All four were diagnosed with congestive heart failure from septic fibrinous pericarditis. Dr. Ronan Chapuis and his research team linked the uncommon cardiac disease to the ingestion of forest tent caterpillars. The owners of all four horses noted that the pastures in which the horses grazed were infested with forest tent caterpillars in 2016 and 2017. 

In 2001 and 2002, veterinarians in the United States determined Eastern tent caterpillars — closely related to forest tent caterpillars — were responsible for mare reproductive loss syndrome (MRLS) in multiple states, including Kentucky and Ohio. MRLS causes abortions in mares, though some of the affected mares also experienced fibrinous pericarditis.

Three of the four horses referred to the WCVM clinic were euthanized. One mare underwent treatment, which involved inserting a needle to draw off the fluid in the pericardium. The vets drained over four gallons of fluid during the procedure. Intravenous fluids, anti-inflammatories, and antibiotics were also administered and the horse was eventually sent home. 

The mare relapsed 11 days after discharge and was euthanized. A necropsy showed that a thick, fibrinous material had formed on the pericardium's inner layer, which likely caused heart disease. When necropsied, all four horses had fibrin in their pericardium.  

The research team notes that these four cases don't allow for a definitive connection between forest tent caterpillars and pericarditis, but they recommend horse owners and caretakers be aware of the risk and work to minimize horse's ingestion of these insects. 

Read more at the Western College of Veterinary Medicine.

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