EDITOR’S NOTE: This is the first of two parts on rabies in horses and cattle.
Rabies is one of the most feared diseases because it can occur in all warm-blooded animals and is almost always fatal. It is caused by a virus that affects the nervous system, transmitted by the saliva of an infected animal, usually through a bite or by saliva coming into contact with mucous membranes (such as eye or mouth) or opening in the skin. This is an uncommon disease in horses and cattle but can be a serious health hazard for humans who handle the sick animals.
Dr. Amy Johnson, of the New Bolton Center in Pennsylvania, has treated two horses for rabies.
“The last horse I treated was a retired racehorse,” she says.
Racehorses are vaccinated against influenza, herpes and some of the other viral diseases but some trainers don’t vaccinate against rabies because they think the risk is low, with horses in stalls rather than out in a pasture where they might encounter wild animals, dogs or feral cats. Horses in barns can still be at risk, however, from bats or barn cats.
Rabies vaccination is now listed by the American Association of Equine Practitioners as one of the core vaccines that should be required for all horses, yet many horse owners do not vaccinate for rabies.
“In our area, the strain that’s endemic is raccoon rabies, the most common one in the eastern U.S,” Johnson says.
In other areas there are different variants, in skunks, foxes, coyotes and bats. There are several strains of rabies, just as there are multiple strains of flu virus.
“Pennsylvania has a high incidence of rabies. We see it in all species, including cows and horses, but especially cats and dogs. We’ve also seen whitetail deer with rabies. Human risk is generally considered fairly low when handling horses or cattle with rabies, because they don’t usually bite and puncture the skin,” she says.
But there is always risk when coming into contact with saliva.
“If you get the animal’s saliva in your eyes, mucous membranes or cuts, you could get rabies,” Johnson says. “There is risk when handling the mouth of any animal that might have rabies.”
Signs of rabies vary, and people often try to treat the animal for something else. Often the inability to swallow is mistaken for choking and the owner or veterinarian may become exposed when examining the mouth and throat.
“The trouble with rabies is that early signs are often vague, and in horses may include lameness and/or colic. You might not think of rabies,” she says.
In one study, 10 percent of the horses showed colic as an initial sign and 24 percent showed lameness. This might lead a veterinarian off track, but then the animals always progress to other signs, becoming increasingly abnormal in behavior and possibly uncoordinated or disoriented.
It may take up to four or five days or longer until they progress to the point where they are euthanized or die. Incubation time (between exposure and start of symptoms) is variable, depending on location of the bite. An animal bitten on the nose or face, for instance, will usually start showing severe signs much sooner than an animal bitten low on the hind leg. In that situation it might be several months between the bite and the onset of illness. The virus has to travel farther along the nerves before it gets to the brain.
“We can only diagnose rabies definitively after the animal is dead.” Johnson says. “If you have a horse that dies or is euthanized after a short course of unexplained progressive neurologic disease, it should be tested for rabies. If you don’t test, you wouldn’t know, and humans may have been exposed.”
If an unvaccinated horse was known to be bitten by a rabid animal, with the offending animal killed and tested, it may be possible to treat that horse (like we treat humans) and not have to euthanize it.
“Laws regarding unvaccinated, exposed animals vary from state to state,” Johnson says. “Check your state’s public health guidelines to determine what you can do with that horse.”