West Nile Fever

The evening news has already started to buzz with the latest on West Nile Fever. Surveillance for infected mosquitoes and birds has begun, and your local drugstore has had a run on mosquito repellents. What is West Nile Fever and how does it affect your horse? Can your horse give it to you? These are but a few of the questions that horse owners pose to their veterinarians every day.

What Is West Nile Fever?

West Nile Fever is one of many diseases categorized as an encephalitis. This merely indicates that the disease causes an inflammation of the brain. It can also cause inflammation of the rest of the central nervous system. It is caused by a virus that belongs to a large family called the flaviviruses. Most of the other flaviviruses also cause encephalitis in various different species.

How Is West Nile Fever Spread?

West Nile Fever is carried in birds. The birds are bitten by mosquitoes, which are subsequently considered fomites, meaning that they are considered to be the reservoir for the virus. This is known as a bird-mosquito cycle. When the mosquito bites a horse or a human, there is a small chance that that horse or human will develop West Nile Fever.

Can West Nile Fever Be Transmitted?

West Nile Fever can't be caught directly by an infected individual. It must be passed through blood, via mosquitoes. People, even when they are sick, have low levels of virus in their blood. This means that even if a mosquito bites a sick person, there won't be enough virus in the blood to infect another horse or person. There is some evidence that horses can sometimes have a high enough level of virus in their blood to infect another individual through a mosquito bite – the jury is still out on this one, and studies are being carried out to determine if horses do become sufficiently viremic to serve as reservoirs. Infected birds, on the other hand, have a very high level of virus in their blood, and in this way, they serve as a reservoir for the disease.

Do All Mosquitoes Carry West Nile Fever?

No. The big culprit has the impressive name of Culex pipiens, or C. pipiens. It is very common in most urban and suburban environments, and delights in habitats such as containers that hold stagnant water – think of an untended water or feed trough, large puddles left from melting snow, your childrens' playthings left out in the rain, wading pools, clogged rain gutters, storm drains, and stagnant ponds. Contrary to popular belief, these mosquitoes don't all die – many of the females, with young ready to be born in the spring, will survive the winter. They're just waiting for the warm weather. When the mosquitoes do emerge, they need a blood meal. Although birds are the natural host for these mosquitoes, they are more than willing to try equine or human blood!

The chances are very slim indeed that your horse will get West Nile Fever from a mosquito. First of all, very few mosquitoes actually carry the virus (estimates are less than 1 one in 1,000). Even if a virus-carrying mosquito bites your horse (or you), it is still unlikely that he will get the disease. Most researchers estimate that there is only a 1 in 300 chance that an individual will develop the disease.

Is West Nile Fever the Same Thing as Eastern Equine Encephalitis?

No. Eastern Equine Encephalitis is caused by a virus that belongs to a different family. It can cause similar signs, and is transmitted from birds through mosquitoes just as West Nile Fever is, but it is not the same disease.

Where Did West Nile Fever Come From?

Veterinarians, doctors and scientists have actually known about West Nile Fever since the 1930s. It was known as Near Eastern Equine Encephalitis in Egypt, and as lourdige in France. There have been several outbreaks of West Nile Fever in Europe. The recent West Nile Fever outbreak in the United States probably originated in the Near East. Because there had been a mild winter and a very wet spring preceding the summer of 1999, the virus was successful in persisting in mosquitoes.

What Areas of the Country Are Affected by West Nile Fever?

So far, the virus has been seen all along the Eastern seaboard, Midwest and Gulf Coast States. It is thought that soon the virus will spread to the West Coast.

How Can I Protect My Horse and Myself?

The best way to protect your horse and you is to use common sense to reduce mosquito populations in your area, and to avoid being outside when the mosquito population is high. There is also a vaccine now available that offers additional protection. Unfortunately, the vaccine has not yet been thoroughly tested so its effectiveness is unknown.

What Are the Symptoms of West Nile Fever?

West Nile Fever causes encephalitis (inflammation of the brain) or encephalomyelitis (inflammation of the brain and other portions of the central nervous system). It can be very difficult to distinguish from other causes of encephalomyelitis.

Some of the diseases that can be confused with it include:

What Is the Treatment Regimen?

If your horse is so severely affected that he is down in his stall, then you have your work cut out for you. Horses are by nature difficult to care for when they are recumbent.

How Does My Veterinarian Diagnose West Nile Fever?

If your veterinarian suspects West Nile Fever, he will be in touch with the state veterinarian. He will probably take serum (blood) to look for antibodies to West Nile Fever. In some situations, your veterinarian may choose to take a sample of the cerebrospinal fluid – antibodies in the CSF is good evidence of West Nile Fever.


West Nile Fever is an uncommon cause of neurologic disease in the horse, but we still know relatively little about how this disease will progress and spread in the United States. Use the procedures discussed above to keep down your mosquito population – but don't panic. Remember that the odds of you or your horse being bitten by an infected mosquito are low, and the odds of becoming infected are even lower. Call your veterinarian immediately if you see signs of abnormalities in your horse – weak gait, especially in the hind end, recumbency, or generalized weakness may be the first signs. Watch for large-scale die-offs of birds, especially crows, in your area. This may be a warning that West Nile Fever is in your area.