Editor's note: This article is part of TheHorse.com's ongoing coverage of topics presented at the 2012 American College of Veterinary Internal Medicine Forum, held May 30 - June 2 in New Orleans, La.
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, associate professor of equine internal medicine at Auburn University, presented a review of the invariably fatal disease at the 2012 American College of Veterinary Internal Medicine Forum, held May 30-June 2 in New Orleans, La.
During the presentation Stewart touched on some key points owners should know and remember about the rabies virus and horses.
Rabies is fatal to all mammals. Stewart 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.
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);
- Superlibido;
- Regional pruritus (itchiness);
- Belligerousness;
- 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.
Take-Home Message
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.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.