What is the difference between EAFL and MRLS?

Mare Reproductive Loss Syndrome (MRLS) refers to early pregnancy loss and abortions in pregnant mares and a range of other conditions in horses, that were first described in Kentucky (USA) in 2001-2002. MRLS was subsequently found to have been associated with the Eastern Tent caterpillar (ETC). There are numerous web pages of information about MRLS (see links pages). MRLS was associated with a much larger impact in terms of losses in the first year or two when it was noticed.

There are both similarities and differences between the ETC and MRLS and our processionary caterpillar and EAFL. Both caterpillars are large and hairy. The ecology of these two caterpillars also has similarities and differences which are intruiging. The ETC moth lays eggs in spring/summer and the eggs lie dormant through the winter and hatch early in the following spring. The caterpillars form prominent silken nests (tents) and emerge from these tents to feed, returning to the tents to sleep. The tents get larger as the caterpillars grow and tents accumulate frass and shed exoskeleton. Caterpillars grow rapidly and within a couple of months they move from the tent to secluded spots and spin a cocoon and pupate, hatching as adult moths about 3 weeks later.

Both conditions involve horses inadvertently consuming caterpillars or caterpillar exoskeleton and caterpillar setae seem to penetrate into the body of the mare and allow a range of bacteria to gain access to the pregnant uterus and result in abortion. ETCs were also associated with other conditions in non-pregnant horses including uveitis (eye conditions) and some other unusual conditions. These conditions have not been noticed in Australia with the processionary caterpillar.

The ETC is a different caterpillar to the processionary caterpillar. Australia does not have ETCs. The processionary caterpillar is urticarial (causes itching on contact) and the ETC is not. The ETC appears to have a more restrictive host tree range, and is found mainly on cherry trees and apple trees. The processionary caterpillar is found on wattles and many other Australian native trees.

One of the many interesting things about these two conditions is that they involve two very different species of hairy caterpillar and yet there are many important similarities between the way the caterpillars develop and the diseases they can cause in mares. This information may suggest that there could be other hairy caterpillars in Australia (and elsewhere) that might be capable of causing similar conditions.

Caterpillars have been shown to cause abortion in horses and are suspected of inducing abortions in camels, however, there have been no reports of similar abortions in cattle, sheep, or goats.