Studies at the University of Queensland and field research at the Hunter Valley Equine Research Centre have demonstrated that exposure to hairy Processionary Caterpillars or their nest material can cause abortion and placentitis in pregnant mares.

Exposure to the caterpillars results in damage to the tissues of the amnion and umbilical cord of the developing foetus. This tissue damage results in poor development of the foetus and can lead to abortion from mid to late-term pregnancy or the birth of premature and weak, underdeveloped foals. This condition is known as Equine Amnionitis and Foetal Loss Syndrome (EAFL).

The species of moths which produce these caterpillars emerge from the ground in late spring (October –November). The moths are short-lived; they mate, lay eggs in nests associated with native trees and die.

There are at least three nest types depending on the various type of moth.

• Bag-type nests that are dew-drop shaped and appear high in eucalypt trees;
• Tree-hugger nests which have a similar appearance to the bag nests but are attached to a tree branch or trunk;
• Ground-based nests which are attached to the base of the tree trunk and are mainly reported in wattles (acacia species).

The caterpillars hatch from the eggs in the nest, feed on the leaves of the tree at night and return to the nest during the day. Caterpillars progress through several larval stages (shedding their exoskeleton in the nest as they grow from one stage to another). The nest will grow larger to accommodate their larger size and the nest will also contain accumulated frass (shed exoskeleton and waste). If the caterpillars eat all the foliage on a host tree they will move in classic head-to-tail processions to another host tree.

By about May-June the caterpillars are mature and ready to pupate. They leave the trees and move into adjacent pasture for pupation, lying dormant until they hatch as moths in Oct-Nov.

Mares are likely to be exposed to caterpillars while grazing around trees where nests are present, encountering head-to-tail processions when caterpillars are moving from one tree to another, when mature caterpillars are leaving trees to pupate, and by disturbing old nest material after caterpillars have pupated. It is currently believed that mares are most likely to be at risk of exposure from about March to June, and beyond that due to exposure to old nest material.

Current field studies have shown that there are a significant number of caterpillar nests now appearing on trees. Now is the time to identify the nests and areas of the farm affected.

It is likely that removing pregnant mares from paddocks which contain caterpillar nests will be effective in preventing EAFL. Due to the widespread distribution of the nests it is often impractical to remove mares from affected paddocks. In this case removal of the nests may be a more practical option.

It is important to note that all caterpillar nest material is capable of causing severe itching or allergic reactions in people on contact with skin, and may cause severe irritation if material gets into eyes, nose or mouth. Nest material is also very light and can easily be dispersed on the wind. Farms should develop protocols for safe handling of nest material & management of inadvertent human exposure.