Early diagnosis of deadly disease is nearly impossibleThe incidence of equine rabies is low; only 44 cases were reported to the Center for Disease Control in 1991 from a total of 6,972 nonhuman animal cases. However, equine rabies is of major importance because of the infectious nature of this disease, the potential of exposure of a number of individuals to a single rabid horse, and the difficulty in diagnosing this disease because of its extremely variable and nonspecific clinical signs.
Raccoons and skunks are the wildlife species that account for approximately 75% of all reported rabies cases. During the last several years, reporting records indicate that the skunk population in this country is declining while the raccoon population appears to be on the rise. In 1990, the number of rabies cases from raccoons exceeded that from skunks for the first time since the 1950s. This trend is expected to continue.
Bats with rabies are also a nationwide problem with all but two of the contiguous United States reporting rabid bats in 1991.
The Journal of the American Veterinary Medical Association reported the number of rabid domestic animals increased by 11.8% in 1991, the most recent year statistics are available, to a record number of 618 cases. Presently there are five live rabies epizootics recognized in the United States. Three of these are centered in the North and South Central states and Northern California and are being attributed to skunks. The other two, centered in the Southeastern and Mid-Atlantic states, are being attributed to raccoons.
These reports of the increasing incidence of rabies in our wildlife are important reminders that we must not become complacent about this serious disease. Rabies can be described as the zoonotic disease of veterinary medicine with the highest fatality rate whose diagnosis can only be confirmed at necropsy. The public must be continually educated about this disease and made aware that our first line of defense against rabies is vaccination.

Pathogenesis
Rabies is caused by a relatively fragile, enveloped neurotropic lyssavirus, a member of the family Rhabdoviridae. Antigenically the rabies virus can vary from different geographic areas and the wildlife host. Rabies is considered to be distributed worldwide with the possible exception of some island countries.
Once the horse becomes infected with the rabies virus, generally by the bite of an infected wild animal (skunk or raccoon), the virus replicates locally, looking to spread to the peripheral nerves within that area. The time it takes for the virus to spread and invade the nerves and reach the central nervous system can vary greatly, depending upon the age and overall resistance of the affected horse and the degree of innervation (number of nerves) of the area that was affected.
An ascending paralysis may develop as the rabies virus infects and damages the peripheral nerves on its journey to the spinal cord and brain. Once the virus enters the spinal cord or brain stem, it moves bilaterally and directly to the brain. Once in the brain, the virus replicates rapidly due to its neurotropic nature and spreads outward along the peripheral nerves affecting the entire nervous system of the horse, as well as other tissues, including the salivary glands.

Clinical signs
The clinical signs of rabies in the horse are extremely variable, making an early diagnosis very difficult. The incubation period is often difficult to document (two weeks to several months) and the medical history may or may not indicate exposure to a wild animal vector. An accurate vaccination history of the horse is important to obtain. In most cases any horse that has been properly immunized against rabies can be considered protected.
Presenting complaints generally include a horse that is ataxic and exhibiting a very erratic or abnormal behavior pattern. This behavior pattern can vary, depending upon the individual nature of the affected horse. While some horses exhibit intermittent or continuous signs of aggressive behavior, other horses can be more typically depressed or coma-like. Horses exhibiting aggression have been reported to actually attack humans, other animals, or inanimate objects around them. Occasionally these animals will self-mutilate themselves at a particular site on the body. It is felt that this may represent a localized neuritis at the site of viral entry. An obscure lameness is often reported as an early sign. Fever can be variable with this disease, as well as the degree of body hyperesthesia. A continuous grinding of the teeth with saliva drooling from the mouth can also be observed. Some horses may become anorectic and refuse to drink. However, the hydrophobia that is commonly associated with rabies is generally not seen in the horse.
Although the initial signs of rabies in the horse can be very misleading, the progression of this disease is usually very rapid (death within three-to-ten days). Death is generally preceded by a very consistent pattern of neurologic signs. As the infection of the brain progresses, most horses worsen rapidly, proceeding to sternal or lateral recumbency, followed by a paddling of the legs, convulsions, coma, and death. Some rabid horses may actually eat and drink until shortly before their death.

Diagnosis
As discussed previously, because of the variable signs of rabies in the horse, an early diagnosis is extremely difficult. Therefore, it is usually necessary to support the horse medically and await the progression of the disease. If horses are primarily affected in the spinal cord and exhibiting signs of ataxia and ascending paresis, attempts to rule out other conditions such as Equine Protozoal Myelitis, Equine Herpes-1 infection, Polyneuritis Equi, and spinal cord trauma must be made. Horses showing the more common encephalitic form must be differentiated from other diseases such as Eastern, Western, and Venezuelan encephalomyelitis, bacterial meningoencephalitis, mycotoxins, hepatoencephalopathy, and trauma.
In all potential cases of rabies, measures should be made to limit the contact of unnecessary personnel with the affected horse. All body fluids should be handled with care and in most cases the evaluation of them is unwise due to the lack of any real diagnostic value and high risk of human death. Most cases of equine rabies need to run their course to ensure an accurate diagnosis.
A diagnosis of rabies can be confirmed by postmortem examination of the brain. Fluorescent-based rabies antiserum (FA) is used to identify the active rabies antigen in the brain tissue. If necessary, the results of these FA tests from the brain can be further confirmed by mouse inoculation. Once a case of rabies is confirmed by FA testing, it is generally not necessary to perform further necropsy due to human health considerations. The remaining carcass should be disposed of by deep burial or incineration.
In rare cases in which rabies is highly suspected and FA examination of the brain was negative or equivocal, further examination of the spinal cord and sciatic nerves should be accomplished to confirm diagnosis.

Prevention
Currently, there are three killed vaccines approved for use in the horse by the United States Department of Agriculture. A single primary vaccination is performed at three-to-four months of age, with annual boosters thereafter. Vaccinated horses that are bitten by a rabid animal should be revaccinated and observed for 90 days. Postexposure vaccination of previously unvaccinated horses is not recommended. These animals should be euthanized or placed under observation for six months. Rabies is a zoonotic disease that is invariably fatal in the horse, for which there is no treatment, and the diagnosis can only be confirmed upon necropsy. The variable signs that accompany this disease make an early diagnosis almost impossible. It is usually necessary to supportively treat the horse until a number of other neurologic diseases are ruled out and the disease has sufficiently infected the spinal cord and brain so that a positive diagnosis can be confirmed.
The three killed rabies vaccines currently approved by the USDA for rabies immunization in the horse may be administered to animals three months or older with single annual revaccination necessary. Vaccinated horses that are bitten by a rabid animal should be revaccinated immediately and observed for 90 days. Unvaccinated horses that have become exposed to rabies should not be vaccinated. Such horses should be euthanized or quarantined under observation for six months.
Although there are no documented cases of human rabies due to exposure from a rabid horse, proper precautions should be employed when confronted with such an animal. When a definite diagnosis of rabies is confirmed, all exposed persons should be contacted and promptly referred to a physician for advice on postexposure treatment.