The rabies virus as seen through the microscope. (University of South Carolina photo)
From the Animal Health Lab at the University of Guelph in Ontario, Canada comes this sobering report:
Recently in Ontario, Canada, a 15-month-old Thoroughbred filly was suspected of suffering the muscle disorder commonly known as "tied up" just one hour after appearing normal and being brought into the barn. Over the next 42 hours the horse's condition worsened drastically until she was unable to stand.
The horse was euthanized and her remains were sent to the Animal Health Laboratory (AHL) at the University of Guelph for necropsy.
Testing of spinal cord samples showed that the horse was infected with rabies virus, and specifically the Arctic fox strain. This type of infection was very widespread in foxes in the second half of the 20th century, but is now seen mostly in skunks in restricted parts of Ontario.
It is important to note that this horse had been vaccinated with a killed 3-year vaccine product at 4 and 5 months of age. Newer DNA vaccines that are now used in horses for West Nile disease, and have been studied for use in equine rabies, may hold promise for better protection in the near future.
Efforts by the Ontario Ministry of Natural Resources have greatly reduced rabies in Ontario to the point where in 2007 there were only 106 confirmed cases of rabies in Ontario (with no horses affected). This case illustrates the need to consider rabies as a rule-out in any horse showing neurologic signs, regardless of vaccination status.