United States

Texas Faces 'Worst Year Ever' for West Nile Virus

8/17/2012 8:50:05 AM

Officials in Dallas coordinated aerial insecticide spraying last night following a sharp rise in West Nile virus infections in the region.

Tiger mosquito can transmit the West Nile Virus. Photo courtesy of Camponotus Vagus

The measure came after the 10th death from the illness in Dallas County, leading Dallas Mayor Michael Rawlings to declare a state of emergency Wednesday. "Unless actions toward response and prevention are immediately initiated, as hereby provided, avoidable serious illnesses and loss of life is likely to occur," said Rawlings in a proclamation.

The surge of illness follows a relative lull in West Nile infections in 2011, when Texas health officials reported 27 illnesses and two deaths. "This year we are having our worst year ever for West Nile in Texas since the disease emerged in 2002," said Christine Mann, spokeswoman for the Texas Department of State Health Services.

The health department's data show 465 cases and 17 deaths from infections this year so far. Typically, health officials report around 100 cases of the mosquito-borne infections annually, according to Mann. The disease season runs from the beginning of the summer until the first frost, usually at the end of fall.

Roughly 80 percent of people who are bitten by infected mosquitoes don't develop any symptoms, but 20 percent develop West Nile fever, which can cause high body temperatures, headaches, rashes and fatigue, according to the Centers for Disease Control and Prevention. Less than 1 percent of infections can lead to West Nile neuroinvasive disease, a potentially fatal condition.

Mann noted that West Nile tends to harm people over the age of 50 the most, as well as people with compromised immune systems. In Texas, the average age for serious West Nile illnesses is 67 years and the average age of fatally infected patients is 79.

A medical puzzle

This new outbreak is especially puzzling because weather conditions this season have been almost identical to those last summer, with record heat waves and drought parching many parts of the state. Weather has long been known to influence how insects spread diseases, and researchers are now trying to piece together why similar conditions yielded two different extremes.

"It's strange that we see pretty typical yearly West Nile activity in Texas, then all of a sudden, this year we have a record number of human cases," said Gabriel Hamer, an assistant clinical professor at Texas A&M University. He explained that newer diseases tend to infect more people when they first emerge and then taper off as the population becomes resistant, a boom-and-bust cycle also seen in some invasive species.

One reason may be that the disease has changed. West Nile is caused by an RNA virus that is prone to mutations. It first arrived in the United States in 1999, but infections rose dramatically after 2002, when the virus mutated. However, it will take some time before scientists can determine if the Dallas outbreak is from a new strain.

The vector is another variable. Hamer said West Nile is spread largely by the southern house mosquito, Culex quinquifasciatus. The insect prefers to feed on birds, which are one of the main harbors for the virus. The female mosquito transmits West Nile after biting an infected host and later biting an uninfected individual.

Extreme weather can then drive this process to create more infections. "In general, the virus does circulate faster when it's hot," said Hamer. "The hotter it is, the quicker it becomes an infectious mosquito."

Weather extremes may be a cause

The drought can also work in the mosquitoes' favor. "These Culex mosquitoes are by definition a container-breeding mosquito," Hamer said. "A container-breeding mosquito actually wouldn't need near as much water as a flood water-breeding mosquito." In fact, heavy rain can reduce their numbers, as planters, puddles, drains and ditches are washed out. As a result, these insects are closely associated with human development, especially in urban areas.

Last summer was too dry in Texas for these mosquitoes to form vast swarms, but a warm winter, heavy rains this spring and high temperatures this summer formed plenty of small pockets of standing water, creating a comfortable abode for mosquitoes to rear their young. The mosquito falloff in 2011 also meant fewer birds were infected, creating a large population of crows, ravens and jays that have no immunity to West Nile.

These factors may now be converging to create an unprecedented outbreak. "That chain of events -- really hot summer, mild winter, really wet spring -- is a chain of events that has not happened in 10 years," Hamer said.

It also means the Texas outbreak is unlikely to be matched next year. "I think the conditions were really rare. That probably wouldn't repeat itself," he said, observing that a large segment of birds that usually carry the virus will likely be immune by next summer.

Still, with a warming climate and more extreme weather events, the ingredients for another outbreak may be thrown in the mix, with unpredictable results. Diseases like West Nile can spread to new areas where people have not been exposed before, creating new infection opportunities (ClimateWire, Nov. 21, 2011).

"As far as where an outbreak is going to occur, that is something we can't really foresee," said Mann. For now, she recommends that Texans wear long clothes and insect repellent and dump any standing water.

Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500. E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.

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