Rabies and Horses
Table of Contents
Equine rabies is a sporadic, but highly fatal, zoonotic disease caused by a virus (Lyssavirus). Rabies affects both animals and humans. It persists in Ontario wildlife, primarily in the red fox and striped skunk. All warm-blooded animals can be infected with rabies but it is most often transmitted in Canada by foxes, raccoons, skunks and bats. Variation in susceptibility is noticeable. Foxes, rats and coyotes are extremely susceptible; cattle, rabbits, and cats are highly susceptible; dogs, sheep and goats are moderately susceptible; and opossums little, if at all (1). The virus is almost always spread by saliva from the bite of an infected animal. In addition, but more rarely, rabies can be spread when virus from saliva is introduced into open cuts or wounds or comes in contact with a mucous membrane in areas such as the mouth, nasal cavity, or eyes (1).
The disease persists in wildlife populations throughout the United States, Canada, Mexico, and other parts of the world. Wildlife infected with rabies may show no fear of man, be aggressive, or be incoordinated. Animals that are normally nocturnal may be active during the day. In the past 50 years, less than 10 humans have contracted rabies in Canada. Five cases of rabies in humans were reported in the United States and Canada during September and October 2000. The most recent Canadian death from rabies was a 9-year old Quebec boy who died in October 2000. He was suspected of being bitten by a bat (2). Rabies costs governments and the people of North America millions of dollars each year for: diagnosis, investigation of animal bites, treatment of humans in contact with rabid animals, compensation for loss of livestock, cost of quarantine, research, and vaccination. A single course of post-exposure treatment in the USA costs $1600 - 2800 US. Ontario has been the hotbed of rabies in North America. Figure 1 shows the number of rabies cases over the last 12 years (3).
Number of Cases of Rabies in Ontario for the Years
Since the rabies baiting program began in 1989, rabies has dropped to its lowest level in Ontario since 1961. During the 1980's, Ontario averaged 2,000 cases of rabies per year. By the end of 1998, there were only 78 cases of rabies; including 34 cases in skunks; 29 in bats; 9 in cattle, and only four in foxes (3). This dramatic change over the last 8 or 9 years has greatly reduced the risk of humans, pets and livestock acquiring rabies, but the risk is not zero.
The number of rabid horses in Ontario was zero from 1997 until the year 2000, when three horses were diagnosed with rabies. The horses resided in Bruce, Grey and Huron counties. One case of horse rabies occurred in Dufferin County earlier this year (2001). Table 1 shows the cases of rabies in horses from 1988 to date (3).
Table 1: Number of Cases of Rabies in Horses from 1988 to Date.
In a United States study, the average incubation period for a horse infected with rabies was 12.3 days and the average time period from first signs to death was 5.5 days. Those animals which had no previous vaccination history had significantly shorter incubation periods and died sooner. Muzzle tremors were the most frequently observed (81%) and most common initial sign. Other common signs included: difficulty swallowing (71%), incoordination or paralysis (71%), weakness or drowsiness (71%). The "furious form" was noted in 43% of rabid horses and clinical signs in some of these animals initially appeared as the "dumb form". The paralytic form was not observed (4). Horses which develop the "furious form" show excitement, become vicious, bite, kick, exhibit blind staggers, suddenly fall and may chew themselves or foreign objects (1).
There are three different strains of rabies in North America which are differentiated by their carrier species. The three strains include Arctic fox, raccoon and bat rabies. There are no differences between the strains as far as their ability to kill their carrier species, other animals and possibly humans.
The Arctic fox strain of rabies came from the Arctic and moved down the Quebec side of Hudson and James Bays in the 1950's. There was a peak in the number of cases in the Cochrane area in 1955. No new cases occurred in northeastern Ontario until 1991 and 1992. Rabies moved south and has been a continuous problem in Southern Ontario since 1959. Southern Ontario has reported more cases of animal rabies than any other province or state in North America (1,3). The counties of Grey, Wellington, Simcoe, Bruce, Middlesex, Lambton, and Waterloo, in descending order, have been the traditional hot spots for rabies over the last 12 years. The great news is that the wildlife vaccination program, administered by the Ontario Ministry of Natural Resources (OMNR), has virtually eliminated the Arctic fox strain. Skunks however, remain as a reservoir of this strain.
The newest hot spot is northeastern Ontario, where the disease has not been seen since the early 1950's. Since 1990, sporadic outbreaks of rabies have occurred in the farming communities of northeastern Ontario. In the fall of 2000, the District of Cochrane was a hotbed of rabies with 35 cases reported (3,5).
The newest threat to Ontario comes from the raccoon variant strain of rabies. Raccoon rabies was first noted in Florida in the 1940's. Since then, it has been travelling slowly northward toward Ontario. It is well-established on the New York side of the Niagara River and the first case was reported in Ontario, in an area north of Brockville, in July 1999. To date, 49 cases have been reported in the counties of Leeds, Grenville, Frontenac and one case in each of Renfrew and Peterborough counties (3).
Bats carry their own form of rabies. Since bats in Ontario are small and insect-eating, they are not affected by the rabies vaccination program. In southern Ontario, normally 15-45 rabid bats are confirmed each year (6).
In 1989, the Ministry of Natural Resources Rabies Unit, in conjunction with scientists from Connaught Laboratories, Queen's and McMaster Universities and the Universities of Toronto and Guelph, attacked rabies at its source, the red fox. Research has centered on immunizing wild foxes. Immunization of a large percentage of the "carrier species" population that spreads rabies may cause the disease to die out completely (3).
A rabies baiting program has been implemented. Current baits contain 1.8 millilitres of rabies vaccine sealed in a small plastic blister-pack embedded in a bait (a mixture of fats, wax and flavouring). The rabies vaccine is a living virus and, therefore, quite delicate. It has to be absorbed in the mouth. If it were released in the stomach, it would be destroyed.
Three Twin Otter aircraft are outfitted with special bait-dispensing machines. Navigation is done using the latest satellite and computer technology so that millions of baits are spread uniformly and at a predetermined density. The number of baits delivered is computer controlled and carefully recorded. This helps ensure a minimal number of baits are wasted. Since the early 1990's, OMNR has air-dropped one to two million vaccine-baits over Southern Ontario. A similar program has been launched against raccoon rabies. There is no bait that will immunize skunks reliably. The control of rabies in bats, which are insectivores, presents a greater challenge.
There is no effective treatment for rabies. Once clinical signs appear, rabies is fatal. Therefore, it is best to prevent exposure to the virus. If an animal is bitten or comes in contact with a suspected rabies case, contact a veterinarian or the Canadian Food Inspection Agency (CFIA) immediately. If a person is bitten by an animal, wash the wound or exposed surface with soap and water, remove clothing that may be contaminated, contact your doctor immediately or go to the nearest hospital emergency.
Vaccinate your horses, all pets and livestock, especially any free-roaming cats which live in the barn. Vaccinating against rabies is good insurance and an inexpensive way of protecting your livestock and pets, as well as yourself, from the disease.
Enjoy wild animals (raccoons, skunks, foxes) from afar. Do not handle, feed, or unintentionally attract wild animals. Secure garbage cans and their lids. Do not approach animals showing abnormal behaviour. Never adopt wild animals or bring them into your home and do not try to nurse sick animals to health. Teach children never to approach and handle unfamiliar animals, wild or domestic, even if they appear friendly (7).
Manufacturers of the two rabies vaccines marketed in Canada, Rabvac 3® (Ayerst) and RM Imrab 3® (Merial), recommend that horses receive a single 2 ml dose of vaccine starting at three months of age and repeated at one year of age, followed by annual revaccination. Merial recommends that RM Imrab 3® be given by either intramuscular or subcutaneous injection. Ayerst recommends that Rabvac 3® be administered by intramuscular injection only. Neither vaccine has a caution for use in pregnant mares (8). Rabies vaccine can only be sold to a licensed veterinarian.
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