RABIES

� Definition: an acute viral disease that affects the central nervous system.

� Transmission: Rabies is caused by a lyssavirus, a neurotropic rhabdovirus that infects neural tissue. Transmitted via the saliva of an infected animal, the virus does not travel through the bloodstream; instead, it enters the neural tissue at the site of the bite and then migrates from nerve cell to nerve cell until it reaches the spinal cord and brain. Upon entering the central nervous system, the rabies virus reproduces rapidly, spreading back out through the peripheral nerves and into the salivary glands ready to be passed to a new host.

Raccoons, bats, foxes and skunks are the most common carriers of rabies in North America, but other mammals can transmit the disease as well. Horses are most often bitten on the lower legs or muzzle. A horse shows no outward signs of illness during the incubation period---the days, weeks or months it takes for the virus to migrate from the bite wound to the brain. The length of the incubation period depends on the location of the bite on the horse’s body. After a bite to the hind leg, it can take weeks or months for the virus to reach the brain, but if a horse is bitten on the muzzle or head, the incubation period can be as short as a few days.

� Signs: Rabies is difficult to diagnose because the signs are variable and can mimic those of other diseases or problems. After the virus reaches the brain, a horse may have a low-grade fever, mild depression, loss of appetite and slight weakness or incoordination. Within four or five days, however,

the signs can progress rapidly to convulsions, excessive salivation and more severe neurological impairment. Pain and sensitivity can recur at the initial site of infection; if the horse was bitten in the leg he may violently gnaw at the affected limb.

Acute rabies develops into two forms: The furious form (mad dog syndrome) is the type most often dramatized in books and movies, characterized by agitation, aggression, hyperactivity and paralysis of the face and tongue. This form is more commonly seen in horses who were bitten on the head. The paralytic (dumb) form causes depression, excessive salivation, ataxia, paralysis and eventually recumbency. Some horses may even show a combination of signs from both forms of the disease.

A definitive diagnosis of rabies can be made only with a postmortem examination of the brain tissue.

� Treatment: There is no treatment, and rabies is invariably fatal, usually within one to five days after the first signs of illness appear. Because of the public health risk, any horse suspected of having rabies must be quarantined or destroyed, and everyone who had been in contact with that animal must seek immediate medical care---they may need to receive a series of shots to protect them against the disease. The current postexposure prophylaxis (PEP) for people exposed to rabies is a series of five shots given over a 14-day period.

If a horse had been inoculated against rabies and then is bitten by a rabid animal, the current protocol is to revaccinate him immediately and then observe him for 45 days for clinical signs of disease.

� The vaccine: Three rabies vaccines are available for horses. All are inactivated tissue-culture-derived products where the virus is grown in cell cultures and then killed to make the vaccine. A single dose is enough to induce strong immunity, and the current recommendation is that mature horses be vaccinated once a year.

According to the AAEP, mares vaccinated prior to breeding develop enough immunity to produce sufficient levels of antibodies in their colostrum to protect the newborn. Alternatively, a pregnant mare can be vaccinated four to six weeks prior to foaling. If the mare was vaccinated, a foal won’t need his first rabies shot until he is 6 months old. Foals get a two-dose series, spaced four to six weeks apart, and thereafter are vaccinated once annually as mature horses.