Crows are highly susceptible to West Nile Virus (WNV), which was first identified in the Western Hemisphere in 1999. The virus is transmitted by mosquitoes, which acquire it from infected birds. And because researchers believe that migrating birds may play a role in spreading the disease, animal health officials are carefully surveying for the virus in a wide range of wild birds.

So far, West Nile Virus has been confined to five northeastern states—New York, Rhode Island, New Jersey, Connecticut and Pennsylvania. There were 62 human cases of WNV-induced encephalitis in 1999, including seven deaths. A total of 25 horses were diagnosed; nine had to be euthanized. Through mid-October of 2000, there were 29 confirmed cases of WNV in horses; 16 were euthanized.

Clinical signs of West Nile Virus infection in the horses included listlessness, stumbling, lack of coordination, weakness of limbs, partial paralysis or death. Fever was not generally observed.

Horses are incidental hosts; they do not transmit the disease.

"It is highly unlikely that mosquitoes feeding on an infected horse could ingest enough of the virus to transmit it to other animals," say representatives from the Veterinary Services (VS) division of the U.S. Department of Agriculture’s Animal and Plant Health Inspection Service. "Horses are humanely euthanized only when they are suffering from illness from which they will not be able to recover."

Likewise, it is unlikely that affected horses would have to be quarantined since they do not appear to be carriers of the disease. However, there have been trade implications. During the 1999 U.S. outbreak, the European Union, Hong Kong, Brazil, Argentina and the United Arab Emirates restricted importation of horses or poultry from all or part of the United States.

No vaccine or treatment is currently available for the West Nile Virus; however VS is working to assist all companies interested in producing a vaccine.

Meanwhile, APHIS has developed a diagnostic test for WNV and is on the lookout for virus activity along the Atlantic seaboard and in other parts of the country. The program investigates any horses showing clinical signs of encephalitis in which other common causes such as rabies can be ruled out.

Inoculation studies have been conducted to determine the effects of WNV on other animals. The tests suggest that sheep, chickens and pigs could be affected by the virus; cows may show antibodies, which means they have contracted the virus without showing any clinical signs or becoming ill.

The wildlife services program of APHIS continues to collect specimens of wild birds for WNV testing, initially focusing on East Coast states from Connecticut to Florida. People who find dead birds should notify their local health officials.

"The key to preventing or controlling future outbreaks of WNV among horses is to prevent animals from being exposed to mosquitoes, which are vectors for the disease," says Alfonso Torres, deputy administrator for VS.

The following recommendations are based on current knowledge about WNV:

The most important step any property owner can take to control mosquito populations is to remove all potential sources of stagnant water where mosquitoes might breed, Torres advises. Mosquitoes can breed in any puddle that lasts more than four days. Precautionary steps include:

• Dispose of any water-holding containers, including discarded tires;
• Drill holes in the bottom of containers that are left outdoors;
• Clean clogged roof gutters annually;
• Turn over plastic wading pools or wheelbarrows when not in use and do not allow water to stagnate in bird baths;
• Aerate ornamental pools or stock them with fish;
• Clean and chlorinate swimming pools that are not in use; and
• Thoroughly clean livestock-watering troughs monthly.

Use of insect repellents may be of some value in decreasing exposure of horses to adult mosquitoes; however, repellents alone should not be relied upon to prevent mosquito exposure.

Housing animals in structures with well-maintained insect screening can reduce exposure to adult mosquitoes. Be sure to eliminate mosquitoes from inside the structure first by using pesticides that kill adult mosquitoes.