Sentinels set for new, mosquito-borne virus

Blood is drawn from hundreds of chickens around Florida and tested for West Nile encephalitis in a Tampa lab.

By WES ALLISON

© St. Petersburg Times, published March 12, 2000

TAMPA -- In a dingy state laboratory in the shadow of Legends Field, Dr. Lillian M. Stark and a team of technicians search hundreds of samples of chicken blood each week for a deadly disease they've never seen.

The chickens are Florida's first defense against West Nile encephalitis, a potentially fatal mosquito-borne virus that was first discovered in the United States last summer when it killed at least eight people in New York and likely infected hundreds more.

No one knows how it got here, but public health officials from Maine to Florida have turned their attention to tracking its spread and trying to prevent more deaths. Birds carry the disease, and many that spent last summer in the Northeast would have headed south for the winter. Specialists at the federal Centers for Disease Control and Prevention fear West Nile may have migrated with them.

If it did, more outbreaks could occur this spring and summer as mosquito populations rise.

"This was a total shock, a totally new virus in the United States," said Stark, director of the state Department of Health's lab in Tampa. "We're concerned. It's not more dangerous than some of the ones that we have here, but we don't know how it's going to act."

Last week, the Centers for Disease Control and Prevention reported that traces of the virus have been found in hibernating mosquitoes in New York City. Mosquitoes transfer the disease from birds to humans and other animals, and Stark's lab examines blood taken from small groups of chickens, called sentinel flocks, posted around the state.

Although West Nile appears about as dangerous as St. Louis encephalitis, another mosquito-borne virus that occasionally strikes the South and Midwest during the summer, health officials fear the unknown: They don't know how easily it is transmitted to people, or which mosquitoes are the best carriers, or how widespread the virus could be.

And because last summer's outbreak occurred in New York, where any form of encephalitis is rare, they worry West Nile may thrive in a warm, wet environment such as Florida's.

"With the level of mosquito populations in Florida and our climate, certainly we are more hospitable," said Dr. Sandra G. Gompf, chief of infectious diseases at the James A. Haley VA Medical Center in Tampa.

"I wonder whether it's sort of out there, lurking and multiplying, and when it gets warm we might see some."

West Nile is native to West Africa and parts of the Middle East and recently has spread to Eastern Europe. The virus attacks the central nervous system, and its early stages feel like the flu, with high fever and pounding headache. As with the flu, doctors can treat only the symptoms, and most people's immune systems beat the virus in several days.

But if it advances, West Nile can cause brain and neurological problems, including seizures, lethargy, paralysis and coma. The disease kills up to 15 percent of its victims, and the very young, the sick, and people older than 50 are at the highest risk.

Doctors vary in their opinions of how seriously West Nile will threaten the United States, but they don't dispute the potential for trouble. Their concerns have prompted what insiders describe as a rare display of cooperation and hustle, led by the CDC, to help Eastern and Gulf Coast states set up systems for detecting the virus in birds before it attacks people.

Most states are only now getting started, the CDC says. But Florida, which endures local outbreaks of St. Louis encephalitis every four to seven years, is simply racheting up its existing surveillance network for mosquito-born diseases.

If West Nile makes it here, it likely will be discovered in the Tampa laboratory, a former tuberculosis hospital in a run-down government complex off Martin Luther King Boulevard. On Monday, Stark locked the front door -- the lab lost its secretary to recent budget cuts -- and trudged up a dim stairwell to the crowded second floor, where technicians test samples of blood from the sentinel chickens.

The lab has been screening the birds for St. Louis encephalitis since 1962, following an outbreak that started near Lake Maggiore in St. Petersburg and killed 35 people. The lab also looks for dengue fever and Eastern equine encephalitis, a less common but more deadly virus that thrives in the Florida swamps.

This winter, the lab began testing for West Nile as well. Stark says the technicians haven't found it, although the encephalitis season doesn't usually begin for another month. Most of the sentinel chicken flocks haven't been pressed into service yet, either, but from May to December -- when mosquitoes are most active -- blood is drawn weekly from 900 birds in 29 central and southern Florida counties where encephalitis historically has been most active.

Neither St. Louis nor West Nile hurts the chickens, but their immune systems generate antibodies to fight it. If those antibodies show up in the tests, Stark knows the birds were bitten by mosquitoes that are carrying the disease.

Health officials then can warn residents to avoid mosquitoes, and local mosquito control units can intensify their spraying. At times when the lab has detected the St. Louis virus, including 1997, when nine were sickened and one died, such efforts seem to have kept major outbreaks at bay, officials say.

But Dana Land, entomology lab supervisor for the Pinellas County mosquito control unit, noted that the mosquito carrying the virus in New York, culex pipiens, doesn't live in Florida, so officials won't know which bug to target until after the disease strikes.

A cousin of pipiens does live here, and the mosquito that carries St. Louis encephalitis is numerous. It attacks after dusk, and is so small that most people don't even realize they've been bitten.

"If West Nile shows up, yes it would (be difficult)," said Land, whose department monitors seven flocks of sentinel chickens scattered around Pinellas. "We'd have to work to find out which type of mosquito is actually carrying the virus."

Last summer, health officials in New York City and its suburbs confirmed 61 cases of West Nile encephalitis. But as a rule of thumb, epidemiologists say, up to 300 people are infected for every confirmed case of a mosquito-borne illness, and many likely got the virus but brushed it off as just another bug.

Reaction to the disease was swift. By early November, representatives from Eastern and Gulf Coast states had gathered at the CDC in Atlanta to develop guidelines for handling West Nile, and the final version is expected to be released to states this week. A CDC workshop on testing, which Stark will attend, is scheduled next month in Colorado.

Dr. Lisa Conti, public health veterinarian in the Department of Health's bureau of epidemiology, said veterinarians are being asked to test sick horses for the disease, and her department has asked the CDC for money for improved testing for West Nile in mosquitoes in the field.

And because an outbreak in humans quite likely will be predicated by a fatal outbreak in wild birds, the state Fish and Wildlife Conservation Commission is developing a plan for reporting dead birds, she said. In the Northeast, dead crows proved excellent indicators that West Nile was in the neighborhood.

Gompf said it's also important that family doctors and infectious disease specialists at hospitals be open to the possibility that a patient has encephalitis.

"It's scary, but it's interesting," Gompf said. "Now we're aware of the possibilities. If we do start seeing an outbreak, or some cases, we can look at it more objectively, and truly analyze what happens."

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