Caterpillars that Cause Abortion in Mares

Photo: Courtesy Dr. Joan Carrick

By CJ (Kate) Savage, BVSc(Hons), MS, PhD, Dipl. ACVIM, WEVA Junior Vice-President and Oceanic Board Member

The horse industry was shaken in 2001 and again in 2004 when abortion storms occurred in two Thoroughbred breeding strongholds with none of the usual reasons implicated. Central Kentucky was hit with this mare reproductive loss syndrome (MRLS) in 2001, while Scone, Australia, was impacted in 2004. Both locations lost a substantial portion of their foal crops.

Scientists quickly started looking into the cause of these outbreaks. Researchers in Kentucky, using an epidemiological study to ascertain what factors could be considered risk factors, developed a few interesting theories. One of the most surprising to many horse people was exposure to Eastern tent caterpillars (ETC)—one of the many types of hairy, bristly caterpillars.

How Is This Possible?

During the 2001 MRLS outbreak, more than 20% of Kentucky's pregnant mares suffered abortions. However, it is likely that prior abortion storms in the 1980s were also due to the ETC, but technology (e.g., ultrasonography) at the time was not sufficient to detect problems with the fetus until abortion occurred.

In Australia, researchers at the University of Queensland have identified processionary caterpillars as a cause of equine amnionitis (inflammation of the amnion, or placental membrane) and fetal loss (EAFL). Veterinarians Kristen Todhunter, DVM, MANZCVS; Judy Cawdel-Smith, BVSc (Hons), DipVetClinStud, GradCertEd (Higher Ed); Nigel Perkins, BVSc, PhD, Dipl. ACT; and Joan Carrick, BVSc, MVSC, PhD, Dipl. ACVIM, have been major players in this quest for information. As with MRLS, it is likely that previous abortion outbreaks were also due to caterpillar infestation.

The two syndromes differ in that there were no known cases of early pregnancy loss due to EAFL, whereas early pregnancy loss was the most substantial problem associated with MRLS. This is likely due to the timing of the breeding season in relation to the suspected exposure period for their respective caterpillars.

In Brazil, another caterpillar species—the giant silkworm moth—has been implicated, as well. Other species of hairy caterpillars could also be capable of causing abortion in mares.

It appears that the caterpillars' hairs, or setae, are key to causing disease. They pierce the mouth, nose, esophagus (the tube from the mouth to the stomach), stomach, and even bowel after mares inadvertently eat or inhale them. Because the hairs carry bacteria (from the environment and the mare’s mouth, for instance), they can cause enormous problems (including abortions) should they wind up in organs such as the uterus or heart.

The caterpillar life cycle is very important, and seasonality and timing will vary depending on the species and the region's climate. Processionary caterpillar moths in Australia emerge in late spring, lay eggs, and then die. The eggs hatch, and by late fall or early winter the caterpillars are mature and ready to pupate (or form a cocoon and turn into a moth).

In Australia, mares are most likely to be exposed to caterpillars while grazing around trees where nests are present or when caterpillars form long lines (“processions”) as they move between trees. In the Southern Hemisphere (including Australia), mares appear most at risk of exposure to these caterpillars from about early fall to early winter (March to June).

Decreasing The Risk

To decrease the likelihood of Australian mares consuming or inhaling processionary caterpillars, aim to prevent exposure and provide safe pasture environments by removing nests from trees and disposing of them. For the latter task, deep burial is better than burning, as nest material appears to be dispersed by burning. Nest material is very light, so avoid working in windy conditions that could spread it further.

In the United States, Eastern tent caterpillars appear to prefer to nest in black cherry trees. Thus, decreasing the number of these trees on a property can be effective but could be seen to be too drastic. Consequently, muzzling mares at pasture during exposure times and feeding them in safe locations or even avoiding pastures with known caterpillar populations can also be effective.

But remember: All caterpillar nest material is capable of causing irritation to humans. Allergic skin reactions and eye, nose, and mouth irritation can occur. Individuals with asthma should be hypervigilant and avoid such material. Further, personal protective garb—including disposable coveralls, gloves, head cover, etc.—should be worn when working with nests.

Finally, it remains imperative to thoroughly investigate every case of abortion on a property.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More