Tent caterpillars possibly linked to rare heart condition in horses

A new study from the University of Saskatchewan has identified a possible link between forest tent caterpillars and deaths in several horses.

The case series of four horses was published in the July 2020 issue of the Canadian Veterinary Journal. All four horses were presented to the Western College of Veterinary Medicine’s Large Animal Clinic in Saskatoon with signs of heart failure. Their clinical signs included fever, increased heart rate, weight loss, rapid breathing, edema along the underside of their body, lethargy and diarrhea.

Further tests included blood work and ultrasound examinations. This led to a diagnosis of pericarditis, which is the medical term for inflammation in the sac that surrounds the heart. This inflammation stopped the heart from expanding properly, leading to heart failure and the noted clinical signs.

Readers may be more familiar with pericarditis as a condition of cattle. Known as hardware disease, it occurs when cattle eat metal objects such as wire and these poke through the reticulum (first stomach chamber) and into the sac around the heart with disastrous consequences.

Three of the horses were euthanized following the diagnosis, while treatment was pursued for the fourth. This horse had an astounding 16 litres of fluid drained from its pericardial sac and survived the intensive treatment.

However, less than two weeks after discharge from the hospital, it developed heart failure again and was then euthanized. The vets determined that fibrin was the culprit. Fibrin is a sticky yellow material that helps blood clot, among other things. The yellow fibrin was released into the pericardium during inflammation and it subsequently hardened into scar tissue that prevented the heart from expanding properly, leading ultimately to heart failure.

All four horses underwent a complete autopsy examination at Prairie Diagnostic Services. This confirmed the pericarditis as a cause of illness.

Bacteria were present in the pericardium and other tissues, confirming the presence of a bacterial infection. Most surprising of all, microscopic examination of gut tissues found foreign material that had a similar appearance to caterpillar parts. It is possible that the gut damage allowed the bacteria to enter the body and travel via the bloodstream to infect the pericardial sac around the heart.

The authors note that pericarditis is a rare condition in horses, so it was quite unusual to have four cases in one summer. The timing also coincided with a severe infestation of forest tent caterpillars and all four horses were kept on pasture with these tent caterpillars present. The study checked records of horses admitted to the hospital with heart failure between 1996 and 2018. There were only 35 horses with heart failure during this time. Of these, 17 had a final diagnosis and none had pericarditis (besides the four cases in the study). It is possible many more horses were affected during the summer of 2017, but these horses were probably euthanized on farm and did not receive an autopsy.

The association between horse disease and caterpillars is not without precedent. In the eastern United States, including the state of Kentucky, there is an unusual form of abortion in mares associated with consumption of the eastern tent caterpillar. Some of these horses also develop pericarditis.

Similarly, processionary caterpillars in Australia are associated with abortion. The study does not describe abortions in Saskatchewan horses but it may have been unrecognized or it is possible that differences in caterpillar species and horse breeding season may be a factor.

The authors conclude that the relationship between the pericarditis condition and the tent caterpillars is not proven definitively. But they build a strong case for an association in a remarkable bit of veterinary detective work. All horses had exposure to tent caterpillars in a year with bad pasture infestations and all horses had a similar condition that is otherwise exceedingly rare in the horse population. The recommendation is to limit horses’ exposure to infested pastures if another bad tent caterpillar spring should occur.

Research out of the United States associated with the tent caterpillar-abortion connection suggests that grazing horses with cattle may also be protective, possibly due to their differing grazing habits.

Dr. Jamie Rothenburger, DVM, MVetSc,PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger

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