West Nile Virus (WNV) has been in this country for more than a decade now, and has spread across the entire country. This is a disease of birds that is spread by mosquitoes and sometimes affects horses and humans as end-stage hosts. The virus first entered this country in 1999, probably with birds brought to a zoo in New York. It is a very common disease in Africa, west Asia, Europe and the Middle East.

The form of the virus we now have in the U.S. is very similar to that found in domestic geese in Israel. The 1999 outbreak that started at the Bronx Zoo affected 62 humans, with seven deaths. There were also 25 horse cases that first year. This disease quickly spread, reaching the west coast by 2003.

Birds are the reservoir for this disease—the source of infection—and mosquitoes are the vectors that spread it from one animal to another. Crows, blue jays, magpies, etc., are very sensitive to the disease and often die. Some other species of birds seem to tolerate it better and continue to fly around, without clinical signs.

Humans and horses are incidental infections. The only way horses or humans get the virus is through a mosquito bite (a mosquito that has bitten a sick bird; the virus level in the blood of a sick horse or human is so low that is virtually impossible for a mosquito to transmit it from horse to horse or human). One exception was a case in which a woman with WNV died and her organs were transplanted into four different people. Three of them developed WNV infection and one of them died.

Horses with WNV may be unable to stand up on their own. The horse below is supported by a sling.

The signs of WNV in horses are usually neurologic. The horse may or may not have a fever. He may be depressed and lethargic. Any neurologic disease can be confused with WNV—EPM, rabies, the neurologic form of the herpes virus, EEE, WEE, botulism, etc. The diagnosis, to determine the cause of a neurologic problem is much simpler now than it was at first, because tests have been developed to check for WNV. Horses with clinical signs can be tested for WNV.

Treatment includes I.V. fluids, DMSO (given orally or intravenously), non-steroidal anti-inflammatories, mannitol (an alcohol given I.V., to reduce cerebrospinal fluid pressure and volume), and dexamethasone. The horse should be supported in a sling, if it cannot stand up.

The first vaccine for WNV was created very quickly, and marketed under a conditional license (because it had not been fully tested for efficacy). Since then, several other vaccines have been created. Some require a two-dose series the first year, with an annual booster thereafter. One WNV vaccine produces adequate immunity with just one shot—and an annual booster. Your veterinarian can answer questions about vaccines and which ones might be appropriate for your horses.

Most horses are vaccinated in the spring, ahead of mosquito season. Risk for WNV is not high during early spring, but increases with warm weather. The disease has to get established each year and become amplified in the mosquito population. If there are warm days in late fall, however, horses could still be at risk. It only takes a week of warm weather to re-establish a mosquito population. If mosquito season is long in your geographic area, you may want to give another booster later in the year. Veterinarians recommend two shots per year for horses in Florida, for instance.

Vaccination is fairly effective in preventing the disease. Some of the unvaccinated horses that are exposed to WNV develop severe illness and die (even with extensive treatment), or recover after being treated, while others have such mild illness that the owner may not know the horse was sick. There is evidence that many horses (and humans) that have never shown signs of illness have encountered WNV; serology studies have shown that many of them test positive although they have never been sick.

In one study, a number of people were interviewed and tested, and for every diagnosed hospitalized human, there were 150 subclinical infections. Either the people didn't get sick enough to go to the hospital or did not feel ill, but they did have antibodies for WNV.

With horses, testing has shown that there are many positive but healthy horses on farms where there have been sick horses. Some of the horses that were brought to veterinary hospitals for things like colic, unidentified fever, lack of appetite, etc. have tested positive. Veterinarians have seen colic cases that they decided to run tests on, and the horse had no neurologic signs but tested positive to WNV. The horse was just colicky for a couple of days. There may be a number of horses that show clinical signs of illness, but they aren't recognized as this disease, because the signs are not typical for WNV.

Some medical entomologists feel that WNV is usually in an area about a year before we start recognizing it (before we are able to isolate it from mosquitoes, or see dead birds, or clinical cases in horses and humans). When we start finding dead birds and clinical cases, we have to assume that WNF has been there for a while.

The vaccine is cheap insurance and very effective in preventing this disease. Since it can be deadly, the wise thing to do is keep your horses vaccinated, giving an annual booster ahead of mosquito season in your area.