Creepy-Crawlies Invade Kentucky Horse Farms: Are Pregnant Mares At Risk Again?

A sure sign of spring: Eastern tent caterpillars are hatching in Kentucky. (South Dakota forestry image)

What on earth could possess them? Researchers tell us today that horses in Kentucky pastures actually do eat eastern tent caterpillars, properly known as Malacosoma americanum (Fabricius). You know them by the damage they do; these little caterpillars spin thick webs on tree limbs...and then munch their way to metamorphosis on emerging young leaves. Once hatched, they fly away, leaving a denuded tree limb behind. But for pregnant mares, they could pose a much greater risk.



Experts at the University of Kentucky today reported that eastern tent caterpillars have begun hatching in central Kentucky and that their population numbers are trending up.

It seems like only yesterday that the horse industry in central Kentucky was devastated by an event known as Mare Reproductive Loss Syndrome (MRLS). The 2001-2002 event caused the loss of an estimated 30 percent of that year's Thoroughbred foal crop, with serious losses suffered by mares of all breeds of horses. After several false tries, the finger of guilt was finally pointed at the caterpillars, which were especially populous that year.

University of Kentucky College of Agriculture entomologist Lee Townsend will be closely monitoring caterpillar development over the next two to three weeks. He and his colleagues anticipate full-grown larvae by the third week of April. From the end of April to the beginning of May, caterpillars will likely leave the trees where they’ve eaten the available foliage and search for additional food to complete their development.

Once the caterpillars have reached these dispersing stages, controlling them becomes much more difficult, Townsend said. If needed, control should target caterpillars while they are gathered together in the trees. Apparently they love ornamental cherry trees, the bright pink-purple blossoms of which are such an exclamation point on spring landscapes.

However, Kentucky's Townsend cautions against spraying too early. That won't work, either. Obviously, timing is everything.

Studies since the 2001-2002 MRLS outbreak revealed that horses inadvertently will eat the caterpillars in the grass. When they do, the caterpillar hairs embed into the protective lining of the alimentary tract. Once that barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death or weak foals from these roaming alimentary tract bacteria are hallmarks of MRLS.

UK entomologists recommend that unless horse farm managers have been aggressive in managing eastern tent caterpillars, or removing host trees, they should keep pregnant mares out of pastures bordered by cherry trees or other hosts for the next several weeks.

At many farms, steps have already been taken to cut limbs that overhang or border paddocks. But eastern tent caterpillars are found in many states and horse breeders should be aware of the danger they pose. Apparently the ingestion of the caterpillars does not have known health risks for horses other than pregnant mares.

For a fact sheet about eastern tent caterpillars, as well as periodic updates, please visit the University of Kentucky's special web page on the caterpillar problem. Ohio State University has a helpful web page on how to wage war on eastern tent caterpillars.

Australian Research Confirms Link Between Caterpillars and Equine Fetal Death

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.