Climate is Ripe for Deadly West Nile Virus Attack on Texas

By Michael D. Lemonick

Texas dodged one major bullet this year: the crushing drought that seared the Lone Star State in 2011 shifted its sights northward in 2012, frying the nation’s agricultural midland but leaving Texas at least a little wetter than it was last summer.

But that little bit of extra moisture has put Texas in the crosshairs of another climate-related disaster. A major outbreak this month of West Nile virus — the worst since the brain infection first reached America in 1999 — has infected some 200 people so far, and killed 17. And on Wednesday, a public-health emergency was declared in Dallas, which has seen 10 deaths from West Nile. Dallas’ mayor, Mike Rawlings, authorized aerial pesticide spraying over the city for the first time since 1966, when an outbreak of encephalitis hit the city.

Culex mosquito, a known transmitter of the West Nile virus.
Credit: Wikipedia

The prime culprit in the spread of West Nile is mosquitoes, which transmit the virus to humans when they bite (there’s no direct human-to-human transmission, fortunately). Climate is a major accomplice, however, according to David Dausey, a professor of public health at Mercyhurst College, in Erie, Penn. “One of the things we’ve worried about for some time is that a changing climate could lead to more mosquito-borne disease,” he said in an interview. And while you can’t attribute a single outbreak to climate change alone, he said, “climate theory tells us that weather extremes will become more common.”

Drought is one such extreme, and drought can be very good for mosquitoes. They breed in standing water, but these can actually become more prevalent during a drought than in rainier times. Creeks that would normally be flowing freely stop flowing and become a series of standing pools. A truly severe drought, like the one Texas saw in 2011, can cut down on mosquito populations, but, Dausey said, “mosquitoes are able to repopulate more quickly than other insects. The warm temperatures make it so that the mosquitoes reach maturation quicker and decrease the incubation period of the virus. This means that the mosquitoes can spread the virus quicker.”

Another climate factor that makes for bigger mosquito populations is the fact that spring is coming earlier and winters have been milder in recent years, both of which give insects of all kinds a running start at the breeding season. It’s not just happening in Texas, and neither is the West Nile outbreak: according to the Centers for Disease Control and Prevention, 43 states have reported infections in people, birds or mosquitoes. Human infections stood at 693 cases as of August 14, and 26 people had died, including six in Louisiana.

Aerial spraying of pesticides.

It’s not entirely clear why Texas and Louisiana have been struck harder than other states, but drought and heat are probably only one factor in the current outbreak, and public health officials are unsure of what else may have helped trigger it. “We don’t know if the number of cases is going to drastically increase, but we do expect more cases,” the CDC’s Dr. Lyle Peterson told the New York Times.

Since all of these trends — warm winters, early springs, hot summers and periods of drought interspersed with heavy rainfalls — are likely to increase in a warming climate, it’s reasonable to worry that insect-borne diseases, including West Nile, malaria, dengue fever, and more could increase as well, or at least move into areas where they’re currently rare.

West Nile, for example, was first identified in tropical Uganda, but it seems to be having no trouble thriving in the U.S. It may just be coincidence that the disease has established itself in North America during the hottest decade on record. But Dausey isn’t buying it. “I don’t see how you can deny the possible connection,” he said. “Could this outbreak be due to just a strange coincidence of events? Yes. Could climate change be one variable? I would say yes, absolutely.”