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May marked the beginning of another West Nile Virus season in Tennessee, and state health officials predict that, if recent trends continue, 2007 will be worse than the previous few years.

"While numbers of West Nile virus cases are hard to predict, with the rise in cases last year, it is possible we could see a more severe season in Tennessee this year," said Abelardo Moncayo, state medical entomologist. Dr. Timothy Jones, the Department of Health's deputy state epidemiologist, concurs and has a recommendation for Tennessee physicians: "I think, in the summer, if a patient has a fever illness and there's no other obvious cause, it's probably worth considering West Nile."

In 2006, the number of human cases in Tennessee increased from 18 to 22, which was the largest number since 2003, when 26 cases were reported. Last year, there was one fatality. "We have had a scattering of cases all over the state in the past years, but the majority every year are in Memphis. From the very beginning, that's been our hot spot," said Jones, surmising that the particular breed of mosquitoes common to the Memphis area, coupled with their sheer numbers, make the region most susceptible.

West Nile Virus is one of several mosquito-borne viruses that can infect people, and the mosquitoes become infected when they feed on infected birds. West Nile reared its head in this hemisphere in 1999, when it was introduced into New York. "Each year since then, it further spread, and in 2001, it hit Tennessee," Jones recalled.

Yet, do just 22 cases statewide last year render the disease a public health risk? "Even though the numbers are pretty small, older people particularly are more susceptible to severe infection and death. Also, particularly in the early years, it created a lot of panic, got a lot of media attention and was forefront in everyone's mind," Jones said, then added, "Any kind of mosquito-borne or vector-borne disease is of concern because, basically, anyone is at risk. There are a lot of diseases where you can control your exposure, but mosquitoes bite everybody."

Mosquito control is the first line of defense, but that's not the state's job. That falls to local governments — and some tackle the job while other, usually smaller, jurisdictions don't. "Memphis has a very mature program," Jones said, as well as most of Tennessee's larger cities and counties.

The first step is usually larvae sighting, when insecticide is applied to areas where mosquitoes lay their eggs. "The public doesn't see this. You put it in ponds, down drains and where there's standing water, and hopefully that will knock them off before they even hatch," he said.

The second step is spraying, "but not just spraying willy-nilly all over the state, because we don't have the resources to do that." What happens instead is local health departments set mosquito traps, collect the insects and send them to the state laboratory, where they are tested for West Nile. "If we detect the virus in mosquitoes in a certain area, then they target that area to spray," Jones explained.

A dead bird may be another clue that West Nile has arrived; thus, the state encourages citizens who find a freshly dead crow, blue jay or – for the first time this year — American robin — to call the local health department.

"We only test birds in a county until they find positives. Once they've detected the virus in the area, there's no need to continue testing because we know it's there," Jones said. "Basically, we consider the birds to be an early warning system. We almost always find birds before we start having human cases."

An ounce of prevention is certainly the rule during Tennessee's summers, and Jones encourages physicians to mention preventative steps to their patients. "Number one in my mind is wearing insect repellant every time you go out at a time when you might get bitten in the summer. That's cheap and easy and there are a lot of choices now," he said, including unscented varieties and repellants that can be sprayed on clothes rather than skin. Other steps include wearing long sleeves and long pants when practical, getting rid of standing water and using screens on open windows.

"The good news is that 80 percent of the people who get West Nile Virus have no symptoms. So, obviously, those are the cases that we never know about," Jones said. "About 20 percent of the people will have mild flu-like symptoms in the summer, and may not see a physician at all. Less than 1 percent of people will get severe infection and wind up having meningitis. Those patients are usually pretty sick and in the hospital, and those are the ones where the physician is probably thinking about West Nile."


June 2007

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