West Nile virus survives winter; no surprise, says CDC

Testing of mosquitoes in New York City and a dead red-tailed hawk in Connecticut have led health officials to conclude that the West Nile encephalitis virus has survived the winter.

Now, they are left to wait and wonder whether the warming temperatures will usher in a new rash of infections in the Northeast and possibly new regions of the United States, and about the effectiveness of the multistate surveillance and prevention program, developed in the aftermath of last year's deadly outbreak.

In the New York City area last year, encephalitis infections with the West Nile virus were blamed for seven human deaths and 61 known nonfatal cases. Hundreds of birds, mostly crows, died throughout New York state, Connecticut, and New Jersey. Several horses on Long Island died or had to be euthanatized because of illness (see JAVMA, Nov 15, 1999, page 1415).

The national Centers for Disease Control and Prevention reported early this March that an experimental test had detected low concentrations of West Nile viral ribonucleic acid in three of 67 poolings of wintering Culex mosquitoes collected between January and February in Queens, NY, where human infections had been documented.

Questions about the test's high sensitivity had the CDC cautioning that the results could be anything from false positives to evidence that the West Nile virus resided in the insects. Additional testing would show that the mosquitoes were, in fact, carrying the live virus, which was "the expected result," according to Nick Komar, ScD, vertebrate ecologist in the CDC's arbovirus disease section.

"There was no evidence that the virus disappeared," Dr. Komar told JAVMA News, about speculation the virus would not endure cold weather. Flaviviruses such as the St Louis and West Nile viruses are known to survive the winter in the Culex species, he said.

Shortly after the CDC publicized its initial findings, scientists at the University of Connecticut announced that a red-tailed hawk, found dead Feb 6 in Westchester County, NY, and brought to Connecticut for necropsy, had died from West Nile encephalitis.

When it was learned last year that a foreign zoonosis had been introduced into the United States, government officials mobilized. The CDC and USDA, in cooperation with numerous state and local health departments and other agencies, devised a surveillance network stretching from the Northeast to states along the Atlantic and Gulf coasts.

Birds are natural hosts for the West Nile virus, although they do not pose a direct threat to humans, instead serving as reservoirs for mosquitoes that infect humans and other mammals. The Culex appears to be the primary vector for the virus.

The federal National Wildlife Health Center says the virus has so far been detected in 19 bird species indigenous to the Northeast. Dr. Linda Glaser, a wildlife disease officer with the center, said that American crows and fish crows account for most of the bird deaths.

Because of bird migration patterns, states have been warned to monitor for bird die-offs and to test for West Nile activity in sentinel bird species. Mosquito surveillance and general alerts were also issued to veterinarians to watch for neurologic illness in animals, especially horses. Human health care providers are to be sensitive to encephalitic infections.

Dr. Herbert Van Kruiningen, director of the Northeastern Research Center for Wildlife Diseases in Connecticut, said the red-tailed hawk was one of hundreds of dead birds brought to the center since September. A necropsy revealed the hawk had focal encephalitis, and viral isolation attempts yielded West Nile virus. A second laboratory, at the Connecticut Agricultural Experiment Station, confirmed the virus isolation.

The most likely scenario, Dr. Van Kruiningen and his colleagues reason, is the hawk became ill and died after eating an infected animal, probably a crow. "We know that the tissues of the [infected] bird are rich with virus," he said. "What do these raptors do but feed all day? They stay alive by killing other birds and some small mammals."

When the JAVMA was going to press, the CDC had not received samples of the hawk, and was unwilling to confirm that the West Nile virus had indeed killed the bird.

That the virus could be transmitted orally is one of a number of possibilities, but one that has not yet been closely examined, according to the CDC's Dr. Michael Bunning, an epidemic intelligence service officer in the vector-borne infectious diseases division. "No one's done any work with raptors and hawks, so anything right now would be speculation," he said.

Poultry studies by the USDA have found that chickens and turkeys were susceptible by injection to the West Nile virus but did not develop clinical signs, and they appear to pose little threat to humans. CDC studies found similar results.

Researchers infected turkeys with the virus, said Dr. David Suarez, veterinary medical officer with the Agriculture Research Service in Athens, Ga, but the birds' titers did not reach a concentration high enough that would enable them to serve as vectors.

Testing of chickens indicates that, unlike the turkeys, there seems to be a brief window when their West Nile virus titers climb to a level where the chickens could serve as potential reservoirs, according to Dr. Beverly Schmitt, chief of the diagnostic virology laboratory at the National Veterinary Services Laboratories in Ames, Iowa.

Over the coming months, health officials will watch for signs that the West Nile virus has again become active; veterinarians are an essential component to the surveillance network.

"We need to keep the vigilance up and keep looking," Dr. Bunning said. "Veterinarians are going to be paramount in listening to phone calls about unexplained bird deaths or, in the off chance we see some more of this disease in equids, listening to that information intently and then submitting samples to verify [the virus]."