An Eastern Tent Caterpillar, typical of the eastern USA; this is the species linked to 2002’s Mare Reproductive Loss Syndrome (MRLS) problem on Kentucky horse farms.

Researchers from The University of Queensland have found hairy caterpillars are responsible for causing abortions in Australian mares. Dr. Judy Cawdell-Smith and Professor Wayne Bryden, from UQ’s School of Animal Studies, found that mares exposed to caterpillars were likely to miscarry.

“This is an unusual form of abortion that was first reported in Australia in 2004 and is similar to a condition reported in Kentucky in 2002,” Dr. Cawdell-Smith said. “Researchers in Kentucky identified Eastern Tent Caterpillars as the cause of the US equine condition, (which came to be known as) Mare Reproductive Loss Syndrome.”

Similar equine abortion cases were reported in Australia’s Hunter Valley in New South Wales in 2004. The Hunter Valley is Australia’s largest Thoroughbred breeding area.

“Studies conducted by veterinary epidemiologist, Professor Nigel Perkins, suggested the abortions were caused by caterpillars or poisonous plants,” Professor Bryden said. “No poisonous plants were found on any of the stud farms where mares aborted. Caterpillars were identified as the cause of the US problem but the same caterpillars don’t exist in Australia. However, other related caterpillars were found on the affected Australian stud farms.

“If you’ve ever seen a hairy caterpillar, it is unlikely that a horse would eat a whole one,” he continued. “What’s more likely is that the caterpillar’s exoskeleton – which is much harder to see in the grass – is picked up by the horse while it is grazing. In our studies, both whole caterpillars and exoskeleton caused mares to abort.”

The researchers believe ingestion of the caterpillar changes the permeability of the intestinal wall, allowing bacteria to pass into the horse’s circulation and through the placenta.

“The subsequent infection caused by the bacteria in the fetus results in abortion,” Dr. Cawdell-Smith said. “These bacteria are found in the intestine of mares and normally don’t cause a problem. Interestingly, mares that abort have no ill effects or evidence of illness.”

Thanks to the University of Queensland for this report.