Horses: Infected horses most commonly show weakness, usually in the hindquarters. Widened stance, stumbling, leaning, toe dropping, and even paralysis can occur. Occasionally, they are febrile, depressed, or fearful. To date, more than 3,400 horses in 32 states have been infected throughout the United States. About 1 in 10 infected horses show symptoms of viral encephalitis, and about one-third of these die (mortality rate is about 35%).

Humans: Most human infections are relatively mild, with flu-like symptoms such as fever, headache, body aches, and sometimes skin rash and swollen lymph glands. In severe infections, signs include high fever, neck stiffness, muscle weakness, convulsions and paralysis. Death rates in such cases range from 3-15%, and are highest among the elderly. To date, more than 40 humans and countless wild birds have died of West Nile virus infection.

Other Animals: Most infected wild birds are found dead, and so descriptions of clinical signs in those birds are not readily available. Dogs and cats rarely show signs of illness, although there are recent reports of potentially infected dogs and cats. Affected animals are usually elderly and may be immunocompromised.

Laboratory findings in infected horses may include lymphopenia and mild CPK or BUN elevations. CSF analysis reveals increased protein concentration and mononuclear pleocytosis. Differential diagnoses include rabies, Eastern and Western Equine Encephalitis, protozoal myeloencephalitis, bacteremia, and liver failure.

Horses: The test of choice is a serum IgM capture ELISA. A positive test result indicates recent infection. A PCR test for detection of West Nile virus is also available. Postmortem diagnostic specimens of choice include removal of the hindbrain and lumbar spinal cord, but precautions should be taken to avoid exposure to infected tissues.

Horses are typically given anti-inflammatory agents (e.g. flunixin meglumine, DMSO, dexamethazone), mannitol, fluids, analgesics, sedation, slinging, and other supportive and nursing care.

Equine Vaccination: A federally licensed West Nile virus vaccine is available for horses (Fort Dodge Animal Health, Overland Park, KS). The standard protocol is for a two-dose series followed by an annual booster, although in climates with year-round mosquito exposure, more frequent boosters may be given. The efficacy of the vaccine is uncertain.

Reducing exposure to mosquitoes is the focus of preventative measures. Eliminating stagnant water and using screens on windows and doors will help control the mosquito population around homes and animal housing areas. Confining dogs and cats to areas where mosquito populations are lower should help. Avoid being outdoors at dusk and dawn during warm weather. If outdoor exposure is unavoidable, cover arms, legs and feet with clothing which should help reduce mosquito bites. For humans, use mosquito repellents on exposed skin and clothing (permethrin or 35% DEET). For pet animals, do not use human products that contain DEET (unsafe as they are too strong for dogs and cats). Use products labeled safe for dogs and/or cats.