Q: What is West Nile virus?


A: West Nile virus is an arbovirus (short for arthropod-borne virus) that causes encephalitis (inflammation of the brain). Arboviruses, including West Nile virus, are transmitted by blood-feeding insects such as mosquitoes. Most infections with West Nile virus have been identified in wild birds, but the virus can also infect humans, horses, dogs, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and domestic birds.

Q: Where did West Nile virus come from?
A: West Nile virus was first identified in the West Nile district of Uganda in 1937, and has since been found in Africa, Eastern Europe, West Asia, the Middle East, and the United States. The strain of virus found in the United States most closely resembles that found in the Mediterranean and Middle East.

Q: How is West Nile virus transmitted?
A: Mosquitoes obtain the virus by feeding on infected birds. The level of viremia in animals other than birds is too low to infect mosquitoes. Infected mosquitoes then transmit the virus to animals and humans through bites, and West Nile viral encephalitis develops in animals and humans when the virus multiplies and crosses the blood-brain barrier. Ticks infected with the virus have been found in Asia and Africa; however, there are no verified reports of ticks spreading the virus and their role in transmission has not been determined. There is no documented evidence of animal-to-animal or animal-to-person transmission of West Nile virus. Suspect cases of human-to-human transmission through organ transplantation or blood transfusions have recently been reported. It is theoretically possible that animal-to-animal transmission of West Nile virus could occur through blood transfusions but this has not been demonstrated. People who directly handle sick animals should take precautions to protect themselves from contact with saliva from sick animals that may actually have rabies rather than West Nile virus infection.

Q: What's the risk of becoming ill from West Nile virus?
A: In areas where mosquitoes carry the virus, less than 1% of mosquitoes are actually infected. Even if mosquitoes are infected, less than 1% of people bitten and infected by those mosquitoes become severely ill. The risk of becoming ill from a single mosquito bite is extremely low.

Q: What clinical signs are associated with West Nile virus infection?
A: Horses - The most common sign is weakness, usually in the hindquarters. Weakness may be indicated by a widened stance, stumbling, leaning to one side, and toe dragging. In extreme cases, paralysis may follow. Fever is sometimes evident, as are depression and fearfulness. Approximately 40% of cases of West Nile viral encephalitis in horses proved fatal during the 1999 outbreak. Humans - Most infections in humans are relatively mild, with flu-like symptoms including fever, headache, body aches and, in some cases, skin rash and swollen lymph glands. Signs of more severe infections include high fever, neck stiffness, muscle weakness, convulsions and paralysis. Death rates associated with severe infection range from 3% to 15% and are highest among the elderly. Other animals - Wild birds infected with West Nile virus in the United States are most often found dead; therefore, descriptions of clinical signs in wild birds are not readily available. Nor have clinical signs associated with West Nile virus infection in dogs, cats, bats, chipmunks, skunks, squirrels, domestic rabbits, and domestic birds been well described. It appears that, although they may be infected, many of these latter species may not develop clinical signs of disease.

Q: Can West Nile virus cause illness in dogs or cats?
A: Rarely. There is a report of illness and death from West Nile virus encephalitis in an elderly dog in Illinois in September 2002. The dog may have been immunocompromised. Infection with West Nile virus had been previously demonstrated in dogs but none were clinically ill. West Nile virus has been isolated from a sick stray kitten in New Jersey in 1999 and two cats in upstate New York in 2000. The three cats were severely ill and died. Because West Nile virus illness in pets is rare, dogs and cats with neurologic disease should be first evaluated for other more likely causes of illness, including rabies. If a pet shows signs of fever, depression, incoordination, muscle weakness or spasms, seizures or paralysis, a veterinarian should examine the animal.

Q: How do dogs and cats become infected with West Nile virus?
A: The same way that people become infected: by the bite of an infected mosquito.

Q: How is West Nile viral encephalitis diagnosed and treated?
A: Diagnosis of West Nile viral encephalitis is based on a history of exposure, clinical signs, and results of diagnostic blood tests. As for all viral diseases, treatment consists of providing support (e.g., hospitalization, intravenous fluids, respiratory support, prevention of secondary infections, and good nursing care) while the affected individual's immune system responds to the infection.

Q: Can the risk of West Nile viral encephalitis be reduced?
A: A vaccine is now available for horses. For other species including humans, reducing exposure to mosquitoes is the focus of preventive measures. The following actions may reduce the risk of mosquito bites and possible exposure to West Nile virus: -Reduce the mosquito population around the home and where animals are housed by eliminating stagnant water. -Do not allow dogs and cats to roam to other areas where mosquito populations may be higher. -Check the integrity of screens around your home, porch and patio. If possible, use screens in areas where animals are housed. -During warm months, avoid being outdoors at dusk and dawn. -If you must be outdoors during hours when mosquitoes are most active, cover up with shoes, socks, long pants and long-sleeved shirts. Clothing alone will not prevent all mosquito bites, however. -For humans, use mosquito repellant on exposed skin and spray clothing with repellents containing permethrin or 35% DEET (N,N-diethyl-meta-toluamide) since mosquitoes may bite through thin clothing. When using insecticides or insect repellants, be sure to read and follow the manufacturer's directions for use. -For pets, DO NOT use human products that contain DEET on animals. The concentrations of DEET used in commercial mosquito products for humans are too strong to be safely used on cats and dogs. There have been cases of dogs and cats exposed to DEET that have developed significant neurologic problems. Apply products that kill or repel mosquitoes and are approved and labeled for dogs and cats. (Be careful, not all products labeled for use on dogs can be used on cats.) A veterinarian can help decide on the appropriate product to use depending on species, age, health status, and degree of exposure. -Insecticides-repellants containing pyrethrins or permethrin are sometimes labeled with mosquito repellant claims.

Q: Should an animal infected with West Nile virus be euthanatized?
A: No, there is no reason to destroy an animal just because it has been infected with West Nile virus. Full recovery from the infection occurs in most cases, and humans and other animals cannot be infected by contact with the infected animal.