What's Causing the West Nile Virus Outbreak?

This year is one of the worst ever in the US for this mosquito-borne illness. Here are a few possible explanations.

 

 

 

By now, you've probably heard that it's been a hell of a year for West Nile virus: The mosquito-borne disease has spread to every one of the lower 48 states except for Maine. Across the country, a total of 4,249 cases and 168 deaths have been reported so far this year—and it's only October. Texas alone has had 1,520 cases and 54 deaths. These numbers are unusually high; last year, the United States had only 712 cases total. So why the sudden uptick in West Nile?

The short answer is that nobody really knows. "We're not quite sure exactly what the perfect combination of environmental and host factors are," says Ingrid Rabe, a medical epidemiologist with the Centers for Disease Control and Prevention (CDC). But experts have a few guesses. It's a complicated disease, so let's start from the beginning.

If a mosquito bites someone who has West Nile virus then bites me, can I get the virus?
No. Mosquitoes don't carry the disease from human to human. Instead, Rabe explained to me, they carry it from birds to humans. That's because birds accumulate much higher concentrations of the virus than humans. Scientific American has a good diagram of the West Nile transmission cycle:



Is West Nile a death sentence?
No. In fact, the majority of cases are very mild: According to a piece published in the New England Journal of Medicine (PDF), only between a fifth and a quarter of West Nile patients experience any symptoms at all. An even smaller portion—less than 1 in 150—develop neurological effects such as meningitis.

Is this the worst year ever for West Nile virus in the United States?
No. From the headlines, you'd think that 2012 had the highest number of West Nile cases since the virus first showed up in the US in 1999. And indeed, it's pretty bad...

http://www.motherjones.com/environment/2012/10/west-nile-virus-maps