West Nile Virus

Pet Tales
       by Barry Burtis D.V.M.

West Nile Virus and other Springtime Concerns

Most of us are anxiously awaiting the return of spring. However, this year, while still eagerly anticipating this marvelous season, many fear the returning problem of the West Nile virus. In the short time this health threat has been in our part of the world, we have become very familiar with it. Newspaper and other media reports remind us that the respite we have had from this disease will end with the arrival of mild temperatures and mosquitoes. Naturally, people often ask about the danger West Nile virus poses for their pets.

West Nile virus is an arbovirus that causes encephalitis. These viruses are transmitted by blood-feeding insects such as mosquitoes. Most infections with West Nile virus have been identified in wild birds, horses and humans, but the virus can affect other wild and domestic animals. West Nile virus was first identified in the West Nile district of Uganda in 1937 and has since spread to many other parts of the world.

Mosquitoes draw the virus in the blood when they bite infected birds and transmit it to animals and humans when they bite them. In areas where mosquitoes carry the virus, less than 1% are actually infected. Even when 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.
Can the West Nile virus cause illness in dogs and cats? The answer seems to be rarely. There is a report of illness and death from West Nile virus encephalitis in an elderly dog in Illinois in September 2002. It is possible the dog may have been immunocompromised or in an already weakened condition. Infection in dogs had been demonstrated previously but none were ill, none were showing any signs of the disease. West Nile virus has been isolated from a sick kitten in New Jersey in 1999 and two cats in New York State in 2000. These numbers would suggest West Nile virus is a very low risk danger to dogs and cats. There are many more likely causes of neurological disease. If a pet shows signs of fever, depression, incoordination, muscle weakness or spasms seizures or paralysis it should always be examined by a veterinarian.

Weakness, usually in the hindquarters, is the most common sign in horses infected with West Nile virus. They may stand in a widened stance, stumble, and lean to one side and toe drag. Paralysis may follow. Fever is sometimes present and affected animals may show depression or fearfulness.

How can we best protect animals in our care? A vaccine is available for horses. For other species, limiting exposure to mosquitoes is the best prevention. Make sure screens help to keep the inside of our homes free from mosquitoes. Ensure that no stagnant water - in bird baths, ponds, wading pools, buckets or pails - be allowed to develop in your outdoor surroundings. It may be wise to restrict outdoor activities at dusk and dawn when mosquitoes are more likely to be encountered. Insect repellants containing DEET, recommended for human use, should not be used on dogs and cats.
There are permethrin containing products licensed for use on dogs to repel mosquitoes but they are not safe to use for cats. In fact, they should not be used on dogs that live with cats. There are really no mosquito repellants, currently, that can be used safely on cats. The best products we have to use with cats are flea shampoos that may have some residual action against mosquitoes.

I would be remiss if I did not mention that this is the time of year when pet owners need to think again about protecting their pets against the other disease brought to them by mosquitoes. Fortunately, there are very safe and effective preventives for heartworm disease. Dogs, in our region in North America, should receive the benefit of these medications from June until November. Owners of cats exposed to mosquitoes should discuss with their veterinarian the nature of heartworm risks for their pet and determine whether they need protection, as well.