West Nile Virus Redux

West Nile Virus: Another Successful Immigrant Story In June 2007, the Institute of Medicine’s Forum on Microbial Threats met in Colorado to discuss the ecology of vector-borne diseases. Colorado is a good place for such a meeting. The CDC has labs in Ft. Collins. There’s the majestic backdrop of the Rocky Mountains, and Rocky Mountain spotted fever. There are cute prairie dogs, and bubonic plague. Local mice carry hantavirus, some of the elk have chronic wasting disease, and various ticks and mosquitoes can transmit a cornucopia of bacteria, rickettsia, parasites and viruses. I prefer Colorado in the winter when everything is dead or buried under ten feet of snow. My seasonal preference was re-enforced by a local CDC scientist’s encounter with one of Colorado’s disease vectors. He told us how he had walked to the end of his driveway to check his mailbox and say hello to a neighbor. In that brief interval of utterly normal suburban behavior, a mosquito bit him. The mosquito drilled a hole through his skin, tapped a small vein and took a quick blood meal. As a parting gift it left behind a few copies of the West Nile Virus (WNV). Swept along in the bloodstream, the virus eventually crossed the blood brain barrier where it made itself at home and began to produce the telltale symptoms of viral encephalitis: headache, stiff neck, high fever, vomiting, photophobia, muscle weakness, and disorientation. “It’ll ruin your summer,” said the CDC scientist. He was sick for months and later told CNN, “This is not a mild illness, and people should try to avoid it.” Americans did avoid it…until it arrived in New York in 1999, probably incubating in an infected bird or in a mosquito hitching a ride in a jet cargo hold. From New York, it

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spread south and west, wiping out birds by the flock, killing horses, and infecting thousands of people. In 2007, there were 3,630 human cases and 117 deaths reported 44 states. About a third of those cases (1,227) resulted in meningitis, encephalitis or polio-like paralysis. Last year, Maryland reported 10 human cases to CDC’s surveillance system, ArboNet. WNV is part of a family of viruses that also cause Yellow Fever, Dengue, St. Louis encephalitis, Japanese encephalitis, and tick-borne encephalitis. Most of these viruses are transmitted by mosquitoes. There are about 2500 species of these bloodthirsty pests worldwide and about 200 of them live in the U.S. The most common source of WNV is a mosquito called Culex pipiens. But the Culex mosquito has some competition for the title of pesky WNV vector. The competitor is the Asian tiger mosquito (Aedes albopictus), a 1985 stowaway in a batch of used tires imported into the U.S. It’s an urban and suburban creature. It’s out when we’re out, ranging across the landscape, and persistently biting for blood. Over the last 20 years, this winged aggressor has invaded 36 states, including Maryland. So, we have immigrant mosquitoes and immigrant viruses locked in a complex dynamic of ecology, competition, adaptation, and infection. It’s hard to know how things will settle out, but it seems clear WNV and the tiger mosquito are here to stay. About 80% of people who become infected with WNV never show any symptoms. Most of the remaining 20% of infections tend to be mild or “flu-like.” But a few of those infections—usually in elderly patients—can lead to serious neurological disease.

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According to a 2008 study of 150 patients in Texas, 60% of those with neurological disease had significant symptoms after a year. After five years, 42% still had not fully recovered. Some of these persisting symptoms include fatigue, weakness, clinical depression, memory loss, and personality changes. Curiously, a Canadian study also looked at the long-term effects of WNV in 156 patients. The investigators wrote in the August 19 issue of the Annals of Internal Medicine that, “both physical and mental functions, as well as mood and fatigue, seemed to return to normal in about a year.” Two different studies and two different conclusions? How can this be? Were the Texas patients older and sicker? Is Canadian medicine better? Are there different WNV strains circulating in different places? Did the temperature in Texas increase the rate and severity of infection? Clearly, more research is needed. In the meantime, we should take heart from that 80% rate of mild infection, and remember to use mosquito repellents. There is no vaccine (except for horses) and no specific WNV treatment. So like the mosquitoes and the virus, we will have to adapt to a changing world.

For more information about WNV: www.cdc.gov/ncidod/dvbid/westnile

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