Field Notes: West Nile Update

Pat Redig doesn't mince words when he talks about the impact West Nile virus had on Midwestern raptor populations last year.

"It was of an epidemic proportion," the director of The Raptor Center at the University of Minnesota says of the mosquito-borne illness that has spread rapidly across the United States since the 1999 New York City outbreak.

"Based on [the raptors] we received and what we heard from the field, I'm confident saying the raptor death toll in the wild was in the thousands," Redig says. "How many thousands, I don't know." Of the 70 West Nile-infected raptors the center took in last year, 61 died. Raptors appear to be able to contract the disease from infected prey, as well as from mosquitoes.

The Minnesota Department of Health's Dead Bird Reporting System registered 324 positive tests for West Nile virus last year - the first year the virus was detected in Minnesota. Species ranged from cardinals to ducks, though most were blue jays and crows. The virus was reported in every county except Lake and Cook.

Yet the extent of the damage to wild bird populations is impossible to know. Flocks are too peripatetic for accurate surveys. And counting dead birds is unreliable because they are hard to find and most are quickly consumed by scavengers.

Other animals can contract the virus too. Of about 1,000 horses reported to have West Nile virus in Minnesota last year, about one-third died.

Fortunately, the virus didn't take the same toll on humans in Minnesota. Though 277 people died from the virus in the United States in 2002, none of Minnesota's 48 human West Nile cases proved fatal.

"Even during a West Nile outbreak, only a small percentage of mosquitoes actually carries the virus. And most people are able to fight off the virus without displaying any major symptoms," says Dave Neitzel, Department of Health epidemiologist. "Ten to 20 percent get the equivalent of a summer flu - a severe headache, fever, and muscle aches. Only about one out of every 150 people bitten by an infected mosquito develops encephalitis or other severe symptoms."

Why were Minnesotans so lucky, compared with the rest of the nation - Neitzel says our mosquito species may hold a clue. Of the 50 species in Minnesota, our primary suspect for human transmission of West Nile virus is Culex tarsalis, a mosquito that prefers rural areas. In Illinois, where human West Nile deaths made headlines, the virus is primarily transmitted by C. pipiens, a species that frequents densely populated areas.

Although C. pipiens can be found in Minnesota as well, Neitzel says it doesn't appear to feed on humans very often here.

As for this coming summer, Neitzel says it's impossible to predict how West Nile virus will affect Minnesota the second time around.

"Humans and animals exposed to the virus last year have developed antibodies, so it may not be as much of a problem. Then again, Illinois saw its first West Nile case in 2001, and had a much worse summer in 2002," he says.

"Either way, Minnesotans know mosquitoes, so I don't think West Nile should change the summer activities we all enjoy. Just take basic precautions like wearing repellent, long-sleeved shirts, and pants, especially during dusk and dawn mosquito-feeding periods. Over time, I think the situation will stabilize."

DNR pathologist Joe Marcino says Minnesota hunters shouldn't be too worried about West Nile virus. "While it's theoretically possible to contract the virus while handling an infected bird - if its blood came in contact with your eyes, nose, mouth, or an open wound - the risk is extremely small," he says. "There were no documented cases of West Nile transmission from a bird to a hunter in Minnesota last year."

As a precaution, the DNR advises all hunters to wear rubber gloves when handling birds, and thoroughly wash hands and equipment after use.

Even if 2003 turns out to be a mild year for the virus, Redig says there's an important lesson to be learned from West Nile. "This didn't crawl out from under a rock. It was introduced," he says. "I think it shows we need to take a closer look at the environmental implications of trading and moving goods."

Gustave Axelson, freelance writer

If you see a dead bird with no clear cause of death (e.g., predator attack or hitting a car or window), report your observation at www.health.state.mn.us, or call 612-676-5414 or toll-free at 877-676-5414. Only report birds during the mosquito season, from late May to early September.