Horse Disease Surveillance Report, Fall 1999

Reportable Diseases in Canada

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

Rabies:

There were no cases of rabies in Ontario horses during 1997, 1998 or to August, 1999. The wildlife vaccination program of the Ontario Ministry of Natural Resources has virtually eliminated rabies in foxes. Skunks remain as a reservoir of the virus. The newest threat comes from the occurrence of three cases of raccoon rabies in the Brockville area. An extensive control effort, by Ontario Ministry of Natural Resources, has been launched to prevent the spread of raccoon rabies from what is thought to be a point source.

Equine Infectious Anemia (EIA):

In 1997, 111 cases of EIA were detected from a total of 68,347 samples.
In 1998, 96 cases of EIA were detected from a total of 66,177 samples. Only one case of EIA was detected in Ontario in 1997 and 1998 and no cases of EIA have been detected in Ontario up until August, 1999.
The main focus of infection in the past few years has been in Saskatchewan, Alberta, and British Columbia.

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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.

Eastern Equine Encephalitis (EEE):

The EEE virus is spread by a mosquito that normally feeds on birds. Humans and horses are infected during periods of high mosquito populations. EEE virus has a range from southeastern Canada to the south eastern United States as well as the Carribean and South and Central America. Eighty to ninety percent of infected horses develop acute and lethal disease with survivors developing neurological signs. From 1938 until the fall of 1992, there were no cases of EEE diagnosed in Ontario. 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. No cases have been detected to date in 1999.

Equine Viral Arteritis:

There have been no reports that the Ontario EVA outbreak, which affected horses in one barn at a Standardbred track and at two training facilities, has spread to other locations. The prevalence of randomly selected control Standardbred horses with antibodies to EVA was 26%. This prevalence rate is lower than other studies have suggested (70-80%). The difference may be in the sampling technique. The horses that we sampled as controls were randomly selected. Other studies reported the number of horses that were positive for EVA antibodies from horses where there was suspicion of the disease. The other explanation might be that the Ontario Standardbred horse population is currently at a low point in the disease cycle and the majority of horses have not yet been exposed to the virus.

Leptospirosis and Equine Recurrent Uveitis:

One farm in eastern Ontario had four of six horses affected with equine recurrent uveitis. Practitioners periodically see a single horse with equine recurrent uveitis but it is unusual to see uveitis affecting a group of horses. The disease is also known as periodic ophthalmia or moon blindness. The differential diagnoses for corneal disease and uveitis include traumatic injury, Onchocerca cervicalis (a parasite), face fly irritation, and leptospira-induced uveitis. Shapiro and Stevenson recently reported equine abortions and recurrent uveitis associated with leptospirosis infections in horses in Eastern Ontario.

Researchers believe recurrent uveitis to be an immune-mediated reaction and a sequella to a Leptospira pomona infection which occurred months or years previously (1). Clinical signs will often disappear followed by recurrence. The horse presents with signs of a very painful eye - severe blepharospasm, photophobia and lacrimation.

Titres to L. pomona and bratislava in the recent cases of recurrent uveitis in Eastern Ontario exceeded a 1:1500 dilution. In one case, the titre for L. pomona was 1:20,480.

Frequently reported risk factors include flooding of pastures or drinking from contaminated water sources. In addition, the Appaloosa breed seems to have a greater predilection for uveitis and for severity of vision loss associated with the disease. This may be related to specific blood lines within the breed.

Presently, manufacturers have no leptospira vaccines approved for use in horses. Dr. Ann Dwyer of New York State suggests that vaccines for cattle were not useful for horses and may be detrimental. Horses should be restricted from drinking water from water courses such as municipal drains and ponds since these could easily become contaminated by wildlife.

Ulcerative Stomatitis:

In the January 1998 minutes of the Report of the Veterinary Subcommittee of the International Breeders Meeting in South Africa, delegates from New Zealand and Argentina reported that they had experienced a condition of erosive stomatitis. Investigators of vesicular stomatitis in the USA have also indicated that cases of stomatitis of unknown origin have been seen in vesicular stomatitis negative animals.

In Ontario, a case of ulcerative stomatitis of unknown origin was investigated in a group of horses. The horses were EIA negative, vesicular stomatitis negative, but had seroconversions to influenza AE2. It is unknown whether there is a relationship between the oral lesions and the four fold change in titre to influenza AE2.

In Argentina, investigative work has linked the condition to bedding. The wood shavings were made from maruba, a tree in the Simaroubaceae family. Exposed horses developed erosive stomatitis and died as a result of contact with this material (2).

The Simaroubaceae family is largely tropical and semitropical trees. Maruba (Simarouba amara) is a tropical hardwood tree from South America. One plant from that family, the "tree-of-heaven", Ailanthus altissima, is grown as an ornamental tree in the warmer parts of Canada. It, apparently, can cause dermatitis in humans and is also suspected of causing gastroenteritis.

To avoid similar problems, horse owners should know the source of all bedding materials and ask about the tree (wood) species included in the shavings before using the product.

Strangles:

This disease is common to horses throughout the world. The bacteria can be found in the guttural pouch of carrier animals.

White Clover and Colic:

Over the course of the spring, in a group of 15 brood mares, one mare had colic surgery, one mare was euthanised after treatment for colic and two others had gaseous colic. The mares and foals are housed outside 10-12 hours of the day on a pasture consisting of 80% white clover (ladino and white Dutch). The pasture also contains orchard, brome and some fescue grasses. The pasture was in excellent condition and was greatly under-utilized. The clover was in the flowering stage and was 8-10 inches high and lush. The mares are brought into the barn and given hay and grain. Each mare was fed 6 lbs. of western oats and 5 lbs. of a 16% sweet feed with vitamins added.

The cause of the colics was probably due to a very high carbohydrate intake coupled with a low fibre content. The high carbohydrate intake was coming from the clover in the pasture and the high grain intake. The mares were selecting the highly palatable clovers from the pastures and generally leaving the grasses. Ladino and other white clovers have 1/3 less fibre content than other roughage such as brome grass and alfalfa. The white clover, at this lush stage of growth, would be 22-25% protein on a dry matter basis. The digestible energy content would be equally high, especially when grazed from noon until 4:00 p.m.

Horses on pasture 24 hours a day can consume 3.3 % of their body weight in dry matter. The horses were receiving 10 -12 hours of pasture per day plus hay and 11 lbs. of grain/sweet feed while in the barn. From this feeding regime, the mares were estimated to be consuming approximately twice their protein and energy requirements.

Research indicates that horses consuming grain, at levels greater than 10 lbs. per horse per day, have a 4-times greater risk of colic than those that do not. Therefore, the high grain feeding, plus the high carbohydrate level from the pasture, could explain the high incidence of colics.

The high clover level in the pasture was probably due to; poor establishment of the grass species in last year's drought conditions; the level of 10% clover seed in the pasture mix, coupled with a high seeding rate of 25 lbs. to the acre. These factors would result in a predominance of clover in the pasture.

References

1. Rebhun WC. Diagnosis and Treatment of Equine Uveitis. JAVMA 1979; 175 (8): 803-808.
2. Outbreak of Erosive Stomatitis. Equine Disease Quarterly 1999; 7(4):2-3.
   

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