Equine amnionitis and foetal loss (EAFL)

The Eastern tent caterpillar (Malacosoma americanum)
In Australia they are referred to as Processionary Caterpillar (Ochrogaster lunifer)

The Eastern tent caterpillar is a univoltine, social species that forms communal nest in the branches of trees. It is sometimes confused with the gypsy moth, or the fall webworm and may be erroneously referred to as a bagworm which is the common name applied to unrelated caterpillars in the family Psychidae. The moths oviposit almost exclusively on trees in the plant family Rosaceae, particularly cherries (Prunus) and apple (Malus). The caterpillars are hairy with areas of blue, white, black and orange. The blue and white colours are structural colours created by the selective filtering of light by microtubules that arise on the cuticle.

The adult moth lays her eggs in a single batch in late spring or early summer. The egg masses contain on average 200-300 eggs. Embryogenesis proceeds rapidly and within three weeks fully formed caterpillars can be found within the eggs. But the small caterpillars lie quiescent until the following spring, chewing their way through the shells of their eggs just as the buds of the host tree begins to expand.

The newly hatched caterpillars initiate the construction of a silk tent soon after emerging. They typically aggregate at the tent site for the whole of their larval life, expanding the tent each day to accommodate their increasing size. Under field conditions, the caterpillars feed three times each day, just before dawn, at mid-afternoon, and in the evening after sunset. During each bout of feeding the caterpillars emerge from the tent, add silk to the structure, move to distant feeding sites en masse, feed, then return immediately to the tent where they rest until the next activity period. The exception to this pattern occurs in the last instar when the caterpillars feed only at night. The caterpillars lay down pheromone trails to guide their movements between the tent and feeding sites. The insect has six larval instars. When fully grown, the caterpillars disperse and construct cocoons in protected places. The adults (imago) emerge about two weeks later. They are rather strictly nocturnal, they start flying after nightfall, and possibly stop some hours before dawn already (Fullard & Napoleone 2001). Mating and oviposition typically occur on the same day as the moths emerge from their cocoons; the females die soon thereafter.

THE CATERPILLA ON THE MOVE

The Barbed Setae on the Caterpillars
The focus on the caterpillar led us to examine them under the Electron Microscope (EM, Fig 3c)). We noted, in passing, the barbed nature of the very fine setae that cover the exterior of the caterpillar and we noted that in some caterpillar species these setae are venomous and extremely effective offensive weapons. Although not recognized at the time, this was a critically important observation. 

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ARTICLE BELOW - ©2008 Anstead Veterinary Practice Brisbane

Caterpillars and Placentitis - current knowledge

Equine amnionitis and foetal loss (EAFL) is a newly recognised condition in mares in which early and late term abortions occur.  The condition was first recognised in the USA and termed mare reproductive loss syndrome (MRLS).  In Australia it is thought that some of these pregnancy losses are the result of ingestion of parts or all of the Processional Caterpillar (Ochrogaster lunifer), with a different species, the Eastern Tent Caterpillar ( Malacosoma americanum) responsible in the USA.  It has been noted that horses do eat these caterpillars and their larvae when grazing. It has been proposed that the barbed hairs on the outside of the caterpillars carry bacteria into the bloodstream of the mare, resulting in infection of the placenta and foetus followed by a rapid abortion.

Processional Caterpillars hide in communal nests at the base of trees or in branches during the day.  These nests are composed of silk,old skins and other debris.  Hairs from old skins in these nests can be spread over adjacent vegetation resulting in ingestion by grazing mares.  Ingestion can also occur when migrating caterpillars leave their nests at night, travelling over paddocks, to feed or find new host trees.  In Australia the Processional Caterpillar feeds on Wattle, White Box and Beefwood trees, with their eggs hatching in spring to form the caterpillar larvae.  Grazing mares in paddocks away from where these native trees grow may be used in combination with insecticides to control this form of placentitis.  Research is currently being undertaken in Australia into the causal factors and control of this syndrome.

By Dr Tori McGuire BVSc (Hons)

Bibliography

Herbison-Evans, D. & Crossley, S. (2008)  Ochrogaster lunifer.   http:www.australian–insects.com/lepidoptera/noto/lunifer.html 

Tobin, T. et al (2006) The 2001 Kentucky Equine Abortion Storm: The Caterpillar/Setal Hypothesis of the Mare Reproductive Loss Syndrome (MRLS)
http: //www.thomastobin.com/mrlstox.htm

Cummins, C. et al (2008) Ascending Placentitis in the Mare: A review. Irish Veterinary Journal Vol. 61, No. 5 pp 307 – 313.

Knottenbelt, DC, (2004) Infectious Processes in Foals. 26th Bain-Fallon Proceedings