Veterinarians recommend rabies vaccinations for horses

By Dr. Gabriele A. Landolt
Colorado State University

Rabies is an ancient disease that remains one of the most feared infectious diseases affecting both humans and animals.

Despite the fact that rabies control programs have substantially reduced the number of rabies infections in domestic animals in the United States, the disease continues to be a problem in wildlife. Several wildlife species (including raccoons, skunks, foxes, coyotes and bats) provide a reservoir for rabies from which the virus occasionally transmits to domestic animals and humans. At present, raccoons, bats and skunks account for most of the rabies virus-infected animals in the United States.

In Colorado, bats and skunks are considered to be the most important wildlife reservoirs. According to surveillance data published by the Colorado Department of Public Health and Environment, bats and skunks account for 61 and 59 of the 131 laboratory-confirmed rabies positive animals in 2010.

Even more importantly from a horse owner's perspective, two horses housed in Arapahoe and El Paso counties were found to be rabies positive in less than nine months. These numbers clearly indicate that rabies constitutes a serious health concern in Colorado and that spread of the virus from wildlife to domestic animals occurs. Moreover, as rabies is a zoonotic disease, animals infected with rabies also represent a significant threat to human health.

The primary means of transmission of the rabies virus to horses is through the bite of an infected animal. Other, rare modes of virus transmission have been reported and include contamination of an accidental break in the skin with saliva from an infected animal. Following contamination, the virus is thought to replicate initially in the muscle tissue at the site of the bite. Subsequently, the virus enters the nerves closest to the bite wound and then travels through the spinal cord to the brain. Once it reaches the brain, the virus spreads down along the nerves to the salivary glands and can then be detected in the saliva.

Signs of infection

In horses, the time from infection (bite) to the onset of clinical signs can vary from a few days to months. The first clinical signs of rabies in the horse are often nonspecific and variable. These early signs can range from poor performance, lameness, gastro-intestinal signs such as colic to bizarre behavior. Yet, once clinical signs occur, they rapidly get worse and frank neurological signs become apparent. Neurological signs can present as hyperactivity and aggression ("furious rabies"), as lethargy and stupor ("dumb rabies") or as paralysis ("paralytic rabies"). The disease is always fatal with an average survival of five days from onset of clinical signs.

Rabies should be suspected in horses with rapidly worsening signs of neurological disease. Unfortunately, there is no test available that allows the definite diagnosis of rabies in the live horse. Therefore, confirmation of rabies infection can only be made after the death of the horse and requires laboratory testing of the brain tissue of the suspect animal.

There is no effective treatment available for horses with rabies. Horses suspected of having rabies should be isolated to prevent possible exposure of humans and other animals. Moreover, horses that are suspected of having rabies should only be handled by individuals that have been appropriately vaccinated for rabies. Also, a list of all "in-contact" individuals should be made to facilitate potential treatment of humans exposed to the rabid horse.

Vaccination

Vaccination is the mainstay of rabies prevention in the horse. As a matter of fact, the American Association of Equine Practitioners classifies the rabies vaccine as one of four core vaccines and recommends that the majority of horses should receive it. Considering that a rabid animal poses a substantial human health risk, vaccination of horses that are in regular contact with humans should be carefully considered.

In adult horses, rabies vaccination requires administration of one initial vaccine, followed by yearly booster doses. For regulatory purposes, it is important to note that equine rabies vaccines should only be administered by a licensed veterinarian as only then is a horse considered to be vaccinated. The distinction of the horse's vaccination status has important implications if a horse becomes exposed to a rabid animal. Current recommendation for vaccinated horses exposed to a rabid animal is revaccination and observation for 45 days. In contrast, the current recommendation for exposed unvaccinated horses is euthanasia.

More information is available from the Colorado Department of Public Health and Environment at www.cdphe.state.co.us/dc/zoonosis/rabies/ and Centers for Disease Control and Prevention at www.cdc.gov/rabies.

Dr. Gabriele Landolt, DVM, PhD, is an assistant professor of equine medicine at Colorado State University's James L. Voss Veterinary Teaching Hospital, specializing in infectious disease, endocrine disease and health problems in neonates. She can be reached at 297-5000.