Rabies Vaccination Revisited

Each year the Centers for Disease Control and Prevention (CDC) releases a comprehensive report detailing the number of reported rabies cases in 49 states and Puerto Rico. The most up-to-date report, which details 2003 cases, reveals that more than 7,000 cases were reported in a variety of wild and domestic animal species across the country. (Researchers currently are compiling 2004 data, and scientists say it's too early to speculate on reported U.S. rabies cases in 2004 or 2005.)

Eighteen states reported rabies cases in horses and mules, with seven of these states reporting more cases than the previous year.

No one knows for sure why there was an increase in some areas and a decrease in others, say the researchers, who report their results annually in the Journal of the American Veterinary Medical Association. They add that rabies vaccinations are the best insurance that a horse (or other animals that can be vaccinated for rabies) will not develop the disease.

Lead author on the 2004 study covering 2003 rabies cases was John Krebs, MS, a public health scientist at the CDC cautions that many cases of rabies remain unreported, and the number of cases is actually greater than the reported amount. He says that while dogs, cats, and cattle are generally the domestic animals most "frequently reported rabid," State Health Departments report around "50 plus or minus 10" equine rabies cases per year. "Scanning previous years of horses, I can see back in 1994, for example, that it was down in the 40s," says Krebs. "We had a banner year in 1998 with 82 horses (an exception to the prior 50 +/- 10 rule). I can't really say why, but looking at the broad span, probably the highest numbers are usually from the Western states, and that's primarily skunk rabies habitat, and there could have been just more infection due to interaction with rabid skunks during that period of time."

Rabies control programs were first put in place in the United States more than 60 years ago, with one key feature being massive vaccination of domestic animals, particularly pet dogs. The idea was to prevent dog transmission of the canine rabies variant, says Krebs. The role of wildlife as a reservoir for other rabies variants was not yet known. While livestock and other large animals can become infected with rabies, it has never been economically feasible or particularly important from a public health standpoint to require vaccination in this group of animals. However, the CDC recommends that all animals, including horses, that have "regular contact with human beings," be considered for vaccination (all mammals are susceptible to rabies).

Rabies virus variants can periodically shift in their distribution and the wildlife reservoirs that keep rabies circulating in an area can change from year to year. For example, states that primarily report bat rabies might have an increase in reported terrestrial animal cases, such as raccoons, skunks, foxes, and bats. Some states experience cases sporadically, while other states experience such heavy wildlife infection that the virus is considered enzootic, or affecting animals in the state year after year. In 2003, 20 states and the District of Columbia were designated as enzootic in their raccoon population. In some states, rabies infections will probably never go away, but they might dip below detectable limits.

The CDC report is a wake-up call to horse owners that the rabies virus is alive and on the move in the United States. While it is second-nature to most dog and cat owners to have their pets vaccinated for rabies, the horse is sometimes overlooked. It only takes a single bite from an infected animal to kill a valuable and beloved performance horse and put in jeopardy the health of everyone who comes in contact with that horse. As the CDC continues to monitor the shifting patterns of this deadly virus, horse owners should discuss rabies vaccination with their veterinarian and consider adding this important vaccine to their horse’s wellness program.

"People develop a great attachment to their domestic livestock, especially horses, and it would seem a shame for a valuable companion animal such as a horse to get rabies if it could have been prevented," adds Krebs.--Sue Piscopo, DVM and Stephanie L. Church

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