Hairy caterpillars and abortions in horses

Hairy caterpillars are now proven causes of horse abortions and the birth of sick foals.

Breeding horses is a roller coaster ride and over the years the improvements in management by owners, information and research available to veterinarians, vaccinations and the availability of advanced equipment, has lead to improvements in breeding success and efficiency. 

One thing that has recently got a lot of interest is abortions related to the hairy caterpillars.  The link between these caterpillars and horse abortions was firsts suspected in 2001.  Now it is recognised in many areas with several different breeds of these caterpillars.

The current research suggests that mares can abort from eating the whole caterpillars and also from eating parts of the caterpillars.  The important parts of the caterpillars are the skins that they shed when they are growing.  These skins are left behind in the ‘bags’ when the caterpillars move out of the trees and can blow about in the breeze for large distances as each skin can weigh as little as 0.01 of a gram. The abortions in mares can be early term or late term abortions.  The early term abortions often have no signs that the mare has lost her foal and have been reported between 32-140days of pregnancy.  Some mares loosing their foals at this stage will show a vaginal discharge as the only external sign.

The late abortions have been found from mid-term pregnancy through to full term pregnancy and most mares do not show any signs prior to aborting.  Some foals go through the full pregnancy and then are delivered with intact membranes (‘red bag’ deliveries) and the foal may not survive even with intensive care.

Prevention of these losses:

• Prevent grazing on caterpillar infested pastures
• Supplement feeding during pregnancy to try and reduce grazing under trees
• Don’t use feeders for mares under trees or anywhere near the caterpillar nesting areas
• Remove any possible nests from trees into plastic bags (commonly seen as ‘bags’ in the trees). These generally occur February and March.