — rural water sources dry up, so mosquitos and birds are going to come into urban areas where they’re more likely to find water.”


And that puts humans into closer contact with infected mosquitos, he says. That’s because West Nile is spread through a transmission cycle where mosquitos contract the virus by biting infected birds, then pass the virus along by biting birds not yet infected.  


“So the virus maintains itself in this cycle,” Cline says. “Incidentally humans are infected. If a human is bitten by an infected mosquito, then the human can become infected with the virus.”


Last year, about 60 West Nile virus cases were reported in the North State. That may not seem like a lot, but once numbers are adjusted for population, Glenn, Colusa and Butte headed the list as being the top three counties in California with the highest rate of West Nile virus. For every 10,000 residents, Glenn had a rate of four infections. Colusa, two. And Butte, one. 


Since the drought began in 2012, Sutter, Tehama and Yuba counties have all also seen a rise in West Nile virus. Except for Tehama, all of the North State counties that have seen a significant increase in cases of the virus are large producers of rice, where fields are flooded during different times of the year. 


For more information about West Nile virus cases in California, visit the state's West Nile virus website.