While there have been seven confirmed cases of Eastern Equine Encephalitis (EEE) to date (Sept 2) within the counties of Stormont, Dundas, Glengarry, Frontenac and Lanark, additional suspicious deaths from progressive neurological diseases are being reported by veterinarians in those regions.
The problem with confirming reports of EEE is that euthanized horses are not routinely tested for the disease unless the owner requests it, but after a few cases veterinarians are able to identify a typical presentation. Few neurological diseases cause such a quick demise.
“This has been a troubling year for EEE,” said OMAFRA Lead Veterinarian in Animal Health and Welfare veterinarian, Alison Moore. “There is some speculation that the wet weather in combination with a relatively warmer than normal summer for the eastern part of the province has led to the proliferation of the mosquito vector,” she told Ontario Farmer. “That in association with a naive horse population has likely led to the devastating results.”
EEE presents with neurological signs such as circling, head-pressing, ataxia and depression, which are similar to several other neurological diseases such as rabies, West Nile, botulism, hepatic encephalopathy, equine protozoal myeloencephalitis (EPM) and equine herpes myeloencephalopathy. EEE can affect any horse and is invariably fatal in unvaccinated horses.
The virus is normally only found near wooded swamps, transmitted between birds by a certain species of mosquitos known as the “black swamp mosquito”, Culiseta melanura, that feeds on songbirds in that habitat, and to other invertebrate hosts by the bridge vector “salt-and-pepper mosquito”, Coquillettida pertubans, that thrives in cattail and grassy marshes with a mucky bottom. The mosquito flies no further than five miles from its breeding sites, meaning that most cases of EEE will happen within five miles of such swamps.
Horses and humans are ‘dead end’ hosts, meaning that sick horses or humans cannot transmit the disease back to mosquitos.
The virus has proliferated in the eastern part of the province, where flooded woodlands provide breeding grounds for mosquito vectors. Moore has not heard of any cases yet from her Quebec colleagues, but she suggests that anyone travelling to the eastern regions of Ontario consider vaccination and using mosquito repellants for themselves and their horses. As she pointed out, “most showgrounds are well-groomed and relatively dry so again the risk is much lower than for horses who live near flooded forests”.
A vaccine is available but as Moore explained, previously unvaccinated horses need to receive a booster three weeks after the initial vaccine in order to mount an effective immune response. If a horse is exposed shortly after the initial dose, they will not be protected. Even properly vaccinated horses can become infected but they show much milder symptoms such as lethargy, depression and mild loss of control of body movements.
“This would not deter me from vaccinating,” said Moore, “however, one must be aware that vaccination will not be protective if the horse is infected around the same time or shortly thereafter being given the vaccine.”
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