Etiology and Epidemiology

The rabies virus belongs to family Rhabdoviridae, genus Lyssavirus. Horses are exposed to this disease through saliva containing the rabies virus. The most common route of exposure is through a bite from an infected animal, although transmission is also possible by means of infected saliva (or nervous tissue) coming into contact with a mucous membrane or open wound. Feces, blood, and urine do not contain virus and are noninfectious; the same applies to skunk spray.

With the exception of Hawaii, rabies cases were reported throughout the United States during the 5-year period of 2008 through 2012. Of the samples submitted for testing, the species with the most frequent positive results for rabies were raccoons (primarily from the eastern United States, where raccoon rabies is enzootic because of an epizootic that began there in the late 1970s that involved the raccoon rabies virus variant), skunks, bats, and foxes. These species serve as reservoirs for specific rabies virus variants; horses do not. Rabies infection in a species other than the reservoir species is considered “spillover.” An example of spillover would be a horse infected with a skunk variant of rabies virus. The usual source of exposure to rabies for horses is by interaction with wildlife, most commonly skunks, raccoons, and foxes. During 2008 to 2012, 196 cases of rabies in equines were reported in the United States; of the 72 cases for which a rabies virus variant was determined, 87.5% were various skunk variants and 12.5% were the raccoon variant.

Bites from wildlife are more likely to occur around the nostrils or muzzle of horses, possibly because horses, especially foals and yearlings, tend to investigate wildlife in their pasture or pen; bites in the lower limbs are also common. Barns make a perfect habitat for many wildlife species. Rabid wildlife become uninhibited and, with loss of fear of a larger animal, have been found inside stalls with horses. Herbivore-to-herbivore transmission is rare.

After rabies is introduced into a susceptible animal by a bite or other means of contact with infected saliva, the virus replicates in muscle cells for a variable length of time before it enters the nervous system and travels along peripheral nerves to the brain. After reaching the brain, the virus travels along peripheral nerves to the salivary glands. Thus the incubation period varies in length, but on average is 2 to 6 weeks, with reports generally ranging from 2 to 9 weeks. This interval can be shorter or considerably longer.

Clinical Signs

The clinical signs of rabies in horses are diverse and variable in expression. A rabid horse can have any number of the following: behavior changes (including distress, aggressiveness, extreme agitation, depression, and lethargy), paresthesia at the site of the bite (which causes the horse to rub and chew at the area), photophobia, hyperesthesia, ataxia, circling, head tilt or pressing, tail weakness, ascending paralysis, lameness, lack of anal sphincter tone, bladder incontinence, tenesmus, recumbency, convulsions, teeth grinding, drawing lips back and forth, hypersalivation, pharyngeal paralysis, abnormal vocalization, inappetence, erratic drinking characteristics, and fever. The appearance of choking is another possible clinical sign; this manifestation can lead to human rabies exposures as individuals reach into the horse’s mouth in an attempt to remove an assumed foreign object or rectify an anticipated dental problem. Clinical signs usually progress quickly, over 4 to 7 days, possibly less. Death usually results from cardiorespiratory failure.

In a study of 21 cases of experimentally induced rabies in horses, the average incubation period was 12 days, and the average morbidity period was nearly 6 days. Muzzle tremor was the most frequently observed clinical sign; other common signs seen in more than 70% of animals in this study included pharyngeal spasm or paresis, ataxia or paresis, and lethargy or somnolence.