Vertebrate Ecology

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Transmission Cycle

West Nile (WN) virus is amplified during periods of adult mosquito blood-feeding by continuous transmission between mosquito vectors and bird reservoir hosts. Infectious mosquitoes carry virus particles in their salivary glands and infect susceptible bird species during blood-meal feeding. Competent bird reservoirs will sustain an infectious viremia (virus circulating in the bloodstream) for 1 to 4 days after exposure, after which these hosts develop life-long immunity. A sufficient number of vectors must feed on an infectious host to ensure that some survive long enough to feed again on a susceptible reservoir host.

People, horses, and most other mammals are not known to develop infectious-level viremias very often, and thus are probably "dead-end" or incidental-hosts.

Birds

West Nile virus has been detected in dead birds of at least 317 species. Although birds, particularly crows and jays, infected with WN virus can die or become ill, most infected birds do survive. Click here for more information on species of dead birds in the U.S. in which West Nile virus has been detected.

There is no evidence that a person can get WN virus from handling live or dead infected birds. Persons should avoid bare-handed contact when handling any dead animals, and use gloves or double plastic bags to place the bird carcass in a garbage bag or contact their local health department for guidance.

Dogs and Cats

West Nile virus does not appear to cause extensive illness in dogs or cats. There is a single published report of WN virus isolated from a dog in southern Africa (Botswana) in 1982. West Nile virus was isolated from a single dead cat in 1999. A serosurvey in New York City of dogs in the 1999 epidemic area indicated that dogs are frequently infected. Nonetheless, disease from WN virus infection in dogs has yet to be documented.

There is no documented evidence of person-to-person or animal-to-person transmission of WN virus. Because WN virus is transmitted by infectious mosquitoes, dogs or cats could be exposed to the virus in the same way humans become infected. Veterinarians should take normal infection control precautions when caring for an animal suspected to have this or any viral infection. It is possible that dogs and cats could become infected by eating dead infected animals such as birds, but this is undocumented.

There is no reason to destroy an animal just because it has been infected with WN virus. Full recovery from the infection is likely. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.

Horses

Cases of WN virus disease in horses have been documented, either by virus isolation or by detection of WN virus-neutralizing antibodies in 1999, 2000, and 2001. Approximately 40% of equine WN virus cases results in the death of the horse. Horses most likely become infected with WN virus in the same way humans become infected, by the bite of infectious mosquitoes.

In locations where WN virus is circulating, horses should be protected from mosquito bites as much as possible. Horses vaccinated against eastern equine encephalitis (EEE), western equine encephalitis (WEE), and Venezuelan equine encephalitis (VEE) are NOT protected against WN virus infection. A West Nile virus vaccine for horses was recently licensed, but its effectiveness is unknown. Horses infected by WN virus develop a brief low-level viremia that is rarely, if ever, infectious to mosquitoes. There is no reason to destroy a horse just because it has been infected with WN virus. Data suggest that most horses recover from the infection. Treatment would be supportive and consistent with standard veterinary practices for animals infected with a viral agent.

Other Vertebrates

Through December 2001, CDC has also received a small number of reports of WN virus infection in bats, a chipmunk, a skunk, a squirrel, and a domestic rabbit.