Today, the threat of rabies to American horses may seem remote. Vaccination against the disease is extremely effec-tive and affordable. And rabies is rare in the United States: Only 25 cases were reported among horses and mules in 2014, according to the Centers for Disease Control and Prevention. You need not lie awake at night worrying about rabies.
But you don’t want to become complacent, either. Usually transmitted through the bite of an infected animal, rabies is invariably fatal---the virus ravages a horse’s nervous system and there is no cure. In fact, rabies has the highest mortality rate of any infectious disease---functionally 100 percent since euthanasia is the only option once signs of illness appear. And reservoirs of rabies virus continue to exist in the wild, causing periodic outbreaks of the disease that pose a risk to both wild and domesticated animals.
All of which means that even as you vaccinate your horse against rabies, it’s wise to remember the threat the disease poses and remain vigilant.
How infection happens
The rabies virus is spread through the saliva of infected animals. In the United States, the main reservoirs for the disease in the wild are raccoons, skunks, foxes and bats. Rabies has been reported in every state except for Hawaii. Wild animals commonly pass the virus to others of their own kind, but susceptibility to the infection varies among species---for example, to become infected an opossum needs a dose of rabies virus 50,000 times higher than the amount it takes to infect a fox.
Horses typically become infected when a rabid animal wanders into their pasture or enclosure. Rabid animals don’t necessarily attack horses, but a curious horse may sniff the visitor, startling it and leading to a bite.
Equine rabies infection is considered “spillover” of the virus, meaning horses can contract rabies but only rarely pass it on. Routine handling or exposure to a horse’s blood, urine or feces generally does not pose a risk. However, rabies can be transmitted to a person or another animal if an infected horse’s saliva comes in contact with the eyes, nose or mouth or a skin wound or abrasion. In some cases, a rabid horse at first seems colicky, exposing people as they care for him. Later, when the true nature of the problem is determined, the owner, veterinarians and others who had contact with the horse may need to undergo preventive measures.
The damage done
Once the rabies virus enters a horse’s body, it invades the nearest peripheral nerve cell. It then replicates, passing from cell to cell, working its way to the brain. In most cases, a horse won’t show any sign of disease during the incubation period, which can last for weeks or months depending on the dose of the virus and the location of the bite. For example, a horse bitten on the muzzle may show signs of rabies within a day or two, while one bitten on the leg may not become ill for weeks because the virus must travel farther to reach the brain.
There are two forms of rabies in horses. In the “furious” form, generally seen after a bite to the head, the horse becomes aggressive and agitated before paralysis of the face and tongue sets in. The inability of these horses to drink, and their frustration with that, often gives the impression that they are afraid of water. In fact, rabies was historically and mistakenly referred to as “hydrophobia.” But a rabid horse is not afraid of water; he simply cannot drink it. A horse with the furious form of rabies can be extremely dangerous---unpredictable, aggressive and violent.
In the “dumb” form of rabies, typically seen after a bite to a limb, the horse becomes gradually more depressed and weak until he is unable to rise. With this form, the characteristic paralysis of muscles on the face and head takes longer to appear than in the furious form; when it does, it is often signaled by drooling as the virus enters the salivary glands.
Regardless of the form, the earliest stages of rabies can be confused with other diseases, particularly those with a neurological component, such as equine0 protozoal myeloencephalitis (EPM) or West0 Nile encephalitis. Misdiagnosis is more likely to occur if the bite wound goes undetected. Within a few days, however, the rapidly spreading paralysis makes it clear something more sinister is at work.
Safeguards against infection
Obviously, the best way to protect your horse from rabies is through annual vaccinations. The American Association of Equine Practitioners classifies rabies as a core vaccine, which means it is recommended for all horses regardless of life stage, lifestyle or location. Currently, three licensed rabies vaccines are available for horses; all are killed-virus products administered annually to mature horses.
Rabies is a reportable disease, which means that a veterinarian is required by law to notify public health officials about potential cases, and management of those cases must conform to state and local health regulations.
If a vaccinated horse may have been exposed to rabies---perhaps a bite wound is found on his leg, for example, or a sick raccoon is discovered in his field---a veterinarian will likely recommend the administration of a “booster” vaccination to ensure that his immune system can fight off the pathogen. (Laboratory tests to check titers of rabies antibodies in a horse’s blood are not a reliable indicator of protection, and there is no risk of overdose with another vaccination.) After the booster is administered, the horse will also be observed for at least 45 days for any signs of rabies.
If an unvaccinated horse is bitten by a rabid animal or otherwise exposed to the disease, the course of action will depend on several factors. In some cases, immediate euthanasia may be recommended. An option in other situations may be immediate vaccination followed by strict isolation and observation for signs of disease for a minimum of six months.
Finally, research by Texas state public health officials has shown that the rabies postexposure prophylaxis protocol (PEP) for domestic animals mandated by the state can be effective in preventing the disease. The Texas PEP calls for immediate vaccination against rabies, a strict isolation period of 90 days, and the administration of booster vaccinations during the third and eighth weeks of isolation. However, because of the public health implications, states and localities have regulations specifying how cases of rabies exposure must be handled; a PEP protocol may not be allowed under those regulations.
SIDEBAR: Rabies: In Brief
© Definition: viral disease of the central nervous system
© Signs: infection produces no signs until the virus reaches the spinal cord and brain. Once that happens, the horse may show acute (also referred to as “furious”) signs such as aggression, agitation, hyperactivity and paralysis of the tongue and face. He might also show paralytic (or “dumb”) signs, such as depression, weakness, ataxia, recumbency, paralysis and excessive salivation. If not euthanatized, the horse will usually die within days.
© Causes: The rabies virus is excreted in saliva of infected animals and usually transmitted through bites.
© Diagnosis: No definitive laboratory test can identify rabies in a live horse. Diagnosis on a living horse is done through a history and observation of signs. Postmortem examination of the brain can confirm the diagnosis.
© Treatment: Nothing can be done once clinical signs appear, which can take days, weeks or months. Immediately after known exposure, a previously vaccinated horse will be given a booster shot and monitored for a minimum of 45 days. In some jurisdictions, an unvaccinated animal may undergo a rabies postexposure prophylaxis protocol (PEP), which entails immediate vaccination, followed by booster vaccinations, and a strict isolation period lasting a minimum of 90 days.
SIDEBAR: Defending the perimeter
Keeping all wildlife away from your horse may seem like a logical defense against rabies. But as a practical matter, that’s just not possible. A better approach is to make your property as uninviting as possible to such visitors. Here are a few suggestions:
• Store grain securely and neatly. Spilled or otherwise accessible feed is the wildlife equivalent of an “open for business” sign on your barn. Apply the same standards to your hayloft, keeping stacks tidy and doing a thorough cleaning at least twice a year.
• Tear down abandoned buildings. A dilapidated shed you never venture into is a haven for wildlife that could harbor rabies. If you have a shed that’s unused but still in good repair, find a use for it that has you entering several times a week in a loud and obvious fashion. Regular disturbances will likely cause animals to relocate.
• Consider adopting a dog. If you don’t already have a regular canine patrol of your property, look into that option. Of course, you never want a dog to encounter a rabid animal, but a vigilant presence may discourage animals from moving into the territory in the first place.
This article first appeared in the June 2017 issue (#477) of EQUUS magazine