Originally published in the November 2015 issue of Veterinary Practice News. Did you enjoy this article? Then subscribe today!
One would think that pretty much everyone knows that rabies vaccinations are required for small animals in every state in the U.S. Rabies, of course, is a serious zoonotic disease, and with the number of dogs and cats in the country, the rationale for rabies vaccination is obvious.
Perhaps less well known is that horses can get and transmit rabies, too. While rabies is certainly uncommon in horses, every year 30 to 60 are confirmed with rabies in the United States. Horses that contract rabies typically:
- Live in an endemic area.
- Have no history of vaccination.
- Live outside 24 hours a day.
- Can be of any age.
- Can be of any breed or gender.
- Are the only ones on the farm that contract the disease.
Rabies in horses is not necessarily easy to diagnose clinically. Signs can be confusing and nonspecific, and misdiagnosis is not uncommon. Typical clinical signs include colic, obscure lameness, ataxia, paralysis, incontinence, tremors, fever, depression, aggressiveness, increased sensitivity to touch and convulsions.
On the other hand, many horses infected with rabies are depressed, don’t eat or drink and show signs of bruxism, or teeth grinding.
In most cases, once clinical signs begin, the horse dies within three to five days. Rabies infection in unvaccinated horses is uniformly fatal.
In endemic areas, wildlife provides a natural reservoir for the rabies virus. Horses are not attacked by rabid animals; rather, since horses are naturally curious, they may reach down to inspect an animal entering their stall or pasture and get bitten.
The time between the bite and the onset of clinical signs can be quite long — longer than six months in some cases — depending on where the horse is bitten. No definitive antemortem test for rabies exists in horses, or in any animal. Blood tests are not specific, and cerebrospinal fluid analysis may also be normal.
The National Association of State Public Heath Veterinarians recommends that all horses that are in frequent contact with humans, especially those that travel interstate, be vaccinated against rabies. The American Association of Equine Practitioners considers it a core vaccine for equine health.
Happily, the available inactivated vaccines, given by intramuscular injection, all produce a strong serologic response, so all make good choices for use in rabies prevention. Most are labeled for the first dose at 3 to 6 months of age, with a second dose at 1 year. Vaccination annually is recommended.
Many veterinarians also recommend boosting pregnant mares before breeding or foaling. For foals from mares with an unknown vaccination history, veterinarians have two options.
First, they can assume the mare is protected and begin vaccination of the foal at 3 to 6 months of age. Alternatively, they can document the foal’s antibody status, either by testing serum from the foal collected after the first 24 hours — that is, after passive antibody transfer has occurred — or by testing serum from the dam in the immediate post-partum period, and proceeding accordingly.
Of course, in all circumstances, veterinarians should read the vaccine label for specific recommendations. Vaccination provides highly effective protection against rabies. Challenge studies are required to license vaccines used to prevent rabies, unlike other equine vaccines, and published results are available for vaccines that have been most recently licensed.
While some veterinarians may be inclined to measure rabies titers and use them to help guide vaccination protocols, titers have not been shown to correlate directly with protection. Thus, the Centers for Disease Control and Prevention believes that measuring circulating antibodies against rabies should not be used as a substitute for rabies vaccination.
Testing for rabies antibodies using the rapid fluorescence focus inhibition test (RFFIT) is available through the Kansas State Veterinary Diagnostic Laboratory. Further information is available at www.ksvdl.org.
Of course, when it comes to protecting against rabies — or any disease — vaccination does not guarantee protection. So in endemic areas, horse owners would be well served to try to limit exposure. Trash and food containers should be secured, and grains and other palatable horse feeds should not be left outside.
Rabies exposure or transmission can occur only when the rabies virus is passed via a bite wound, into open skin wounds, or onto mucous membranes, either from saliva or other material that can transmit infection, such as neural tissue. If horses are exposed to confirmed rabid animals, they should follow one of these protocols:
- Horses currently vaccinated against rabies with one of the USDA-approved rabies vaccines should be immediately revaccinated and then observed for at least 45 days to look for clinical signs of rabies. The exposure should be reported, and attending veterinarians should rely on the direction of public health officials.
- If a horse is unvaccinated and has been exposed, public health officials should be contacted immediately. Public health agencies have established requirements and conditions for monitoring exposed animals and for the disposition of animals should they ultimately contract the disease. The agencies dictate the options for an exposed horse; typically these include immediate isolation and post-exposure vaccination.
- Some owners may elect to have horses with confirmed exposure to rabies euthanized immediately. This is not an acceptable option for many owners, and if the owner does not elect for immediate euthanasia, then veterinary supervision and monitoring should take place for at least six months, after approval from public health officials.
Make sure your clients’ horses are protected against rabies if your practice is an endemic area. It’s an important service that protects the health and well-being of everyone.