updated 10/31/2003 6:18:46 PM ET 2003-10-31T23:18:46

A nasty three-day bout of fever is turning out to be a surprisingly common symptom of West Nile virus infection and may afflict about 100,000 Americans this year.

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Dr. Lyle Peterson, who heads the government’s West Nile efforts, said that about 20 percent of people who catch the virus get sick this way, while far smaller numbers — approximately one in 150 — get severe neurological symptoms. Most of the roughly 500,000 people expected to catch the virus this year will show no symptoms at all.

Typical symptoms of the milder illness, called West Nile fever, include three to five days of fever, headache, eye pain, nausea, vomiting and diarrhea. This may be followed by a red rash, then by several weeks of fatigue.

Petersen said he is a personal authority on West Nile fever because he got it in July. “I didn’t feel back to normal for a couple of weeks,” he said.

Petersen, who heads mosquito-borne diseases for the U.S. Centers for Disease Control and Prevention in Ft. Collins, Colo., discussed this year’s outbreak Tuesday at a meeting in Chicago of the American Society for Microbiology.

Petersen said West Nile fever is being reported about 10 times more frequently than last year, but that’s probably because the disease has become easier to diagnose.

Two new tests have helped give doctors and researchers a much better handle on the disease. An antibody test allows doctors to search for signs of recent infection in sick people, and an experimental test that looks for viral genes is being used to screen blood donations.

Dr. Jesse Goodman of the Food and Drug Administration said about 2 million blood donations have been screened so far for the virus, and 527 have turned up positive.

He said the infection rate appears to vary dramatically around the country. In some places, as many as one in 250 donations are infected, while in most areas it is around one in 1,000 or higher.

Petersen said that this year, 66 people have died of West Nile infections, which is about the same as last year’s total at this time. By the end of 2002, 284 people died.

Peterson said the outbreak this summer in Western states may be especially bad because a variety of mosquito there, called Culex tarsalis, is abundant and very good at spreading the virus. The bug breeds in irrigated farm land.

West Nile virus was discovered in Uganda in 1970 and has caused sporadic outbreaks in Africa and Europe. Another CDC researcher, Dr. Robert Lanciotti, said the strain of the virus circulating in the United States is among the most virulent known.

© 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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