Josie Traub-Dargatz DVM, MS, DACVIM
Professor of Equine Medicine
Colorado State University
Animal Population Health Institute (APHI)
Introduction and Background
Rabies comes from the old Sanskrit word rabhas which translated means “to do violence” while the Greeks called rabies Lyssa or Lytta which meant madness or rage. The infectivity of saliva from rabid dogs was described by a Roman writer, so the disease was recognized even in Roman times. Rabies is caused by a virus and the infective virus is contained in the saliva of the rabid animal and any warm-blooded vertebrate animal is susceptible to infection however only a few types of animals serve as reservoirs of the virus in nature. Rabies is a zoonotic disease which is a disease that is shared by animals and man.
The most common means of exposure to rabies virus is through the bite of a rabid animal. Non-bite exposures to rabies virus are very rare. Scratches, abrasions, open wounds, or mucous membranes contaminated with saliva or other potentially infectious material (such as brain tissue) from a rabid animal constitute non-bite exposures. Contact of mucous members of the eyes, nose and/or mouth with aerosolized virus is an uncommon but possible means of exposure. Rare cases of human rabies have been associated with the tissue implantation from a rabid human such as corneal transplants. Veterinarians and others who may be involved in the necropsy and specimens from suspect rabid animals need to take precautions when performing the necropsy in dealing with samples. Other contact, such as a person petting a rabid animal or contact with the blood, urine or feces (e.g., guano) of a rabid animal, does not constitute an exposure and is not an indication for prophylaxis per the Centers for Disease Control and Prevention. For more detailed information on rabies the website for the Centers for Disease Control and Prevention (CDC) is a good resource and can be accessed athttp://www.cdc.gov/rabies/.
Once the virus is in the body it multiplies and the virus spreads along peripheral nerves and eventually infects the central nervous system, causing encephalitis and ultimately death. The time from when the animal or person is bitten and when signs develop can be quite variable. The signs of rabies can vary depending on the location of initial exposure, the strain of rabies virus involved and the animal infected.
Over the last 100 years, rabies in the United States has changed dramatically. More than 90% of all animal cases reported annually to CDC now occur in wildlife; before 1960 the majority of cases reported were in domestic animals. The principal rabies hosts today in the USA are wild carnivores like raccoons, skunks, foxes, coyotes and insectivore bats. Outbreaks of rabies infections in terrestrial mammals like raccoons, skunks, foxes, and coyotes are found in broad geographic regions across the United States.
In this century, the number of human deaths in the United States attributed to rabies has declined from 100 or more each year to an average of 2 or 3 per year. Two programs have been responsible for this decline. First, domestic animal control and vaccination programs begun in the 1940’s and oral rabies vaccination programs in various types of wild animals that occurred more recently have eliminated domestic dogs as reservoirs of rabies in the United States. Second, effective human rabies vaccines and immunoglobulins have been developed. Most of the human rabies cases in the USA are the result of bat exposure which may go unnoticed or the significance of a potential exposure being unrecognized, such as finding a dead bat in the home.
In the spring of 2008 the Colorado Department of Public Health and Environment issued a notice to local public health agencies and veterinarians regarding the occurrence of rabies in 7 skunks and one coyote in Colorado. The occurrence of rabies cases in terrestrial animals such as these is unusual in Colorado. John Pape of the Zoonosis unit of the Colorado Department of Public Health and Environment was quoted as saying, “In eastern Colorado we are seeing the skunk strain of rabies circulating farther west than at anytime in the last 30 years,” Pape said. The problem is skunks are highly efficient at transmitting rabies to other animals like pets.” Terrestrial rabies suggests more risk of exposure to pets as well as livestock than if rabies is occurring only in bat populations. For more information go to the CDPHE website at http://www.cdphe.state.co.us/dc/zoonosis/rabies/index.html. There is an ongoing skunk surveillance program in parts of Colorado and maps of the current number of investigated animals and those positive for rabies are available along with information on the surveillance program at the CDPHE website athttp://www.cdphe.state.co.usdc/zoonosis/rabies/SkunkSurvProtocol.pdf and http://www.cdphe.state.co.us/dc/zoonosis/rabies/skunkrabies_map.pdf.
Domestic animals account for less than 10% of all animal rabies cases in the USA. The number of laboratory confirmed equine rabies cases reported in the USA in the recent past is generally around 50 cases per year across all the continental states. The largest number of equine rabies cases is generally reported in Texas. The majority of rabies cases occur in animals with no rabies vaccination history. Because horses are curious and many types of wildlife with rabies loose their fear of humans and other domestic animals the likelihood of exposure is heightened.
The reported signs of rabies in horses have ranged from poor racing performance to bizarre behavior. Some animals develop an apparent lameness, colic like signs, and genitourinary signs. The classic description is of the encephalic form with evidence of progressive depression (dumb rabies) or aggression with self mutilation and biting at the site of the bite wound (furious rabies). Equine rabies cases may also have an inability to swallow as well as excessive salivation. One of the most consistent signs of rabies in horses in a case series from Canada was irritability when touched (hyperesthesia). Veterinarians who have seem multiple equine rabies cases have said that an equine rabies case can initially look like anything and the classic signs may not initially be apparent or never develop. Once signs develop the course of the disease is usually hours to days but the time from exposure to onset of signs can be quite variable and in some instances quite long. The disease is invariably fatal.
There is no laboratory test that can be done on the live animal to make a definitive diagnosis of rabies in horses. A rabies infected horse can expose owners, care takers, veterinary personnel, and many other people to rabies.
Once the animal dies or is euthanized, a veterinarian should be involved in collection of appropriate samples to make a diagnosis. This includes the collection of the brain to be submitted for specific laboratory testing to detect the rabies virus in this tissue. It is important that those who will be involved in the necropsy take appropriate precautions to avoid exposure to the virus during the collection of tissues and that all samples collected from suspect animals be labeled appropriately so that those performing testing can take appropriate precautions.
There is no known effective treatment of unvaccinated or vaccinated horses with clinical rabies. Thus rabies is a disease that is better prevented. The mainstay of prevention of rabies is through vaccination. Equine owners should have their veterinarian involved in the development of a vaccination plan for their horse(s) including the use of rabies vaccine. Rabies vaccine is now considered a core vaccine by the American Association of Equine Practitioners (AAEP); vaccination guidelines are available at the AAEP website at http://www.aaep.org/vaccinationguidelines. A core vaccine is defined as one “that protects from disease that is endemic to a region, has with potential public health significance, is required by law, is virulent/highly infectious, and/or poses a risk of severe disease. Core vaccines have clearly demonstrated efficacy and safety, and thus exhibit a high enough level of patient benefit and low enough level of risk to justify their use in the majority of patients.” Rabies vaccine meets the criteria as a core vaccine based on the fact that rabies is recognized in wildlife in all geographic areas of the continental USA, the vaccine has been demonstrated to be safe and effective and there is a public health risk from exposure if a horse developed the disease.
A Colorado State University Extension Livestock Management release from October 2008 entitled “Rabies in Horses: Should Horses be Vaccinated in Colorado?” suggests that it is the horse owners’ decision whether to vaccinate or not to vaccinate and that it has become a vital question for horse owners in Colorado to ponder. This information sheet written by Dr. N. Striegel goes onto say that rabies in horses is rare but it would seem after considering the recommendations of veterinarians, the incidence of wild life rabies and the course of the disease, the benefit of vaccinating horses for rabies outweigh other potential reasons for not vaccinating horses in Coloradowww.ext.colostate.edu, number 1.819.
Since this presentation was given in February of 2009, a horse has contracted the rabies virus was subsequently euthanized in El Paso county near Colorado Springs. This is the first case of rabies in horses in over 25 years and it marks the importance of vaccinating both small and large animals against this disease. The following link is the press release from the El Paso County Department of Health and Environment and their recommendations. Skunk Rabies News Release