West Nile Virus

USDA Veterinary Services
March 2003

West Nile is a mosquito–borne virus that was first detected in the United States in 1999. The virus, which causes encephalitis, or inflammation of the brain, has been found in Africa, Western Asia, the Middle East, the Mediterranean region of Europe, and, most recently, in the United States.

Mosquitoes acquire the West Nile virus (WNV) from birds and pass it on to other birds, animals, and people. While humans and horses may be infected by the virus, there is no documentation that infected horses can spread the virus to uninfected horses or other animals. Migrating birds appear to play a role in spreading the disease.

U.S. History

On September 14, 1999, the National Veterinary Services Laboratories (NVSL) of the U.S. Department of Agriculture’s (USDA) Animal and Plant Health Inspection Service (APHIS) isolated a virus from neurological and other tissues of flamingos and tragopans (pheasants) from the Bronx Zoo and crows from the New York City area. Samples of the isolated virus were sent to the U.S. Department of Health and Human Services’ Centers for Disease Control and Prevention (CDC) for identification. On September 27, 1999, CDC officials announced that the isolated virus was very similar to that of WNV, previously unseen in the Western Hemisphere. CDC later confirmed the virus as West Nile and connected it to an encephalitis outbreak that killed 7 people and infected at least 55 others in the New York City area in 1999. The virus has since been identified in horses, mosquitoes, or wild birds in at least 43 States and the District of Columbia.

Because the virus is transmitted by mosquitoes, it has the potential to affect livestock and poultry. In 2002, more than 15,000 equines in 40 States were diagnosed with cases of illness caused by WNV.

Clinical signs of WNV infection in horses include ataxia (stumbling or incoordination), depression or apprehension, weakness of limbs, partial paralysis, muscle twitching, or death. Fever is not often observed.

Although the role of commercial poultry in maintaining or transmitting the virus is not thoroughly understood, no clinical signs have been reported in U.S. poultry. According to scientific literature, chickens can develop a short–lived infection, but clinical signs are not seen.

Federal Actions to Protect Agriculture

Any horses showing clinical signs of severe encephalitis in an area without previous equine cases that year, where other common causes such as rabies can be ruled out, will be investigated by APHIS’ Veterinary Services (VS) program as having a potential foreign animal disease. VS has developed reliable diagnostic tests for WNV and is on the lookout for virus activity across the United States. APHIS’ Wildlife Services (WS) program collects wild birds to test for WNV.

USDA’s Agricultural Research Service has conducted WNV inoculation studies with turkeys. NVSL did the same with chickens and NVSL’s Foreign Animal Disease Diagnostic Laboratory conducted studies with horses. Only chickens showed the likely potential to produce enough virus in their blood to infect mosquitoes. Each species developed antibodies after being inoculated, and encephalitis was not observed in any of the animals.

VS continues to treat all situations where birds show neurological signs as having the potential for hosting a foreign animal disease. Such a response allows NVSL to rule out exotic poultry diseases, such as Newcastle disease and highly pathogenic avian influenza, before testing for WNV.

The CDC, the U.S. Geological Survey’s National Wildlife Health Center, and APHIS are cooperating to survey for the virus in a wide range of wild birds. This Federal working group, in conjunction with relevant State agencies, gathers and analyzes surveillance data to define the extent to which the virus may be distributed in mosquito and bird populations in the United States.

How You Can Protect Your Animals

You can decrease the chances of your animals being exposed to WNV by decreasing their exposure to adult mosquitoes. The best way to do this is by removing any potential sources of water in which mosquitoes can breed. Dispose of any water–holding containers, including discarded tires. Drill holes in the bottom of containers that are left outside. Clean clogged roof gutters on an annual basis. Turn over wading pools or wheelbarrows when not in use, and do not allow water to stagnate in birdbaths. Aerate ornamental pools or stock them with fish. Clean and chlorinate swimming pools that are not in use and be aware that mosquitoes can breed in water that collects on your property; mosquitoes can breed in any puddle that lasts more than 4 days. Thoroughly clean livestock–watering troughs on a monthly basis. Local mosquito-control authorities can help in assessing the mosquito–breeding risks associated with your property.

Pet birds can also be protected by limiting their exposure to mosquitoes. In areas reporting large numbers of wild bird deaths, investigations are conducted and samples collected to determine the cause of deaths. People finding dead wild birds should notify local health officials.

No treatment is currently available for WNV; however, VS is working to assist all companies interested in producing a vaccine. On August 1, 2001, USDA issued a conditional license to Fort Dodge Animal Health of Fort Dodge, IA, a division of Wyeth, for a vaccine intended to aid in the prevention of disease in horses caused by WNV. In November 2002, a full license was granted for this product. Use of the vaccine is restricted to licensed veterinarians.

Additional Information

For more information about WNV, contact:
USDA, APHIS, Veterinary Services
4700 River Road, Unit 41
Riverdale, MD 20737-1231
Phone: (301) 734-8073
Fax: (301) 734-7817

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