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Study Looks At Occurrence Of West Nile Symptoms

October 9, 2010

While many people who become infected with the mosquito-borne West Nile virus do not show signs or develop symptoms, a new study suggests it may actually happen more than has been realized.

West Nile fever was first documented in the US in 1999, after an outbreak in New York City. An investigation suggested that about 20 percent of infected individuals developed symptoms, which included fever, headache, rash and muscle/joint pain.

Most people recovered from the infection with no long-lasting complications. But in about 1 percent of cases, the virus invades the brain and spinal cord, causing potentially fatal swelling.

The US Centers for Disease Control and Prevention agreed with initial New York numbers, stating that about 80 percent of infections cause no symptoms at all.

But in the new study, researchers found that among 576 US blood donors whose blood tested positive for West Nile virus, more than half reported having at least one symptom of the infection within two weeks of their donation, and twenty-six percent said they had 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,” senior researcher Dr. Susan L. Stramer, executive scientific officer for the American Red Cross, told Reuters.

The new study also suggests that infected individuals can develop a wider range of mild symptoms than commonly believed, Stramer explained.

Participants in the study were asked about having any of 14 symptoms. Those symptoms included everything from fever, headache, rash and body aches, to abdominal pain, muscle and joint pain, and vomiting and diarrhea. Despite being named West Nile fever, 44 percent of symptomatic donors reported no fever.

Infected participants were compared to a control group of 615 uninfected donors who were also surveyed about symptoms in the two weeks of giving blood.

Fifty-three percent of the infected group reported at least one symptom. That compared with 11 percent of the control group.

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

Thirty-five percent of women reported symptoms, while 25 percent of men had them. Stramer said the reasons for the findings are unclear.

73 percent of individuals who reported three or more symptoms said they sought out 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; none were diagnosed as having brain or spinal cord infection.

Stramer believed the low diagnosis rate likely reflected the fact that West Nile testing was only performed in serious cases.

The findings will not affect how West Nile infection is managed. There is no specific treatment for the infection, Stramer said, and nearly all infections resolve on their own.

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

Although, people who do have severe West Nile-like symptoms — a spike in fever, severe headache, stiff neck, confusion, or sudden weakness in the limbs — should seek medical attention, since these are potential signs of brain and spinal cord infections.

While the current study findings do not call for more testing, Stramer said they do help define the range of mild symptoms that the infection may cause.

Image Caption: The proboscis of an Aedes albopictus mosquito feeding on human blood. Under experimental conditions the Aedes albopictus mosquito, also known as the Asian Tiger Mosquito, has been found to be a vector of West Nile Virus. Aedes is a genus of the Culicine family of mosquitoes. Credit: James Gathany / CDC

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