Rabies in Horses

Rabies, an old and clever virus, is making a resurgence in parts of the United States. While the number of rabies cases in animals might seem relatively small, and the number of cases in humans minuscule, the public health costs and implications are much larger.

In 1994, the last year for which accurate counts are available, some 8,224 cases of animal rabies (42 in horses) were reported in the 48 contiguous states, Washington D.C., and Puerto Rico. That year, six cases of rabies in humans were reported.

The distribution of rabies cases within the United States is somewhat startling, although easily explained. The Western states show very little incidence of rabies. But in Texas, and in the Northeast, the incidence of rabies has grown.

While the rabies virus has, for centuries and throughout the world, shown an adaptability that has allowed it to spread and survive, humans in the United States have contributed to some of its recent spread and success here. At the same time, there has been some change in the traditional carriers of the virus. Historically, the rabies virus has lived in the host populations of dogs, foxes, and skunks. But widespread vaccination of dogs, begun in the United States in the 1940s and 1950s, has severely reduced the instance of rabies in domestic animal populations. The new carriers, or vectors, are more likely to be coyotes, skunks, raccoons, and bats.

Coyotes account for the resurgence of rabies cases in Texas. Rabies in coyotes was first recognized in Texas in 1988. The incidence of rabies in that popu-lation, which spread to additional counties in the state in the late '80s and in the early '90s, resulted in 446 known cases of rabies in coyotes and dogs by 1994. That in turn caused the governor of Texas to declare the spread of rabies a state health emergency in 1994. The Texas State Department of Health has established a quarantine prohibiting shipping of bats, coyotes, foxes, raccoons, skunks, and wolves out-
side the state, in addition to cats and dogs over three months old that have not been vaccinated.

The implications of the problem in Texas go well beyond the boundaries of the state. Rabies in animals that originated in Texas has accounted for cases found in other states as humans have moved animals between states.

Despite the prohibition, some coyotes and foxes from Texas have been carried to other states to be in "fox running pens," where dogs can chase them. While most states have laws preventing the movement of wildlife by private individuals, the increase of interest in this type of sport has been accompanied by an increase in the transport of these animals. The result has been to give rabies a jump start in the states where the animals have been shipped.

In late 1994, five unvaccinated foxhounds in Florida were diagnosed with the same strain of rabies found in the dogs and coyotes of Texas. Several of the dogs had been in a fox running pen with coyotes imported from Texas. Likewise, rabies found in Alabama has been associated with similar circumstances. These instances reflect not only the spread of rabies, but also the apparent lack of vaccination of the dogs involved.

In the Northeast it is not coyotes but raccoons that seem to account for the rise in the incidence of rabies. New York equaled Texas in 1994 in the number of cases of rabies found in horses, seven. In the same year, New York reported 1,272 cases of rabies in raccoons. Rabies in raccoons was identified in the Southwest for decades. But in the 1970s, apparently because of raccoons carried by humans from one region to another, the rabies virus began to appear in the abundant raccoon populations in the Mid-Atlantic states. Rabies in raccoons moved throughout the Eastern seaboard in the late '70s and '80s until the virus now has been found in raccoons in 16 states, stretching from Rhode Island and New Hampshire to Florida. While a rabid raccoon has not yet been found in Maine, in 1994 a rabid skunk in Maine was found to be carrying the strain of the virus found in raccoons.

"Of greatest concern at the moment is this raccoon rabies in the Northeast," said David Powell, MRCVS, an epidemiologist at the Gluck Equine Research Center at the University of Kentucky in Lexington. Powell said the rabies virus, while it requires a live host to survive, has shown remarkable ability to do just that.

"It's been clever enough to jump from species to species," he said. "The virus is keeping one step ahead of the game and finding a host in which it can survive and multiply," Powell added.

Most cases of rabies in humans in the United States seem traceable to bats, even in cases where there is no recollection of a bite. In some cases, people have awakened to find a bat in the room, but can find no evidence of a bite. It is more difficult to control and identify the spread of the virus in the bat population since bats fly and, in some cases, migrate.

The incidence of bat rabies has been exacerbated by the import of some species from outside the United States that were found to carry the virus.

Fighting The Disease

The battle against rabies has existed for centuries. In the United States there has been tremendous success as vaccination programs for domestic animals have become widespread. But incidences of rabies, and cases in which people are known to have been near an infected animal, still result in tremendous costs. The post-exposure vaccine is no longer painful, but remains tremendously expensive, at a cost of about $1,000 a person. The total cost of post-exposure treatment is put at about $45 million per year. That, of course, is in addition to the cost of preventive vaccines.

A clear need in halting the spread of rabies--and limiting the contact of humans with the disease--is vaccinating domestic animals. While dogs are often vaccinated, in some cases cats are not. Clearly, horses should be vaccinated. And, in no case should wild animals, particularly those in the populations that currently appear to harbor the virus, be moved from their native region.

There is a bright spot on the horizon in the fight against rabies. A vaccine for non-domestic animals, dispensed through baits distributed in the wild, has proven effective in use in Europe and in early trials in the United States. The vaccine has been tested in Virginia, Pennsylvania, and New Jersey. In addition to medical problems to be worked out in transferring a vaccine developed in Europe for foxes to a raccoon population in the United States, the researchers had to determine optimum material to use for baits to carry the vaccine, the appropriate time of year to distribute it, and the impact, if any, on other species in an area. A test across the Northern Cape May Peninsula showed a significant decline of the incidence in rabies in raccoons there and in the rate of the spread of the virus.

About the Author

Jennifer O. Bryant

Jennifer O. Bryant is editor-at-large of the U.S. Dressage Federation's magazine, USDF Connection. An independent writer and editor, Bryant contributes to many equestrian publications, has edited numerous books, and authored Olympic Equestrian. More information about Jennifer can be found on her site, www.jenniferbryant.net.

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