Rabies

Takeaways

• Rabies is a fatal neurologic disease of mammals.
• Rabies is rare in horses, but invariably fatal.
• Clinical signs of rabies in horses vary, are easily confused with other diseases, and often progress rapidly.
• There is no treatment for rabies in horses.
• The rabies vaccine is recommended as an annual core vaccine for all equids.

What is rabies?

Rabies is a neurologic disease caused by a virus in the genus Lyssavirus. All mammals are susceptible to rabies and it has the highest case fatality ratio of any infectious disease. Rabies is relatively rare in horses, with the Centers for Disease Control and Prevention (CDC) reporting 13 cases of rabid horses and mules in the United States in 2018, but all cases are fatal.

Wildlife have accounted for >90% of rabid animals in the United States since 1980. The primary reservoir species responsible for maintaining rabies are raccoons, bats, skunks, and foxes. The disease is a significant threat to horses in South and Central America but is not a major cause of death in North America due to successful vaccination efforts.

Rabies infection usually occurs by the bite of a rabid animal but can also be transmitted when fresh saliva from an infected animal contacts a wound or mucous membranes. Encounters between horses and rabid animals are rarely witnessed, and bite wounds, which may be punctures, are difficult to find. Bites often occur on the lower limbs or face and the virus migrates to the spinal cord and brain where it causes fatal inflammation of the brain (encephalitis).

With the increasing urbanization of areas where the disease is regularly found in wildlife populations, the risk of exposure continues to be a concern for veterinarians and horse owners. Horse-to-human transmission is rare, but exposure to nervous and other tissues from horses that may be infected should be avoided.

Rabies is a reportable disease in the United States; contact your state’s public health officials immediately. Report animals that are acting suspicious to USDA’s Wildlife Services (1-866-4-USDA-WS (1-866-487-3297).

What are the clinical signs of rabies?

The clinical signs of rabies in horses vary and often progress rapidly. They can include inability to stand, uncoordinated movements (ataxia) and muscle weakness, excessive sensitivity to touch, muscle tremor, lameness, anorexia, loss of tail and anal sphincter tone, loss of sensory perception in the hind limbs, fever (>101°F), colic, lethargy, convulsions, self-mutilation, and aggressiveness. Survival time after the onset of clinical signs may range from 2 to 5 days and can be as long as 2 weeks, although that is unusual.

How is rabies diagnosed?

Antemortem diagnosis can be difficult since early stages of rabies can be confused with other diseases. Laboratory tests and postmortem evaluation of brain and salivary gland tissues are commonly used for diagnosis.

How is rabies treated?

There is no treatment for rabies in horses.

What is the prognosis for rabies?

While the incidence of rabies in horses may be low, the disease is invariably fatal and has considerable public health significance.

How can rabies be prevented?

The American Association of Equine Practitioners (AAEP) recommends the rabies vaccine as a core vaccine annually for all equids.

A study by UC Davis researchers reported that vaccinated adult horses can maintain antibody titers for 2 to 3 years, regardless of age (< 20 and > 20 years of age). A small number of horses in the study (7/48) were poor responders to vaccination. Significant differences in response were identified between previously vaccinated horses and those that had never been vaccinated prior to the study. Additional research is needed, but it is possible that rabies vaccine intervals greater than one year are appropriate for horses that previously received an initial two-dose series of vaccines

Individualized vaccination programs should take into account a number of variables, including the health of the animal, probability of exposure to the infectious agent, and severity of the disease. Work with your veterinarian to determine the optimal vaccine schedule for your horses and situation. For horses with reported severe vaccine-related reactions, antibodies can be detected in order to determine if the level of antibodies is compatible with protection.

For more information:

Diseases with horse to human transmission, UC Davis Center for Equine Health

Center for Equine Health Horse Report, Summer 2014 - Zoonoses: What Horse Owners Need to Know

Harvey, A.M., Watson, J.L., Brault, S.A., Edman, J.M., Moor, S.M., Kass, P.H., Wilson, W.D. 2016 Duration of serum antibody response to rabies vaccination in horses. J Am Vet Assoc 249(4): 411-8.

American Association of Equine Practitioners - Rabies

Centers for Disease Control and Prevention - Rabies

*This article may not be reproduced without the written consent of the UC Davis Center for Equine Health. Please email requests to cehadmin@ucdavis.edu.