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Rabies is a fatal viral disease that can affect horses and is present in wild mammals all over the world. [1] Rabies is a zoonotic disease, which means it can be transmitted from animal to humans, and can be fatal if not addressed urgently after exposure to the virus. [2]
This disease primarily affects the central nervous system, leading to progressive and fatal encephalitis. Initial signs in affected horses include fever, colic, and lameness, followed by aggression, incoordination, recumbency and ultimately death.
The rabies virus is transmitted to horses through saliva, most commonly via bites from infected wildlife. Once a horse is infected and displays symptoms, there is no effective treatment for the disease. [3]
There is no reliable ante-mortem method to diagnose rabies in horses. If a horse displays symptoms indicative of rabies, they are often labeled as a “rabies suspect,” prompting the initiation of biosecurity and public health protocols. [4]
Regular vaccination against rabies is the primary preventive measure. Rabies vaccines for horses are both safe and effective. [5]
Rabies Virus in Horses
Rabies is caused by an RNA virus from the Rhabdoviridae family. [6][7] Horses that are infected by the rabies virus experience significant inflammation in the brain and spinal cord. This severe inflammatory response is fatal in unvaccinated horses. [6][7]
Rabies affects mammalian species. Although the virus circulates among various wildlife species, horses are deemed “dead-end” hosts, as they do not transmit it to other animals. [6][8]
According to the Centers for Disease Control and Prevention (CDC), wildlife accounts for 91% of reported rabies cases. [2] Affected animals are often called “rabid”. [1]
The primary reservoir species vary by region, but all animals can transmit the virus to both humans and horses. Notable reservoir species include raccoons, skunks, bats, and foxes. [8]
Rabies has been a nationally notifiable disease since 1938. [8] While the virus exists worldwide, certain countries like Great Britain, New Zealand, and Iceland have successfully eradicated it. [3]
In the United States, domestic animals represent 8% of all reported rabies cases. [3] Horses constitute 3 – 5% of rabies cases annually. [4]
Clinical Signs
Clinical signs of rabies in horses can be ambiguous and vary widely. Once symptoms manifest, they intensify until the horse succumbs to the disease. [9]
Rabid animals typically exhibit symptoms that fall into two primary categories: the “dumb” form and the “furious” form. [3]
Horses predominantly exhibit the “dumb” or paralytic form of rabies. Symptoms associated with this form include pronounced depression, paralysis, drooping of the face or jaw, and excessive salivation. [1][3][4][10]
On the other hand, the “furious” form of rabies is marked by aggressive and erratic behavior or intense agitation. [10] While this form is more typical in carnivores, when it appears in horses, it poses a significant threat to both the animal and its handlers. [3][4]
Clinical signs may include any of the following: [3][9]
- Fever
- Poor racing performance
- Lameness
- Knuckling
- Uncoordinated or stumbling gait
- Paresis or paralysis of the hindlimbs
- Unwillingness to rise (recumbent)
- Colic
- Constipation
- Straining to pass manure
- Dribbling urine
- Lack of tail and anal tone
- Erect penis in males without stimulation
- Frequent mounting behavior
- Self-mutilation
- Itchiness
- Muscle tremors
- Unusually aggressive behavior
- Blindness
- Head pressing
- Frequent bellowing
- Hyperextension or backwards arching of the head and neck (“stargazing”)
- Difficulty swallowing (may be observed as choke)
- Drooling
- Difficulty eating (may be observed as quidding)
- Seizures
- Coma
- Sudden death
Transmission and Disease Progression
Rabies is primarily transmitted to horses through the saliva of an infected animal. Horses usually become infected through bites from wildlife carriers, such as raccoons, skunks, bats, or foxes.
Less frequently, the virus can enter the body if saliva or brain tissue comes into contact with open wounds or mucous membranes, such as the gums, nostrils, eyes, or mouth. [2] Rabies cannot be transmitted through intact skin. [2]
Pathogenesis
Horses contract the rabies virus when it enters their system via a bite wound from an infected animal. The virus is present in the saliva of the infected animal, which is then transmitted to the horse and begins to replicate in muscle tissue. [10] Subsequently, the virus infiltrates the nearby nerves within the muscle tissue. [11]
Once inside the nerves, the virus advances to the central nervous system (CNS), which includes the spinal cord and brain. As the virus multiplies within the CNS, it induces inflammation and disrupts cellular function, manifesting as the clinical symptoms described above. [12]
After reaching the brain, the rabies virus can disseminate to other tissues and organs. Notably, it often migrates to the salivary glands, which facilitates the transmission of the disease if the infected animal bites another mammal. [13]
Typically, the interval between exposure to the virus and the onset of clinical symptoms ranges from 2 to 6 weeks. However, in rare instances, this period can extend up to 6 months. [4]
Diagnosis
While clinical signs may suggest a diagnosis of rabies, there is no reliable way to diagnose this disease in a horse that is alive. [4]
In horses infected with rabies, cerebral spinal fluid (CSF) may show abnormalities, such as elevated protein levels and more inflammatory cells. However, these findings are not specific to rabies. [3][9]
For a conclusive diagnosis, the brain must be sent to a regional laboratory that is approved by the health department for rabies testing. [3] If testing of rabies is required, it is important not to damage the brain or spinal cord during euthanasia. Samples of the brain are shipped refrigerated and not frozen. [4]
Sample containers must be thoroughly decontaminated and labelled as containing potential rabies specimens. This labeling is to inform laboratory personnel of the hazard and ensure proper handling of the specimen. [3]
Diagnostic Tests
To confirm a diagnosis of rabies, several different tests can be performed using brain tissue. Tests such as microscopic analysis, special staining techniques, and PCR tests are commonly available through approved laboratories. [3]
In some cases, a post-mortem exam will show swelling and reddening of the brain, and inflammation and atrophy of the lung tissue. [3][4][9]
On microscopic examination, signs of inflammation, cellular damage, and clusters of the virus may be visible within the brain and neural tissue. While these observations, combined with clinical signs, strongly indicate rabies, special staining of the tissue is still required for diagnosis. [3]
If a horse tests positive for rabies, it is mandatory to inform the state or provincial veterinarian. [3]
Differential Diagnoses
Depending on the clinical signs observed in your horse, other neurologic diseases may be considered. For example, the sudden onset of hindlimb issues, such as uncoordinated gait and lameness, can also indicate a back or pelvic injury, infection, or tumor. [3]
Other serious infectious diseases, such as West Nile virus and Eastern Equine Encephalitis, may also be considered. Toxins can also produce overlapping symptoms with rabies, such as seizures or coma. [3]
Treatment and Prognosis
There are no effective treatments for rabies in horses once symptoms are present. Affected horses typically die or are euthanized within 5 – 10 days following the onset of clinical signs. [3][9]
Management
Upon suspected exposure to rabies, the following steps must be taken to prevent further transmission of the virus: [3][14]
- Healthy vaccinated horse: Re-vaccinate and quarantine for 45 days to monitor for clinical signs
- Unvaccinated horses: Quarantine and monitor for 6 months or euthanize
Any horse with clinical signs consistent with rabies should be considered a rabies suspect. [4] Horses that are rabies suspects should be isolated away from other animals and humans to prevent further viral exposure. [3]
Upon recognizing a rabies suspect, follow these steps: [4]
- Contact government animal health officials for isolation guidelines and case confirmation
- Use personal protective equipment (PPE) when handling the horse
- Minimize the number of people in contact with the horse
Horses displaying symptoms consistent with rabies, and for which other neurological diseases have been ruled out, should be euthanized and their samples sent for rabies testing. [4]
Prevention
The primary preventive measures against rabies in horses are vaccination and avoiding exposure to potential carriers.
Most rabies cases are found in animals that haven’t been vaccinated against the virus. Rabies is also entirely preventable, highlighting the importance of preventative vaccinations. [5]
Vaccines
The rabies vaccine is a core vaccine recommended for all horses. A single vaccination in a horse’s history can provide some level of protection against the rabies virus. [4]
Several vaccines are approved for rabies prevention in horses. While most vaccines typically require annual administration, bi-annual vaccination might be suitable based on the location. [3]
Vaccination Protocols:
General vaccination protocols for horses are as follows: [14]
- Healthy adult horses: Vaccinate annually, and before breeding for broodmares.
- Pregnant mares: may be vaccinated 4 – 6 weeks before foaling to pass immunity to the foal via colostrum.
- Foals from vaccinated mares: Vaccinate at 4 – 6 months and again 4 – 6 weeks later. Provide a booster annually thereafter.
- Foals from unvaccinated mares: Vaccinate at 4 – 6 months and again at 12 months of age. Provide a booster annually thereafter.
Horses are considered immunized against rabies 28 days after vaccine administration. [3] Previously vaccinated horses maintain effective protection against the virus for at least 2 – 3 years. [15]
Vaccinations should only be administered under the supervision of a veterinarian. Veterinarians are trained in the storage, administration, and potential adverse effects of the vaccine, and can sign the vaccine certificate for your horse. [4]
Using a rabies titre, or the count of circulating antibodies against the rabies virus, is not a reliable method to decide the need for vaccination in horses. This is because the titre doesn’t consistently reflect the horse’s actual immunity to the virus. [4]
Other Preventative Measures
Minimizing contact with wildlife can also help decrease the risk of exposure to rabies. Biosecurity measures are also recommended, such as keeping barn doors and food sheds closed to discourage wild animals on the property. [1][2]
Active control programs are present in Canada and the United States to minimize human and pet exposure to rabies. This is done by distributing bait to wildlife that contains an oral rabies vaccine. [1][3][5][16]
The rabies virus is efficiently neutralized or eradicated by drying, UV/sunlight exposure, heat, and standard disinfectants. [1][4] Areas potentially exposed to the rabies virus must be thoroughly sanitized.
Risk to Humans
Rabies poses a significant zoonotic risk to humans. Although rare, there is a risk of virus transmission from infected horses to human handlers. [3]
In humans, rabies is fatal within 7 – 14 days after the onset of clinical signs. [7][17] Horses suspected of having rabies must be regarded as a significant threat to human health. [3]
Only humans immunized against the virus should handle rabies suspects. Humans that must come in contact with a rabies suspect should have their names listed on the horse’s stall and should use appropriate personal protective equipment (PPE), including: [3]
- Eye goggles
- Face shields
- Gloves
For veterinarians conducting post-mortem examinations, more extensive PPE is required such as: [3]
- Rubber boots
- Additional layer of heavy vinyl gloves
- Splash-protectant clothes or Tyvek coveralls
- N95 mask
Rabies is a preventable disease in humans with public education focused on exposure avoidance. [3] Individuals believed to have had contact with a potentially rabid animal should seek medical attention urgently. [3]
Postexposure treatment may be required, depending on the level of exposure and previous vaccination history. [3][17]
Summary
- Rabies is a fatal virus in horses, often transmitted through bite wounds from infected wildlife. [2]
- In horses, rabies infection manifests as severe neurological disease with a variety of symptoms, including lameness, fever, colic, inability to eat and paralysis. [3][9]
- A presumptive diagnosis of rabies is made based on clinical signs and history of wildlife exposure. However, a definitive diagnosis can only be made with laboratory tests of brain tissue after the animal has died or is euthanized. [3][4]
- There is no treatment for rabies and the disease is fatal within days after observation of symptoms.