NEW YORK (Reuters Health) - While many people infected with the mosquito-borne West Nile virus do not develop symptoms, it may happen more often than has been realized, a new study suggests.

West Nile fever, the collection of symptoms caused by infection with the virus, was first recognized in the U.S. in 1999, after an outbreak in New York City. An investigation at the time suggested that about one-fifth of infected individuals developed West Nile symptoms, which include fever, headache, rash and muscle and joint pain.

Most people recover from the illness with no complications, but in about 1 percent of cases, the virus invades the brain and spinal cord, causing potentially fatal inflammation and swelling.

Current estimates from the U.S. Centers for Disease Control and Prevention are in line with the initial New York numbers, stating that about 80 percent of West Nile infections cause no symptoms at all.

But in the new study, researchers found that among 576 U.S. blood donors whose blood tested positive for genetic material from the West Nile virus - indicating they had been recently infected - more than half reported having at least one symptom of the infection within two weeks of their donation.

Twenty-six percent experienced three symptoms, according to findings published in the Journal of Infectious Diseases.

“I think this shows us that (symptomatic infections) are much more common than we’ve thought,” said senior researcher Dr. Susan L. Stramer, executive scientific officer for the American Red Cross.

The study also suggests that infected individuals can develop a broader constellation of mild symptoms than commonly believed, Stramer told Reuters Health.

Study participants were asked about having any of 14 symptoms -- which, in addition to fever, headache, rash and body aches, included abdominal pain, “bone pain,” and vomiting or diarrhea. Despite the name of the illness, 44 percent of symptomatic donors reported no fever.

The infected men and women were compared with a control group of 615 uninfected donors who were also surveyed about symptoms in the two weeks of giving blood.

Overall, 53 percent of the infected group reported at least one of eight “indicator” symptoms -- the widely recognized West Nile symptoms such as fever, rash and headache. That compared with 11 percent of the control group.

According to Stramer, those responses suggest that up to 42 percent of West Nile infections result in symptoms.

Women were more likely than men to report symptoms -- 35 percent, versus 25 percent. The reasons for that finding are unknown, Stramer said.

Of the infected donors who developed three or more symptoms, 73 individuals sought medical care. But only four people received a diagnosis of West Nile, and they were all individuals who had ended up being hospitalized for their symptoms (though none was diagnosed as having brain or spinal cord infection).

This low diagnosis rate likely reflects the fact that West Nile testing is infrequently performed, and would only be done in serious cases, according to Stramer.

None of this will change how West Nile infection is managed. There is no specific treatment for the infection, Stramer pointed out, and 98 percent to 99 percent of infections resolve on their own.

Given that, and the high cost of diagnostic testing, she said, “most doctors would not consider it cost-effective to test.”

That said, people with West Nile-like symptoms that take a turn for the worse -- a spike in fever, severe headache, stiff neck, confusion, or sudden weakness in the limbs, for example -- should seek medical care, since these are potential signs of brain and spinal cord involvement.

Since 2003, blood donations in the U.S. have been screened for the presence of West Nile genetic material, which detects early infections that have not yet caused symptoms.

While the current findings do not call for more testing, Stramer said they do help better define the range of mild symptoms that West Nile may cause -- as well as the infection’s human toll, in terms of lost work days and plain misery.

SOURCE: link.reuters.com/zyh67p Journal of Infectious Diseases, online September 27, 2010.