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Ontario, and 1 horse from Cambridge, Ontario, devel- oped clinical neurological disease suggestive of eastern equine encephalitis (EEE). None of the horsesĀ ...
CORRECTION Eastern equine encephalitis in horses in Ontario in 1994 Can Vet J 1995; 36: 1 74 A typographical error in the March edition of Cross-Canada Disease Report left the incorrect impression that several of the horses had raced in Syracuse, New York, 1 wk before becoming ill. In fact, only the surviving horse from Cambridge, Ontario, took part in the race. A corrected version of the article appears below. Apologies are extended to the authors and CVJ readers.

Ontario Eastern equine encephalitis in horses in Ontario in 1994 August and September 1994, 7 horses from Muskoka, Ontario, and 1 horse from Cambridge, Ontario, developed clinical neurological disease suggestive of eastern equine encephalitis (EEE). None of the horses had been vaccinated for EEE. The horse from Cambridge survived, but the 7 horses from Muskoka died. The brains of 2 dead horses from different premises were negative for rabies, but they had nonsuppurative meningoencephalitis characteristic of EEE. Both horses had perivascular cuffs, predominantly lymphocytes with a few neutrophils, in both white and gray matter, and neuronal necrosis. Formalin-fixed, paraffin-embedded tissue from both brains was positive for EEE viral antigen in indirect immunoperoxidase tests (Histo Stain SP Kit, Zymed Laboratories, San Francisco, California, USA), using polyclonal murine antisera monospecific for EEE. Sera from both dead horses and from the surviving horse contained hemagglutination-inhibiting antibodies (1) to EEE, but not to western equine encephalitis, Highland J, or Powassan viruses. The surviving horse from Cambridge had significant serum antibodies to EEE virus in acute and convalescent sera. This horse had raced in Syracuse, New York, 1 wk before becoming ill. Foci of endemic EEE, bird-mosquito cycles occur near Syracuse, New York; in southwest Michigan; in northeast Indiana; along the Atlantic seaboard; and in the Gulf Coast states (2). Recent outbreaks of EEE in horses have occurred in east-central Ohio in 1991 (3) and in Michigan in 1991, 1993, and 1994 (Dr. E. Walker, Michigan State University, personal communication). Eastern equine encephalitis has occurred only infrequently in horses in Ontario. In 1938, horses from St. George and St. Catharines were affected with EEE. It was not recognized again in horses in Ontario until 1992, when a Belgian mare, which had been pastured near Woodstock adjacent to a hardwood swamp with In

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permanent fresh water, died with meningoencephalitis

characteristic of EEE. The virus was isolated from the brain, and 1 of 9 healthy in-contact horses had antibody to EEE virus (1). Although the EEE virus likely continues to exist in Ontario in migratory bird populations, spillover into the equine population has been rare, with a very low prevalence of disease. The circumstances that precipitated the current epizootic in Ontario are unknown. It is difficult to predict if a similar epizootic will occur this year, for swamplands do not necessarily have infected mosquitoes in ensuing years. Outbreaks in successive years occurred in Michigan in 1993 and 1994. Effective vaccines are available to protect high-risk horses. Horses traveling to endemic areas in the United States or pastured adjacent to permanent hardwood swamps are potentially at risk for predation by mosquitoes infected with EEE virus.

References 1. Carman PS, Artsob, A, Emery S, et al. Eastern equine encephalitis in a horse from southwestern Ontario. Can Vet J 1995; 36: 170-172. 2. Francy DB, Wagner BA. Equine encephalomyelitis in the United States, 1980-1990. Use of weather variables to develop a predictive model for equine cases. Anim Health Insight 1992; (Spring): 1-8. 3. Nasci RS, Berry RL, Restifo RA, Parsons MA, Smith GC, Martin DA. Eastern equine encephalitis virus in Ohio during 1991. J Med Entomol 1993; 30: 217-222.

Susy Carman, Murray Hazlett, Ron Wilson, Tony Van Dreumel, Gary Thomson, Ontario Ministry of Agriculture, Food and Rural Affairs, Veterinary Laboratory Services Branch, Box 3612, Guelph, Ontario NiH 6R8; Tom Mullaney, Michigan State University, Animal Health Diagnostic Laboratory, Box 30076, Lansing, Michigan 48909-7576, USA; and Mohamed Sabet Mahdy, Vector-borne and Special Pathogens Unit, Laboratory Services Branch, Ontario Ministry ofHealth, Etobicoke, Ontario M9P 3TI

Can Vet J Volume 36, May 1995