Affected horses generally start displaying clinical signs two to nine weeks post-infection, and clinical signs range from depression, anorexia, and ataxia to belligerousness (being controlled here with a rope), abnormal spinal reflexes, and death.
Photo: Tomas Gimenez, DrMedVet
Equine rabies is a feared and fatal disease, but there are ways horse owners can reduce the risk of their animals becoming infected. Do you know what they are?
Allison Stewart, BVSc (Hons), MS, Dipl. ACVIM, ACVECC, now an associate professor at Massey University Institute of Veterinary, Animal and Biomedical Sciences, in Palmerston North, New Zealand, shares some key points owners should know and remember about the rabies virus and horses.
Rabies is fatal to all mammals. Stewart, previously an associate professor of equine internal medicine at Auburn University, in Alabama, explained that rabies--caused by a lyssavirus that affects mammals' neurologic system and salivary glands--has at least six genotypes, all of which have different host ranges and pathogenicities.
Handling Rabies Exposure
While owners can vaccinate their horses against rabies to reduce the risk of disease contraction, they can't always prevent a curious equid from walking up to investigate the wrong wild animal--and getting bitten in the process. Allison Stewart, BVSc (Hons), MS, Dipl. ACVIM, ACVECC, now an associate professor at Massey University Institute of Veterinary, Animal and Biomedical Sciences, in Palmerston North, New Zealand, gave some tips on how to handle suspected rabies exposure:
"Livestock exposed to a rabid animal that have an up-to-date vaccination record should be revaccinated immediately and observed for 90 days," she said;
In the event an unvaccinated animal is exposed, he or she should be either be closely observed for six months, or euthanized immediately, Stewart said;
In the event humans have been exposed to an unvaccinated, potentially exposed animal, "Euthanasia of suspect animals already exposed to humans should be avoided until clinical signs are shown, as (disease progression) within the central nervous system is required to obtain a diagnosis," she explained; and
"Research in other countries describes that after natural infection of cattle and horses, the use of four injections of a modified live vaccine were protective," she said. However, there are currently no USDA-approved biologics for protective use post-exposure for animals.
Stewart also reminded the audience that rabies is a reportable disease, so animal health authorities should be notified immediately when a case is confirmed
"Also if any horse bites a person then the local veterinary rabies officer should be contacted," she said. "The horse will be quarantined at home for 90 days, and if the horse dies or is euthanized for any reason then testing for rabies must be performed."
Rabies isn't everywhere, but it's prevalent in the continental United States. Hawaii is the only U.S. state that is rabies-free. In 2010, 6,153 cases of rabies were reported throughout the rest of the nation, she said; only 37 of those cases were identified in horses and mules.
"Similar to other species, equine cases have been gradually declining," she added. "The majority of equine cases were from nonvaccinated animals, although reportedly there have been some cases of equine rabies in horses vaccinated more than 12 months previously." (More on vaccination in a moment.)
Countries considered rabies-free include Australia, Great Britain, New Zealand, Scandinavia, and Switzerland.
Horses contract rabies from other infected animals. For a horse to contract the rabies virus, he must either be bitten by another rabid animal or have a wound contaminated with saliva or blood from an infected animal, Stewart said.
Rabies outbreaks in domestic animals generally coincide with an increased wildlife population. "There is an increased incidence (of infections in horses and livestock) in late summer when wildlife populations have peaked," Stewart said. Additionally, cases are often confirmed during the wild animals' mating season and when young offspring are separated from their mothers or parents. When wildlife moves into new territories the risk of fighting and therefore the spread of rabies increases, she said.
Early clinical signs of rabies in horses "can look like anything." "Encounters between domestic animals and rabid wild animals are rarely witnessed," Stewart said, noting that puncture wounds from teeth are often difficult to locate on a horse. If wounds are discovered, she recommends washing the lesions immediately with a 20% soft-soap solution or Zephiran in an attempt to prevent infection.
Stewart said affected horses generally start displaying clinical signs two to nine weeks post-infection. Early clinical signs, she said, typically are nonspecific and include depression, anorexia, and/or mild ataxia. Clinical signs that arise later in the course of the disease can include:
- Repetitive twitching;
- Hypersensitivity to touch and sound;
- Hypermetria (a condition in which voluntary muscular movement overreaches the intended goal);
- Proprioceptive deficits (lack of physical awareness of limbs and their placement);
- Regional pruritus (itchiness);
- Periods of violence interspersed with periods of normalcy or depression; and
- Normal, increased, decreased, or absent spinal reflexes.
"Rabies is rapidly progressive and uniformly fatal with death from cardiorespiratory failure or misadventure within 10 days," Stewart said.
Rabies can only be definitively diagnosed post-mortem. Stewart explained that the only way to make a definitive diagnosis is to examine the animal's brain post-mortem with a fluorescent antibody test.
"If there is even a slight possibility that an animal died or was euthanized because of rabies, or if a horse had bitten a person prior to death, it is imperative that a post-mortem and testing for rabies is performed," she stressed. "Any person that was exposed to blood or saliva from a rabid horse has a risk of contracting and therefore dying of rabies. The chances of infection from a horse to a human are small, but it is just not worth the risk."
Rabies is a zoonotic disease, meaning it can be passed from horses to humans. For this reason, it's crucial to put strict biosecurity protocols in place if the disease is suspected, Stewart stressed. For starters, if a horse could have rabies, he should only be handled by people who handled him before; avoid contact with as many new or additional people as possible. "Ideally care should be provided by veterinarians and technicians that have been vaccinated against rabies," she said.
"A list of 'in-contact' and 'potential in-contact' individuals, including owners, should be kept on a clipboard close to the stall," she added. "Any person who handles the animal or biologic samples from the animal should sign the list." She also noted that the attending veterinarian should label all samples as "rabies suspect" so laboratory staff will know to exercise due caution as well.
She recommends donning gloves, face shields and/or masks, and eye protection when handling horses suspected of having rabies.
In the event a positive rabies test is returned, Stewart said the owner or veterinarian must contact the state veterinarian immediately, as rabies is a reportable disease. Then, she said, all individuals in contact with the horse should be notified; these people should contact their doctors, as well as state and local health officials, for guidance on post-exposure treatment.
Rabies vaccinations are, on a whole, very effective. "Rabies is an excellent immunogen, and vaccines induce a strong serologic response within 45 days after a single dose," Stewart said. She relayed that the American Association of Equine Practitioners (AAEP) advises that all horses receive rabies vaccinations annually.
AAEP vaccination guidelines recommend that adult horses receive an initial single dose and a booster vaccination annually; foals born to vaccinated mares should receive a first vaccine dose no earlier than at six month of age and a second dose four to six weeks later followed by annual vaccination; and foals of unvaccinated mares should receive a first vaccine dose at three or four months of age and should be revaccinated annually.
Despite the fact that rabies is invariably fatal once contracted, it's a very preventable disease. An understanding of the disease, how it works, and how to prevent it can help keep your horses healthy.
Real-Life Examples of a Real-Life Threat
Although rabies is a rare disease in domestic animals, due to its uniformly fatal nature, encounters remain as vivid memories. These are two real-life examples from my own experiences.
Case 1: Human Exposure
We had a "down horse" emergency case due to arrive at the John Thomas Vaughan Large Animal Teaching Hospital at Auburn University. Prior to arrival, the owners told us that the previous day, the horse was at a 4-H show being ridden by a young girl, and he seemed a little off: slightly colicky in the morning and then lame in the afternoon. The following morning the horse was ataxic (incoordinated) and their regular veterinarian recommended referral to Auburn. The owners set out on the four-hour drive to our hospital, but two hours prior to arrival the horse went down in the trailer. The owner called us to say that the horses' condition had deteriorated and he was now biting viciously at his extended foreleg; he also notified us that he did not think the horse was worth saving and that he was going to return home to humanely shoot the horse and bury it on the farm.
Rabies is a risk with any horse that has progressively neurologic signs, and though rare, this case was concerning. We strongly recommended continuing to the university, but the owner was concerned about expensive care with a poor prognosis. My fears turned to the daughter. Even if the horse was ultimately euthanized, there was a risk the daughter could have been exposed if the animal tested positive for rabies. A drop of saliva while bridling the horse could easily inoculate a small cut on a finger. To my relief the owner agreed to continue to Auburn to have the horse euthanized and a post-mortem performed.
On arrival the horse was still down on the trailer, tongue lolling and biting at anything that moved. We were able to heavily sedate the horse and carefully access a rear leg vein to humanely put him to sleep. All our veterinarians and technicians are vaccinated for rabies, but it was still quite frightening and risky, so we minimized the number of people involved.
We performed a post-mortem exam immediately, which confirmed our biggest fear: The horse was positive for rabies. We notified the state health department, and they were able to ensure all individuals that had contact with the horse--people in the horse's barn and at the show, the referring veterinarian, and the owner and his daughter--received post-exposure hyperimmune plasma and a long series of vaccines against rabies.
Case 2: Rabies vaccines: Important for horses, dogs, and even the barn cat
A friend of mine has several horses and visits her farm daily with her daughter and grandson. One evening while pulling a few flakes of hay from the hayshed, the daughter felt something brush against her face and then over her arm. A flashlight revealed a small bat that was awkwardly fluttering over the hay bales. My friend chased off the barn cat who was very interested in this new, wildly moving "cat toy" and managed to kill the ailing bat with a single whack of a nearby shovel. The daughter then noticed a small scratch across her arm. They washed the wound carefully and wrapped the deceased bat in several plastic bags. Meanwhile the old one-eyed barn cat was outside happily enjoying his evening game with the grandson.
My friend called me for advice. As a veterinarian I am not allowed to give medical advice, but I ensured that they had washed the scratch with water and disinfectant and suggested they pack some ice packs around the bat's packaged remains and head to the emergency room. Luckily they still had the dead bat available for rabies testing and it was delivered to the Alabama State Veterinary Diagnostic Laboratory first thing the next morning, where testing confirmed the bat was positive for rabies.
The daughter was given rabies hyperimmune plasma and post-exposure vaccines. She told me that over the next two weeks, every time she received one of the vaccines the scratch on her arm would redden and hurt for several hours. She was convinced the bat had inoculated her with the rabies virus.
The county rabies veterinary officer was notified of the positive rabies test, and we recommended that all the horses and the barn cat be revaccinated immediately and carefully observed for any possible signs of disease. Fortunately, the cat had not come in contact with the bat; an exposed cat with overdue vaccines, such as this one, would have to be euthanized or quarantined for many months. Initially my friend was reluctant to spend the money to vaccinate the old barn cat. He had lived happily on the farm for many years and, although she never really owned him, he certainly thought he owned the farm.
I walked her through the facts: Rabies is endemic in America, and obviously there were rabid bats on the farm. The individual most likely to contact a sickly bat would be the old barn cat, and the cat was always around the valuable horses and was the 4-year-old grandson's playmate. The old barn cat certainly needed to be vaccinated, even as a means of protecting others potentially more valuable than he. Are all your horses, dogs and barn cats up-to-date on rabies vaccines?
Allison Stewart BVSc(hons), MS, Dipl. ACVIM, ACVECC
About the Author
Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.
POLL: University Equine Hospitals