Rabies virus exposure typically occurs following a bite from an infected animal. Depending on the anatomic site of exposure, an incubation period of variable duration follows as the virus evades the immune response by hiding in the central nervous system. Virus amplification occurs in the dorsal root ganglion (a cluster of neurons in a dorsal root of a spinal nerve) after which it travels towards the brain via the spinal cord. At this point, clinical signs of rabies become manifest and therapy is almost invariably futile. Without early treatment, rabies is nearly 100% fatal.

Rabies is a zoonotic (capable of being transmitted from animals to humans) disease that is distributed nearly worldwide. Attention to the disease is primarily focused on preventive and control strategies. Many countries are considered rabies-free for the purposes of importing dogs into the United States; learn more at cdc.gov/importation/rabies-free-countries.html.

Rabies can be prevented by pre-exposure vaccination in both humans and animals. A human diploid cell vaccine and a purified chick embryo vaccine are available for humans (the latter is mainly used outside of the U.S.) for pre- and post-exposure to rabies, with rabies immune globulin available only for post-exposure treatment in exposed humans.

Pre-exposure vaccination involves administration of three doses of vaccine given over a one-month period. In unvaccinated humans, post-exposure treatment consists of the administration of five doses of vaccine. Vaccines for multiple species of domestic animals, including horses, are available to licensed veterinarians. Wildlife vaccines might be available from veterinarians but are typically used in targeted locations by the Kentucky Department of Fish and Wildlife and the USDA.

Kentucky Rabies Test Results

Results Wildlife Domestic Species (excluding horses) Horses Total
Negative 12,378 16,733 1,034 30,145
Positive 680 28 25 733
Unsatisfactory Specimen 823 675 11 1,509
TOTAL 13,881 17,436 1,070 32,387
From January 1989 through December 2017, Kentucky tested 32,387 animals for rabies virus. Of this total, 2.3% (733 animals) tested positive, 93.1% (30,145 animals) tested negative, and 4.6% (1509 animals) were unsatisfactory for testing (i.e., the sample was untestable due to maceration, degradation, or insufficient material). Of note: Animals suspected of having rabies should not be euthanized by traumatic insult (e.g., gunshot) to the brain because such trauma frequently renders the sample unsatisfactory for testing.

Of the 733 positive animals, only 7.2% (53 animals) were from domestic species (pets and farm animals), and the other 92.8% (680 animals) were wildlife. Twenty-five of the positive domestic animals were horses, which means that horses accounted for less than 1% of the total positive rabies cases in Kentucky over the last 29 years. Rabies-positive horses were primarily located in Central Kentucky (Figure 1, top left).

The terrestrial reservoir for rabies in Kentucky is the striped skunk, and rabies-positive skunks have been located in all counties where infected horses have resided (Figure 2).

As a closing comment, a robust surveillance program involving the University of Kentucky Veterinary Diagnostic Lab, Breathitt Veterinary Center, the Kentucky Cabinet for Health, Kentucky Fish and Wildlife, and USDA is in place to thoroughly monitor rabies in Kentucky. The vast majority of animals testing positive for rabies are wildlife, with little or no exposure to humans, pets, and domestic animals. More importantly for horse enthusiasts within the state, the number of horses testing positive for rabies is extremely low.

CONTACT: Jacqueline Smith, PhD—jsmit8@uky.edu—859/257-7559—University of Kentucky Veterinary Diagnostic Laboratory, Lexington

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.