West Nile Virus in Horses and Humans

West Nile Virus (WNV) is a fairly new disease in the United States, which can infect both people and horses, sometimes causing death. There is no specific treatment available for WNV, which results in an encephalitis (inflammation ...West Nile Virus (WNV) is a fairly new disease in the United States, which can infect both people and horses, sometimes causing death. There is no specific treatment available for WNV, which results in an encephalitis (inflammation ...

Story originally posted by Horsecity.com Staff

West Nile Virus (WNV) is a fairly new disease in the United States, which can infect both people and horses, sometimes causing death. There is no specific treatment available for WNV, which results in an encephalitis (inflammation of the brain), but vaccination and mosquito control should aid in preventing WNV in horses.

On Aug. 2, 2001, the USDA conditionally licensed a WNV vaccine for horses. Conditional licensure of this vaccine means that it is pure and safe, and can be expected to be reasonably effective. This conditional license is good for one year.

Horse owners who take their horses to states/areas where WNV has been diagnosed should use caution (use recommended insecticides on horses and in trailers, keep all equipment clean, and use fans when horses are stalled to reduce mosquito contact). Vaccinate your horses if recommended by your veterinarian. Owners planning to ship horses to other countries should be alerted that the WNV vaccination may not meet the import regulations of some countries due to the presence of certain antibodies in the horse’s blood. These antibodies are not permanent, but could interfere with international shipment of horses.

As with similar diseases, WNV is principally a disease in birds that is transmitted from infected birds to other susceptible birds, people or horses by mosquitoes and occasionally ticks. WNV was discovered in 1937 in Uganda and is commonly observed in Africa, Western Asia, the Middle East and occasionally in Eastern (southern) Europe; however, the first cases in the United States were reported in late summer of 1999 in dead crows in the New York City area. WNV was soon diagnosed in humans and horses in this area. There is no evidence that WNV can be transmitted from horses to humans, or visa versa.

However, bird-to-bird transmission has been proven. This disease is similar to Eastern and Western Equine Encephalomyelitis, which are also transmitted from birds by mosquitoes, but are not transmitted from horses to humans either.

In 2001, WNV was detected in larger numbers over a wider range than in 1999 or 2000. WNV has been detected as far south as Florida and as far west as Louisiana. The first positive horse case in 2001 was reported July 12, where as the first reported case in 2000 was Aug. 17.

Positive WNV cases diagnosed in horses were found in the following states in 2001: Alabama, Connecticut, Delaware, Florida, Georgia, Kentucky, Louisiana, Maryland, Massachusetts, Mississippi, New Jersey, New York, Pennsylvania, Tennessee, and Virginia. About 70 percent of the equine cases in 2001 were diagnosed in Florida.

Horses with WNV lack muscular coordination. They are weak in their hindquarters, stumble and go down with difficulty in rising. They may tilt their heads, have facial or muzzle twitching, paralyzed or drooped lips, grind their teeth and may become blind. They may be lethargic, lack awareness, or be hyperexcitable. Fever has been detected in about 25 percent of all confirmed cases. Some horses are subclinical. These signs are often confused with other diseases such as Equine Protozoal Myeloencephalitis (EPM). Horses that live through two to three weeks of WNV usually recover without any long-term side effects. In those parts of the world where WNV is endemic, a percentage of the equine population has been exposed to the virus, developed antibodies and become immune to the disease.

In 1999, there were 25 WNV equine cases, with nine dying or being euthanized. Of the 60 equine cases in 2000, 23 died or were euthanized. The two-year combined death rate was 38%. Through 2000, there were 20 cases confirmed in humans, with only two deaths.

People who become infected with the WNV may have a fever, headaches and body aches which last only a few days within two weeks of exposure. A few individuals will have more sever symptoms such as neck stiffness, incoordination, disorientation, tremors, convulsions, paralysis and rarely death.

Three studies conducted on humans in areas with high levels of human and equine infections showed a rate of 1.3 infected individuals per 1,000 people. In infected humans, only 1 in 150 gets the encephalitis. So the probability of a person even contacting this disease in an endemic area is very low. This translates to less than one person per 100,000 individuals getting an encephalitis, even in a "hot bed" area. Of individuals getting the disease, the death rate in humans has been low. This data should reassure individuals that this is not a disease with a high-risk death rate in humans.

Culex salinarius, more than other mosquito species associated with WNV, is of concern as it feeds on mammals, including humans. The Culex pipiens complex of mosquitoes, closely associated with WNV, breed in standing water, especially water polluted with organic matter. These mosquitoes also may prefer to bite birds and are active from dawn to dusk. Unfortunately, WNV has been noted in 43 different species of mosquitoes.

Outbreaks normally subside in the fall and winter as mosquitoes die off. With WNV spreading south, there may be areas where the disease occurs year-round if mosquitoes are active in winter months.

Efforts should be implemented to reduce the presence of mosquitoes, especially eliminating their breeding grounds. This can be accomplished by removing any materials such as old tires that can contain standing water from farms. Consult with local mosquito control agencies or health agencies for methods to control mosquitoes in standing water, such as occurs in streams that partially dry-up in summer, low swampy areas, etc. Due to the human health concerns, local health organizations may have greater interest in mosquito control in such areas.

Use recommended mosquito repellents on horses and insecticides in and around stabling and congregating areas, especially at dusk, dawn and night. Many mosquito species are dusk, night or dawn feeders, but some are daytime feeders.
Horse owners can contact their county Extension office for the latest information on WNV in horses.