Potomac Horse Fever Confirmed in Ontario

Potomac Horse Fever Confirmed in Ontario

PHF is caused by Neorickettsia risticii, an organism found in some flukes (a wormlike parasite) that infect aquatic snails and insects (such as caddisflies, mayflies, damselflies, and dragonflies).

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The Ontario Ministry of Agriculture and Food (OMAF) has announced that two Ontario, Canada, horses tested positive for Potomac horse fever (PHF) in early August.

"Samples collected from an anorexic and febrile horse in Essex County confirmed that the horse was suffering from PHF; the horse subsequently died," the department said in a veterinary update. "A horse in the Ottawa area also tested positive for PHF. It was exhibiting profuse watery diarrhea but is reported to be recovering."

Neither horse was vaccinated against the disease, the OMAF alert said.

PHF is caused by Neorickettsia risticii, an organism found in some flukes (a wormlike parasite) that infect aquatic snails and insects (such as caddisflies, mayflies, damselflies, and dragonflies).

The incubation period is 10 to 18 days. Initial clinical signs include anorexia, lethargy, and fever, followed by enterocolitis, dehydration, and diarrhea. This could progress to toxic shock, laminitis, or death (30% mortality rate). The disease can cause abortion in pregnant mares, and endotoxemia, characterized by a bright red or purple line (“toxic line”) on the gums, in any affected horses.

"The Essex County horse had a toxic line," the alert stated.

The alert indicated that PHF treatment methods include intravenous oxytetracycline and, in horses with signs of enterocolitis, intravenous fluids and anti-inflammatory agents.

Early treatment increases the likelihood of survival, and veterinarians in endemic areas often initiate antibiotic treatment prior to diagnosis if a horse is showing signs consistent with the disease in late summer or fall," the alert stated.

The OMAF recommended the following steps to aid in disease control:

  • Vaccination could decrease clinical signs but it will not prevent infection and illness. It protects against only one strain of the more than 14 identified strains of N. risticii.
  • Cover feed supplies to prevent insect contamination and check water buckets and troughs for dead insects; and
  • In endemic areas, turn off outside lights near stables and pastures during summer months to prevent attracting insects.

The OMAF recommends veterinarians in Ontario include PHF in their list of rule-outs when examining horses during the summer and fall that are anorexic and febrile.

"Laboratory samples should include EDTA blood (i.e., blood collected in a tube containing a chelating agent and anticoagulant) from live animals, or lung, liver, and spleen from necropsy," the organization indicated. "Serology is of limited value because of the high prevalence of false-positive titers."

About the Author

Erica Larson, News Editor

Erica Larson, news editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in eventing with her OTTB, Dorado.

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