Killer Mosquitoes

In the age of SARS and mad cow disease, there's another public-health danger to worry about, and health officials think it will kill birds, horses, and maybe even humans this summer. West Nile virus has arrived in the Northwest.

When veterinarian Ken Leisher took the ferry to Whidbey Island last October, West Nile virus was low on his list of suspects. His patient was a 17-year-old gelding with a 105-degree fever. "The owners noticed he was less interested in his feed, mopey, and lethargic," says Leisher. "It sounded like influenza." But then Leisher saw the horse. "He was unsure, unsteady on his hind legs. It was evident that we were dealing with a neurological problem," he says. The veterinarian started supportive medications and sent off blood tests for a handful of diseases. The horse got worse in the next two days. "He was stumbling a little bit, more weak," says Leisher. "By the fourth day he had muscular twitches and he was apprehensive. Everything was just really saying West Nile."

West Nile virus was known to be in Western Washington. A month before, tests on a dead crow in Snohomish County had come back positive. The horse's blood tests confirmed West Nile, making Leisher's patient the third known infection in the state, the second in Western Washington, and the first in a mammal.

The next month, last November, veterinarian Sharon Hoofnagle encountered a case in Whatcom County. "When I went out it sounded like a case of colic," she says of the 14-year-old mare. "But it didn't sound right. Alarm bells were going off. She had a dazed, glazed look in her eyes. Whether she was in pain, I don't knowbut I believe she was. She was fearful. Animals, like humans, do show fear in their eyes."

Both horses were healthy and well cared for. Both had been given the West Nile virus vaccine but came down with the infection before immunity could be established. Both horses have recovered completely, perhaps due to partial immunity from the vaccine. Once horses become sick enough to see a veterinarian, though, they have a 30 percent mortality rate.

Severe acute respiratory syndrome (SARS) and mad cow disease have been getting all the headlines lately. But West Nile virus is more certain to make a mark on this region. It is expected to attack birds, horses, and humans statewide this summer, starting sometime after mosquitoes start hatching. In fact, state health officials said last Friday, May 30, that they think a man in Franklin County has the virus. If that's confirmed in a few weeks by tests, he would be the first human infected in Washington. West Nile first appeared in North America in 1999, in New York, and started marching west. No one knows how it reached New Yorkby ship or plane, or in a mosquito or a bird. Last year the virus reached the West Coast. In 2002, 4,156 humans and more than 14,000 horses across North America got sick enough for a physician or veterinarian to conduct tests that found the virus. Complications from the virus killed 284 people and a third of the horses. No one knows how many more thousands of humans and horses were not sick enough to be tested.

We do know that, of humans who have antibodies in their blood indicating that they have had the infection, 80 percent do not remember being sick, and 20 percent remember a flulike illness with fever and muscle aches. Only one of 150 infected people develops encephalitis (inflammation of the brain), the most dangerous complication, which is fatal in one out of 10 cases. The only proven risk factor is age. Eighty percent of West Nile encephalitis sufferers last year were over 40. The median age of those who died was 78, with a range from 24 to 99.

Humans and horses sit in the same place in the West Nile virus ecosystem. From the point of view of the virus, mammals are dead ends. Mosquitoes deliver it to humans, horses, and other mammals. But mosquitoes cannot acquire the virus from mammals. Infected mammals just don't build up enough virus in the bloodstream to make a mammal-sucking mosquito a carrier. So where do they get it, and how does the disease spread? Infected birds. They turn into factories for the virus, producing enough viral particles in the bloodstream to infect a mosquito that lands for a blood meal. Wildlife biologists are not sure which birds, or how many, spread the virus.

People can protect themselves and each other with clothing, insect repellent, and mosquito habitat reduction. Animals can't protect themselves, obviously, and are at far greater risk. The virus might turn out to affect people more by killing animals and altering ecosystems than by directly infecting humans. But the risk to humans is real, and while horses can be vaccinated, there is no vaccine for people.