A female Aedes aegypti mosquito is shown in this Centers for Disease Control and Prevention file photo. (James Gathany/CDC via Reuters)

There are now at least 311 human cases of the West Nile virus in California, according to the state’s resource on the mosquito-spread infection. That’s more than twice as many cases as there were last year at this time, and more than triple the five-year average in the state. Twelve of those infected with West Nile in the state have since died.

As it turns out, the historic drought plaguing California could be a major contributor to the increase.

Although it might seem that mosquitoes, which depend on water to breed, would suffer when there’s less water on the ground, the opposite is true under the right conditions. Droughts, along with warm weather, can produce the conditions necessary for an abundance of the insects.

In the Southern California region at the center of the state’s West Nile outbreak this year, the drought conditions are just right for the flourishing of a particular species of mosquito: the southern house mosquito, or ulex quinquefasciatus, which, according to the Centers for Disease Control and Prevention’s Roger Nasci, is a “very good vector for the disease.”

Nasci, chief of the CDC’s arboviral diseases branch, said in an interview that drought conditions this summer have helped “cook down” the pools of water favored by the species, so that those habitats “get nice and stagnant and stinky. This mosquito likes it a lot.” The rainwater collection vessels that are popular with conservation-minded residents of drought-ridden areas, for instance, can be perfect for mosquito reproduction unless steps are taken to discourage it.

Because mosquitoes usually pick up West Nile virus from infected birds, the shortage of water also helps increase the share of insects carrying the disease. “When we have less water, birds and mosquitoes are seeking out the same water sources, and therefore are more likely to come in to closer proximity to one another, thus amplifying the virus,” California’s department of public health chief Vicki Kramer told KQED.

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