Logic might lead you to believe that a mosquito-borne illness like West Nile virus would spread during rainy conditions, as the bugs need water to breed.

But a new study demonstrates the opposite is true.

Researchers found that the severity of West Nile virus outbreaks spike during dry times. This came as a surprise to researchers at UC Santa Cruz, Stanford University and the New York State Department of Health, who thought other climate variables like precipitation and temperature, including days below freezing, played a more significant role.

“But we found that the primary driver of intensity of yearly epidemics of West Nile virus was drought,” says UC Santa Cruz disease ecologist Marm Kilpatrick. ”

He recommends health officials amp up mosquito prevention efforts like spraying and traps when the drought index hits severe.

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More Research Needed

Scientists aren’t sure why dry conditions lead to more mosquito carriers.

There are two current hypotheses. Some suspect mosquitoes and birds frequent the same watering holes when there are fewer puddles around, leading to more disease carriers in the environment.

Others argue that dry conditions lower the immunity of birds because of reduced food supply. Hunger leads to stress, which could make the birds more susceptible to disease.

Kilpatrick says more research is needed to determine the exact cause.

Researchers found that the other factor in predicting the virus was whether the area had experienced a substantial epidemic in the past. If that was the case, the subsequent epidemics were much smaller, Kilpatrick said. He believes both humans and birds build up immunity over time, so outbreaks are lighter in areas that have already endured the virus.

History of West Nile virus in the U.S.

Mosquitoes carry the virus. The insects contract the disease when they feed on infected birds and then transmit it to humans.

North America has experienced West Nile virus outbreaks every year since 1999, but the number of cases nationally has greatly fluctuated. In some years only a few hundred people suffered severe symptoms, in other years thousands fell ill. In each of three years (2002, 2003, and 2012) about 3,000 people contracted meningitis or encephalitis, resulting in brain damage and nearly 300 deaths for each outbreak.

The unpredictable spikes led scientists at the Centers for Disease Control and Prevention to suggest that predicting the size of future epidemics was difficult or impossible. However, public health experts in California suspected drought was partly to blame in 2014, when the number of cases doubled. Now the data proves those suspicions correct.

California Not in the Clear

Though California was hit with more than 500 cases in both 2014 and 2015, the numbers were not high enough to prevent a future epidemic. “There hasn’t been strong enough evidence that we’ve had enough cases, for our very large population–30 million people–to limit subsequent transmission,” says Kilpatrick. “Our analyses suggest as many as 1000 neuro-invasive disease cases [from West Nile virus] are possible in California under future severe drought conditions.”

Severe West Nile virus in California — *WNV = West Nile virus cases in California.* Source: UC Santa Cruz (UC Santa Cruz)

The majority of people infected with West Nile virus will not show symptoms. Twenty percent will experience flu-like aches and pains, and the virus will cause brain damage in 1 percent of cases. Ten percent of those cases are usually fatal.

Even though the disease is more prevalent during drought years, it’s not because there are more mosquitoes. Rather, a higher percentage of the insects carry the disease. “If there are more infected mosquitoes in the air, you have a higher chance of getting infected,” says Kilpatrick.

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