Every week, health officials learn something new about the West Nile virus that exploded across the United States this summer.
When the virus first appeared in New York City in 1999, only humans, some bird species, and horses were thought to be susceptible to the exotic encephalitis virus. Now, dogs, squirrels, sheep, goats, even alpacas, aren't safe. Veterinary diagnostic laboratories in several states are seeing illness and death in mammalian species not known to have been susceptible to it.
This has been the worst year in the virus' brief history in the United States. At the end of 2001, West Nile virus had spread to the Midwest and as far south as Louisiana. Between 1999 and 2001, a total of 149 human cases and 18 fatalities had been documented in the United States.
As of Oct. 9, all but seven states have detected the virus within their borders, with nearly all of them reporting human cases. The Centers for Disease Control and Prevention is attributing 146 deaths and more than 2,500 infections to the virus, with Illinois leading the nation with 38 fatalities.
In August and September, the CDC awarded $18.6 million to several cities and states to help combat the virus. This year alone, the agency provided an estimated $35 million toward this end. Since 2000, more than $58 million has been made available to state and local health departments to develop or enhance epidemiologic and laboratory capacity for West Nile virus and other arboviral diseases.
With news of the apparent epidemic, recent discoveries about the virus having been making headlines. Four recipients of transplanted organs have contracted West Nile virus from a single infected donor. After five recipients of blood transfusions tested positive for the virus, the CDC, along with other federal agencies and blood collection groups, began an investigation into the risks of passing the virus through blood. An infant appears to have been infected through breast-feeding.
In addition, six people in Mississippi and Louisiana sick with the virus developed an acute flaccid paralysis, leading the CDC to encourage clinicians to check patients with AFP for West Nile virus infection. Now, the Food and Drug Administration has approved the first national trial of a drug to treat the infection.
Between Jan. 1 and Oct. 6 of this year, some 9,000 equine cases in 36 states had been confirmed by the Department of Agriculture's National Veterinary Services Laboratories or reported by state officials.
"As this disease makes its way across the continental United States, we are seeing an increasing number of West Nile virus cases in horses," said Bobby R. Acord, director of the USDA's Animal and Plant Health Inspection Service. "Our best estimates are that one in three WNV-affected horses will die."
The USDA is urging horse owners to protect their horses from West Nile virus through use of the approved vaccine or other preventive measures.
Veterinarians are making discoveries of their own about West Nile virus. Since 1999, the general consensus had been that only certain birds and a small number of mammalian species were at risk. Early in the outbreak, however, there were signs that other species—specifically raccoons, cats, and bats—were susceptible to the virus (see JAVMA, Oct. 15, 2000, page 1127).
And now there is growing evidence that the virus is, in fact, killing small numbers of other animal species.
In Illinois, West Nile viral infections were identified in several gray squirrels, a three-month-old wolf, and an eight-year-old dog with an immune-mediated disease. A mountain goat in Wyoming tested positive for the virus, and there are reports of mass die-offs in the gray squirrel population in a region of the state where West Nile virus is active.
The NVSL confirmed the virus in goat and sheep tissue samples submitted by Nebraska. And, finally, a fox squirrel in Michigan was infected, as well as an adult alpaca in Minnesota.
Since September, veterinarians in St. Tammany, La., have been conducting serologic surveys on dogs and cats that the CDC will examine to determine how the virus is affecting the pet population there. The parish is a hotbed of virus activity, with numerous human and equine cases, explained Dr. Brent Robbins, director of animal services in the parish.
Around 400 samples will be collected from cats and dogs during routine examinations. Free testing is also available for pet owners worried about the health of their animal. "What [the CDC] is really interested to know is if [dogs and cats] can be used as sentinels," Dr. Robbins said.
Although there is no dog or cat case of West Nile virus infection in Louisiana, some veterinarians in St. Tammany have seen unusual fevers and neurologic signs they can't explain in cats and dogs, according to Dr. Robbins.
He thinks that, like people, many animals are infected but don't show any signs of clinical illness. Of the small group that do, an even smaller number actually die as a result. "You would think with as active as the virus has been in the area, there would be numerous cat and dog illnesses and deaths," he said. "We don't see that."
The results of the serologic survey in the parish are expected to be ready in December. For continued updates about West Nile virus, visit the AVMA Web site at www.avma.org.