This is an excellent question and one we dont yet know the answer to. All the experimental studies on caterpillars and mare abortion that have been done in Australia to date, have involved only processionary caterpillars.
There are other hairy caterpillars in Australia and it is possible that other hairy caterpillars may be capable of causing the same condition. Examples include the White Cedar moth caterpillar (Leptocneria reducta), which can be seen in large numbers in some years on white cedar trees, and the Mistletoe Brown Tail Moth caterpillar (Euproctis edwardsi). More research is needed to determine if other caterpillars may be capable of causing abortion in mares.
It is interesting to look at the American experience with the Eastern Tent caterpillar (ETC) – see the next section.