The 7-year-old unvaccinated mule experienced onset of clinical signs on Jan. 15. Signs consisted of fever, ataxia (loss of control of bodily movements), recumbency (down and unable to rise), and cranial nerve deficit. The mule was euthanized.
Another equid on the farm was exposed. The area Veterinary Medical Officer performed a field investigation and officially quarantined the other livestock on the premises.
This is Colorado’s first case of rabies in domestic livestock this year and its first case of rabies in an equid since 2013.
Rabies—a zoonotic disease that can be spread from animals to humans—is caused by a lyssavirus that affects the neurologic system and salivary glands. Horses are exposed most commonly through the bite of another rabid animal.
In horses, clinical signs of rabies are variable and can take up to 12 weeks to appear after the initial infection. Although affected horses are sometimes asymptomatic, an infected horse can show behavioral changes such as drowsiness, depression, fear, or aggression. Once clinical signs appear, there are no treatment options.
Rabies can only be diagnosed postmortem by submitting the horse’s head to a local public health laboratory to identify the rabies virus using a test called fluorescence antibody. Thus, ruling out all other potential diseases first is very important in these cases to avoid potentially unnecessary euthanasia.
Because rabies threatens both horses and the humans who handle them, the American Association of Equine Practitioners (AAEP) recommends rabies as a core vaccine every U.S. horse should receive. The AAEP’s 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.