West Nile Virus:
The Latest Threat To Raptors

by Stephanie Streeter

In just three short years, the West Nile Virus (WNV) has spread from coast to coast. The fast moving virus was first discovered and identified in New York in 1999. By 2002 it had made its way to California, showing up along the way in every contiguous state except Arizona, Utah, Nevada and Washington.

According to the Centers For Disease Control and Prevention (CDC), as of October 7, 2002, there have been 2,736 confirmed human cases of WNV, with 146 reported deaths. In the tri-state area the numbers are 35 confirmed cases and 4 deaths in Pennsylvania, 51 cases and 3 deaths in New York, and 6 cases and 0 deaths in New Jersey. Although mosquitoes have tested positive as WNV virus carriers and two crow deaths have been attributed to the virus, to date, no humans have been infected in Pike County, Pennsylvania, home of the Delaware Valley Raptor Center's two facilities. However, not far away, a Scranton woman was positively diagnosed on October 10, 2002 as the first WNV victim in Lackawanna County. The 73-year-old is reported to be paralyzed and on a respirator.

As troubling as these numbers are, birds have taken a much harder hit, with a disturbing number of raptors falling victim to the disease. As of October, 7, 2002, the CDC has positively attributed 4,462 bird deaths to the WNV, involving 111 species. These, of course, are just the birds that have been found and tested. How many more have died from the disease in the wild is not known. Dr. Pat Redig, Director of The Raptor Center at the University of Minnesota who treated the first confirmed case of WNV in that state on August 23, 2002 (a great horned owl), said in an interview with Blade Science writer, Jenni Laidman, "I have a sense of a Tsunami coming through the midsection of this country. I don't think it's a stretch to assume the number of birds killed by this is in the thousands."

Originally, in the Northeast, crows and bluejays were the species most commonly affected by the virus. Now, however, any avian species brought to rehabilitation centers, or in captive situations such as zoological parks, breeding and education facilities, or belonging to falconers, that exhibit symptoms including head tremors, balance problems, leg paralysis, seizures, vision difficulties and unexplained molting are suspect, and are being tested. To date, we have not seen any WNV infected birds at the Delaware Valley Raptor Center, but other centers have, and two of this state's zoos, Zoo America in Hershey, PA and the Pittsburgh Zoo, have lost exhibit birds. The Pittsburgh Zoo suffered significant loses to its raptor population with the deaths of two snowy owls, two golden eagles and one red-tailed hawk.

Raptors in Ohio and midwestern states seem to be taking a particularly hard hit from the WNV. The great horned owl is the species most often seen, but many others have fallen victim including the snowy owl, red-tailed hawk, kestrel, merlin, Cooper's hawk and gyrfalcon. According to Mona Rutger of Back To The Wild, Ohio is experiencing a wildlife crisis with an unheard of number of raptors being brought to rehab facilities. Many of the birds are dying and dying quickly, within 48 hours of admittance.

So, what has changed to make birds of prey suddenly so much more vulnerable to the virus? And, why are more birds dying in the Midwest than in the East? To answer the last question first, the assumption is that eastern raptors have been exposed to the virus for a longer period of time and have therefore had a chance to build up an immunity to the disease. Undoubtedly, some birds in the wild have been infected by the virus but have not succumbed to it. These survivors are the strongest of their species and are passing on their robustness to their offspring; natural selection at work. By contrast, the birds in the Midwest are a virgin population in that this is the first or, at most, the second season they have been exposed to the WNV. The mortality rate will be higher until the population begins to build a natural immunity to the disease.

There has also been some question as to the possibility of the virus mutating which could possibly account for the increase in species involved and the increased mortality rate in the Midwest. But, to date, that has not been proven and, in fact, the strain of the virus in the Midwest is identical to the strain found in Israel from which the New York virus originated.

As to why birds of prey are suddenly so much more vulnerable, the answer to this question is a little more complex. Originally, the vector species for the West Nile Virus was thought to be the mosquito, and although this annoying little blood-sucker continues to be a major player in this game, it is no longer the only one. Katherine McKeever of the Owl Foundation in Vineland, Ontario lost 74 owls at her facility. During this devastating loss, she noticed that the WNV infected birds were infested with an unusually high number of hippoboscid flies, commonly called flat or louse flies; some with as many as 400. This blood-sucking parasite which is often found on raptors, rather than the mosquito, she surmised, was what had infected her birds. Tests conducted at her facility confirmed her suspicions, the hippoboscid flies taken from her sick owls tested WNV positive. In Pennsylvania, testing of hippoboscids began this summer and here too, they have been WNV positive.

Another possible transmission pathway is the ingestion of WNV infected birds and animals. Raptors are not above occasional scavenging and will eat prey they themselves have not killed. Great horned owls are one of the few birds of prey that will readily eat crows, a species especially hard-hit by the virus, which may explain why so many great horned owl cases are showing up at rehab centers. Because transmission of the virus via bodily fluids is now considered highly probable, wildlife pathologists have begun taking extra precautions when performing WNV post-mortems. Ward Stone, the chief wildlife pathologist with the New York State Department of Environmental Conservation said in a posted e-mail that because of the possibility of contamination via bodily fluids he has instituted changes in necropsy room dissection techniques at his facility. Everyone is now required to wear stainless steel mesh gloves over one or two pairs of latex gloves when dissecting to reduce the risk of contamination via scalpel cuts and the piercing of flesh by bone splinters. Although, he admits, it greatly slows dissections, it makes for a safer setting which is important since there is as yet no human vaccine.

Each day seems to bring worse WNV news than the day before - more species dying, greater mortality, continued rapid spread, multiple transmission pathways; but amidst the distressing daily reports came positive news on September 20, 2002. According to a press release, the American Bird Conservancy (ABC) in partnership with the American Zoo and Aquarium Association (AZA) and with support from the Disney Wildlife Conservation Fund have developed a West Nile Virus vaccine for birds. The first round of trials, carried out by The Centers for Disease Control and Prevention (CDC) in Fort Collins, Colorado, have been completed with encouraging results: inoculated birds (American crows who suffer a 100% mortality rate in the wild when infected with the disease were used) showed a 60% increase in survival rates over unvaccinated birds in lab tests.

"We are very pleased with the initial tests and believe this could be an effective tool for protecting some of our most endangered birds such as Mississippi Sandhill Cranes and California Condors," said Dr, Patti Bright, a veterinary epidemiologist, and Director of ABC's Pesticides and Birds Campaign. "We are now looking forward to conducting field trials at participating institutions to see how the vaccine performs in realworld situations, outside the laboratory."

Paralleling the development of the recombinant DNA vaccine in the US, scientists in Israel (where the virus was first discovered) have been working on a killed vaccine, which has also showed early promise in laboratory trials. Concurrently, researchers at CDC, in collaboration with Colorado State University, United States Army Medical Research for Infectious Diseases, Harvard University, and Temple University, are investigating the manufacture of an oral vaccine for use on wild bird populations. Recent successes with injectable vaccines may lead one step closer to this goal.

Until this breakthrough, some of those managing captive raptor populations have been giving their birds adjusted doses of an equine WNV vaccine even though its use on birds was not approved or recommended by the manufacturer. Did it work? No one knows for certain; but, thankfully, there have been no reported deaths due to administering of the vaccine.

As with all other new viral diseases, mortality rates should begin to drop as our avian populations develop a natural immunity to the WNV. In the meantime, cold winter weather will give many parts of the country a reprieve by killing off the mosquitoes, still the primary vector species, and scientists will continue their efforts to get an effective avian WNV vaccine on the market as soon as possible. For those of us working with wild birds, it can't be soon enough.

Editor's Note: Most state and many local government agencies have programs in place to collect and test dead birds suspected of having died of the WNV. A complete list of each state's WNV contacts can be found on the National Pesticide Information website at, http://npic.orst.edu/wnv/stateinfo.htm

Before retrieving a dead bird, please follow these simple steps to insure your safety:

1. Wear plastic gloves when handling the carcass. If you do not have gloves, place your hand in the plastic bag in which you will be placing the bird, take hold of the carcass and turn the bag inside out, enclosing the carcass in the bag while simultaneously freeing your hand.

2. Double bag the carcass in two plastic bags. Securely close each individually.

3. Take the bird directly to a designated drop-off center. If you cannot transport it immediately, refrigerate the plastic wrapped carcass or put it on ice to keep it cool until delivery is possible. Do not freeze.