Exotic virus spreads across Northeast; researchers worry about wildlife

Published on October 01, 2000
information-circle This article is more than 3 years old

With each passing week, the damage wrought by the West Nile virus on the Northeast's ecosystem grows steadily. Those states battling the virus watch as infection rates continue to spike for an ever-expanding spectrum of bird species and mosquito pools carrying the deadly virus. Human cases of West Nile fever, once confined to New York City, are also inching upward.

Shortly after this year's first equine infection was diagnosed on Staten Island, NY, in late August, an 82-year-old New Jersey man became the first person to die of West Nile virus infection this year. West Nile encephalitis had been diagnosed in 14 people by September. In Israel, 10 people have died and more than a hundred infected during a West Nile outbreak this summer.

Yet, while government and health officials here focus their energies on tracking the virus and protecting the public, there is little time or resources to look at how the virus may be affecting wildlife. In New York alone, more than 50,000 bird deaths have been reported this year—an alarmingly high number. There's no way to determine whether West Nile virus is the culprit in each case, but there may be reason for worry.

"Not all of those would have been from West Nile, but that's still a large, huge number of potential [infections], and with the crows in particular, we are concerned that a large proportion of those could be from West Nile," said Dr. Millicent Eidson, New York state public health veterinarian and director of the zoonoses program.

Ward Stone, New York state wildlife pathologist, has found the virus in a growing number of bird species, from sparrow hawks to great blue herons, indicating that all bird species are potentially at risk. In his 31 years of monitoring the state's wildlife, Stone has faced a host of exotic viruses, including the one causing duck plague. But of them all, he said West Nile virus has had the greatest impact on wildlife he's ever seen.

"I am very concerned about what this means to wildlife. It appears that many of our Western Hemisphere species are susceptible, not only to infection with the virus, but it is also pathogenic to them. That being the case, we have to especially worry about those species which have low populations."

He's anxiously monitoring the state's bald eagles for signs of the virus, although none of the approximately 200 birds has been affected so far.

Given the proliferation of West Nile virus in the Northeast, there's little doubt the virus is here to stay. But there is new evidence that has Stone and others wondering whether there's more to this exotic virus that has yet to be learned. Over the summer, a young raccoon and two species of bats found dead in New York turned out to be infected with West Nile virus.

Mosquitoes that have fed on infected birds can pass the virus to warm-blooded animals; of these, only humans and horses are said to be at risk of fatal encephalitis. Birds are highly susceptible to West Nile virus, and are thought to be the only viral reservoirs. But is it possible that the virus, now loosed upon an inexperienced ecosystem, is killing mammals once thought to be safe? And are birds the only viral reservoirs, or could bats also be carrying the virus into new regions?

"If we think bats and raccoons may be dying from West Nile, that could really be something that hasn't been seen before in the Eastern Hemisphere," said Dr. Linda Glaser, wildlife disease officer with the US National Wildlife Health Center in Madison, Wis.

As of press time, what killed the raccoon and bats had not been determined, but Stone "wouldn't be surprised" if West Nile virus has been killing mammals on other continents all along, and just hasn't been looked into. In places where the virus is endemic—Africa, India, and the Middle East—most studies focus on the virus' effects on public health and agriculture, not wildlife. It also might not be so apparent, he said, because animals have had generations to build immunity to the virus.

But is it possible that the virus, now loosed upon an inexperienced ecosystem, is killing mammals once thought to be safe? And are birds the only viral reservoirs, or could bats also be carrying the virus into new regions?

"Obviously the raccoon and bats are very distantly related," Stone said. "And if we've got the virus in these two, it's entirely logical that we're going to find it in a number of other mammalian species in the Western Hemisphere. And that raises another question: which ones?"

This past year's massive crow die-off announced the virus presence; this summer Dr. Glaser and Stone have heard reports of a similar kind of widespread mortality, but in bats in New York, Pennsylvania, and Ohio.

The cause could be any number of things, such as pesticides or harsh weather conditions. Bats are especially difficult to test because of precautions against rabies. But most laboratories are focused solely on testing wild birds as part of the multistate surveillance system, and cannot spare the time or limited resources looking at how West Nile virus may be affecting other animal species.

Stone wonders whether infected bats are contributing to the spread of the virus in New York. If bats are, in fact, viral reservoirs, then on the basis of the migratory patterns of various bat species, the wildlife pathologist sees the potential for West Nile virus to penetrate farther into the United States, and into Central and South America and Canada.

What the future holds and additional investigations will uncover are uncertain. It is hoped that the virus will be contained, the damage to the ecosystem minimal, and that wildlife will eventually build immunity. Vaccines for horses and people, according to Stone, could come along within the next five to 10-years.

Dr. Eidson maintains that veterinarians are key to combating West Nile virus. Because mammals are susceptible to the virus, she advises practitioners to educate their clients on how to reduce the chances of exposure. Some captive and pet birds have become ill, she noted, and cats and dogs are also at risk, although there are no reports of encephalitis infections this year.

"If there's an unusual neurological illness, not only should important diseases like rabies be considered, but there may be cases of West Nile virus occasionally, too."