West Nile Virus in Animals: “Is my dog/cat/horse/bird safe?”

The West Nile Virus has been in the news a lot recently, due to its prevalence in Texas mosquitos. Naturally, this has led to a lot of questions from our clients regarding the safety of their animals. Here are some answers to frequently asked questions about the West Nile Virus.

What is the West Nile Virus?
The West Nile Virus is a mosquito-borne virus that was first isolated from a woman in the West Nile region of Uganda, Africa in 1937. It was determined in the 1950’s that the virus’s life cycle involved mosquitos and birds. Up until 1999, the virus had been found in Africa, the Middle East, Asia, and Europe. In 1999, the West Nile Virus was first discovered in North America by a veterinarian in a New York outbreak. The virus isolated in the 1999 New York outbreak most closely resembled a virus from a goose in a 1998 outbreak in Israel.

Up until the 1990’s, the virus had historically caused only a few cases of mild febrile symptoms in people, with only rare cases of neurologic symptoms. Starting in the 1990’s, there were several occasions of neurologic disease, both in people and in horses, throughout Europe and the Middle East. The 1998 outbreak in Israel was the first reported instance of the virus causing death in birds.

It took three years from its discovery in 1999 in New York for the West Nile Virus to spread nationwide. Most likely, this spread was due to bird migration. The United States form of West Nile differs historically from other outbreaks, because of its increased mortality in birds and its ability to affect more species.

What animals are affected by the West Nile Virus?
Birds are certainly in the biggest danger, with crows, blue jays, raptors, certain waterfowl, and flamingos being at the greatest risk. There are reported cases of West Nile Virus in parrots, but it is much rarer. Birds will usually die a very sudden death, with little warning beforehand.

Horses are also susceptible to the neurologic form of West Nile. The virus affects the brain and spinal cord, so they will have trouble walking normally or will have mental changes. About 10-39% of infected horses will develop symptoms of the disease. Of those that develop symptoms, about 30% will die.

Dogs can get West Nile, but there are only a few reported cases. During experiments, dogs that were bitten by infected mosquitos did not show any signs of illness and had a very low level of virus in their blood.
Cats can also get West Nile, but there are also only a few reported cases. Cats that were fed infected mice in experiments did not develop signs of illness. Cats that were experimentally infected by mosquitos did get a mild fever that went away after a short period of time. Cats do get a higher level of virus in their blood than dogs, but it is still much lower than that of birds.

Alligators– Several hundred farmed alligators became ill with West Nile and many died. This was most likely caused by the fact that they were fed infected horsemeat.

Squirrels– There have been clusters of squirrel deaths caused by West Nile.

How do animals get West Nile?
The most common form of transmission is through mosquito bites, although ticks are also thought to be able to spread the virus. Infected birds have a very high level of virus in their blood, making them responsible for the spread of the disease. Oral transmission (eating tissues of infected animals) has also been documented.

Is there a vaccine?
There is a West Nile vaccine labeled for horses, and we do recommend that horses in Texas get it! Some zoos use the horse vaccine to protect their birds, although it is not known how effective it is. There is no vaccine labeled for dogs or cats. It is not recommended that the horse vaccine be used on dogs and cats, especially since they are not very susceptible to the disease.

What can I do to protect my pet from West Nile?
For horses, the West Nile vaccine is definitely recommended. We recommend giving it in the spring, just before the worst of the mosquito season. Putting fans in stalls that blow over the horses may help decrease mosquito feeding.
Get rid of any standing water, like in flower pots or old tires. Clean water tanks and buckets regularly to prevent mosquito breeding. Keep pets indoors especially during times of the day that mosquitos are most active. Dispose of any dead bird or squirrel, to prevent your dog or cat from eating it. Monthly tick preventions may help, as ticks may be another vector of the disease.

Lichtensteiger, Carol and Greene, Craig. “West Nile Virus Infection”. Infectious Diseases of the Dog and Cat, 3rd ed. Saunders Elsevier, St. Louis, MO, pp 192-195. 2006.
The Merck Veterinary Manual, 9th ed. “West Nile Encephalomyelitis”. Merck & Co. Whitehouse Station, NJ, pp 1077- 1081, 2005.