Horse Disease Surveillance - Spring 2002

Reportable Diseases in Canada

These diseases are named in the Federal Reportable Diseases regulations made under the Health of Animals Act and are the mandate of the Canadian Food Inspection Agency.

No cases of contagious equine metritis, dourine, piroplasmosis (babesiosis), or glanders have been reported.

Equine Infectious Anemia (EIA)

In Ontario, only one case of EIA was detected in 1997 and 1998. A single case of EIA was identified in the Hamilton area in December of 2000. The main focus of infection in Canada in the past few years has been in Saskatchewan, Alberta, and British Columbia.

Rabies

Two strains of rabies are being monitored in Ontario. The Arctic Fox strain has virtually been eliminated due to the wildlife vaccination program administered by the Ontario Ministry of Natural Resources. Skunks remain as a reservoir of the virus. The newest threat to humans and livestock comes from the occurrence of raccoon rabies in the Brockville area.

There were no cases of rabies in Ontario horses during 1997, 1998 or 1999. In 2000, 3 cases of rabies occurred in horses in western Ontario. In 2001, 5 cases (3 in western Ontario, 1 in central Ontario, 1 in northern Ontario) were diagnosed. To March 31, 2002, no cases of rabies in horses have occurred this year. Table 1 indicates the total number of cases of rabies in all species and for all regions of Ontario.

Non Reportable Diseases in Ontario

Epizootic lymphangitis of horses, horse pox, Japanese encephalitis of horses, and Venezuelan equine encephalitis have not been known to occur in Canada.

Botulism

The weather at the beginning of the 2001 haying season was again a little unpredictable for farmers. A number of horse owners purchased or produced round bale silage. Unfortunately, again, a number of outbreaks of botulism were traced back to the feeding of round bale silage. While the quality of forage in round bale silage can be very high, the presence of botulinum toxin is still unpredictable and, therefore, caution is needed when using silage for horses. Owners contemplating using silage should refer to the information sheet Botulism in Horses and Haylage, and talk to their veterinarian about vaccinating their horses for botulism.

Clovers, Diarrhea and Photosensitization

During 2001, two cases of photosensitization were investigated. One was associated with the consumption of hay containing alsike clover in levels above 20% of the hay content. Owners are reminded that the consumption of alsike clover can cause photosensitization and big liver syndrome in horses. The photosensitivity is usually the first sign of toxicity. However, horses with minimal exposure to sunlight exhibited diarrhea and colic as the primary signs of toxicity. The second case of severe photosensitization occurred in September- October 2001 in a group of horses consuming primarily white clover pasture. The pasture was heavily fertilized with chicken manure. The clover was 26% protein on a dry matter basis. A similar condition (consisting of photosensitization and mildly elevated liver enzymes) to alsike clover poisoning was observed. Horse owners who wish more information on alsike clover poisoning or who wish to differentiate between the different clovers should refer to the information sheet Alsike Clover Poisoning, Photosensitization or Photodermatitis in Horses.

Eastern Equine Encephalitis (EEE)

From 1938 until the fall of 1992, there were no cases of EEE diagnosed in Ontario. In 1992, a single case of EEE was diagnosed. Since 1992, there have been three sporadic EEE infections in Ontario. In the fall of 1994, EEE was diagnosed in two horses in the Bracebridge area. Five others died in the 1994 outbreak but were not tested. A single case was identified in the Orillia area in 1997. In the fall of 2001, two horses with clinical signs were confirmed as infected. One was a resident horse of the South Muskoka area. The other had been in competition in the same area, as well as many other locations throughout Ontario.

Methicillin Resistant Staphylococcus Aureus (MRSA)

Recently, a methicillin resistant strain of Staphylococcus aureus (MRSA) was isolated from a foal. Staphylococcus aureus (staph) is a bacterium commonly found on the skin and in the nose of healthy people and animals. Occasionally, it can get into the body and cause infections ranging from minor (pimples, boils, other skin conditions) to serious (blood infections, pneumonia). Methicillin resistant strains of S. aureus are no more pathogenic than other strains. They are of concern because they are difficult to treat should they occur in a hospital setting where there are a large number of sick or immunocompromised animals that are more prone to become colonized and potentially develop disease. Some MRSA strains are resistant to a number of antibiotics. MRSA is sporadically isolated from horses in Ontario. People and horses can be colonized with MRSA and not develop disease. Disease typically only occurs in compromised individuals. MRSA infection is spread via direct contact. Routine infection control measures should decrease the chance of spreading the infection. Hand washing is the most important method of preventing the spread of the bacterium.

Strangles

Strangles is a highly contagious and serious infection of horses and other equids, caused by the bacterium, Streptococcus equi. The disease is characterized by severe inflammation of the mucosa of the head and throat, with extensive swelling and often rupture of the lymph nodes, which produces large amounts of thick, creamy pus. The lymph nodes under the mandible (submandibular) and those behind the jaw (retropharyngeal) are the most commonly affected. The organism can be isolated from the nose or lymph nodes of affected animals and can be found in the guttural pouch of carrier animals.

Strangles has been reported from a number of locations across Ontario and North America this spring. Owners are reminded that strangles is highly contagious and can be transmitted from horse to horse directly, by the sharing of infected equipment between horses and on handlers clothing and hands. Flies can also transmit the disease in the warmer periods of the year. It is essential that horse handlers wash their hands or use disposable gloves between horses when an infected horse is identified. This requires that properly equipped wash stations be located throughout the horse stabling area. The other important aspects of control include: monitoring rectal temperatures, isolation of infected horses to a different barn, biosecurity within farms and between the home farm and training/show facilities. Your veterinarian can provide advice on the need to vaccinate against strangles. For more information on strangles and biosecurity to prevent the introduction of strangles to your farm view the Health Management section under Horses.

West Nile Virus

There were more than 700 equine clinical cases due to infection with West Nile Virus in the U.S. last year, mostly in Florida, Georgia and along the eastern seaboard. Approximately 30% of reported clinical cases died or were euthanised. 2001 saw the spread of West Nile Virus (WNV) southward and westward in the United States, and into Ontario for the first time. The virus was found in 128 dead birds and nine mosquito pools across southern Ontario, from Durham Region to Windsor-Essex. WNV has also been discovered in overwintering mosquitoes in the Windsor area. This might indicate that it has or is becoming established in this province.

Although most infected mammals do not become ill, a few individuals of certain species, especially horses, may develop encephalitis due to WNV. Clinical signs will resemble other encephalitides, but in the early stages may be clinically indistinguishable from rabies. In addition to the general precautions to prevent exposure to arboviruses (mosquito-borne viruses), a killed vaccine is now available. The Canadian Food Inspection Agency has given conditional approval for the importation and sale of the vaccine produced by Fort Dodge Animal Health. It is available in Canada upon request through Ayerst Laboratories in Guelph. For further information about this vaccine, ask your veterinarian or call Ayerst at 1-800-265-7200.

Related Links

• Alsike Clover Poisoning, Photosensitization or Photodermatitis in Horses
• Botulism in Horses and Haylage
• Centers for Disease Control and Prevention
• Equine Viral Encephalitis
• Public Health Units
• Public Health Agency of Canada
• Canadian Food Inspection Agency

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