Equine Amnionitis & Foetal Loss



Reproductive loss is a major cause of concern for all horse breeders, especially the thoroughbred industry where  abortions can cause up to 10% of losses annually. In the absence of other established causes such as Equine Herpes Virus, breeders and researchers in Australia have been searching for answers to this problem. During 2004 in the Hunter Valley region of NSW a consistent pattern of abortions emerged. This became known as Equine Amnionitis and Foetal Loss (EAFL) and comparisons with the syndrome identified in Kentucky as Mare Reproductive Loss Syndrome (MRLS) were made.


 mrls7    mrls2    mrls3   

          Ochrogaster lunifer moth     Ochrogaster  lunifer                Ochrogaster lunifer 
                                                          Processionary Caterpillar                                                          

In 2006 the principals of Equivet Australia – Max Wilson & Robyn Woodward – visited Lexington in Kentucky en route to the UK for the breeding season. They found Lexington to be an incredible place and truly the American home of the thoroughbred, where there are over 450 studs within a 40km radius of Lexington and almost 20,000 mares being bred annually. The area is serviced primarily by two huge veterinary practices, each employing almost 50 veterinarians and 200 lay staff during the breeding season. Stud principals from the area combined with the Gluck Research Centre to compile a data base of information about the problem of Mare Reproductive Loss Syndrome (MRLS) which they attribute to caterpillars. These findings have been made available to Equivet Australia.

KENTUCKY RESEARCH FINDINGS ¨ Mares that aborted (and no other cause was established) were in areas where Malacosoma americanum (Eastern Tent Caterpillar) is found.  There are two syndromes - Early Foetal Loss (EFL) at 15 - 45 days & Late Foetal Loss (LFL) at  9 - 10 months.  Time from access to caterpillars to abortion is 8 - 13 days for EFL or 3-15 days for LFL.


In the Hunter Valley region of New South Wales, mrls8mares often graze native pastures in caterpillar-nest-at-base-of-treeareas populated with trees that provide a habitat for caterpillars. Initial investigations in 2004 showed that the abortion outbreak occurred in late March to May coinciding with the time when processionary caterpillars move from the trees.  

Caterpillar nest in fork of tree                                                Caterpillar nest at base of tree          



 Caterpillar nest (silken bag) showing caterpillars on outside - actual size aproximately 30cm


caterpilar-nest-hanging-resize       cterpillar-nest-in-tree-resize

Caterpillar nests in trees


A procession of caterpillars shifting to a new tree




During 2005/2006 studies at the University of Queensland showed that exposure to preparations made from the processionary caterpillars (or their shed exoskeletons) were responsible for causing pregnancy loss in the mare or deficits in the newborn foal. Shed exoskeletons accumulate in the nests as the caterpillars pupate.  When the caterpillars leave the nest to migrate, the nest frequently disintegrates and falls onto the ground. See Fig.1. The exoskeleton is light and fragile and as it falls can easily drift onto surrounding pasture where it can be picked up by grazing horses.

The results of this Queensland study indicate that the barbed fragments from the setae (small hairs) of the exoskeleton may penetrate the intestinal wall and allow bacteria into the bloodstream thereby causing infection of the placenta and subsequent abortion.


Processionary caterpillars - Ochrogaster lunifer (processionary caterpillar) and Leptocneria reducta (white cedar moth caterpillar) - have been found in large numbers on many broodmare farms in southern Queensland and northern New South Wales. The natural hosts of  the caterpillars are eucalypt and acacia species, including the mountain coolibah, white box, white cedar, and wattles commonly found in eastern Australia.  Late summer / autumn is the season when the caterpillar nests can be seen hanging in the trees as a silken bag. See Fig. 2. A caterpillar colony can consume the foliage from an entire tree before moving on, however the defoliated tree usually recovers.

The caterpillars are grey & hairy with a brown head. See Fig. 3.

When the caterpillars emerge from the nest & go looking for a new host tree they can form a procession of up to 100 caterpillars on the ground and travel long distances. See Fig. 4. Caterpillar exposure is more likely in times of drought when mares resort to grazing areas under trees as feed becomes scarce. 

Exposure to hairy caterpillars can cause an intense allergic reaction in humans resulting in quite severe skin shes. In properties where abortions have occurred it has been reported that some horses grazing under trees populated with caterpillars displayed skin reactions, however not all mares that aborted showed detectible abnormalities. 

PREVENTIVE METHODS  There is some evidence that when mares are known to be in contact with caterpillars giving blanket treatment with antibiotics on a regular basis throughout the pregnancy may be effective in preventing abortion due to infection. However this method would be both time consuming and very expensive.

Another method would be to inject the infected trees with systemic insecticides to kill the caterpillars eggs. This would not be effective unless there is rainfall and the sap is running in the trees, and again the cost and doubtful efficacy would suggest that alternatives would be       preferable. In some cases insecticides can be sprayed on to foliage, which will make direct contact with the caterpillars and cause them to die.



  • Remove mares away from pastures where known caterpillar habitat trees are present, or leave a large margin when fencing off areas with affected trees.
  • Any supplementary feeding of mares should be done away from trees using elevated feeders to reduce the risk of contamination.
  • Remove caterpillar nests from the trees in February/March before the caterpillars leave the nests and dispose of them immediately or the caterpillars may burrow into the ground and disappear. A cherry picker can be used for this purpose if the trees are tall, but cost  may be prohibitive.
  • Replace the trees with others that are NOT the natural food of the caterpillars.


Bright lights can be used to attract the moths to bug zappers before they turn into caterpillars.  


Ochrogaster lunifer moth



The Rural Industries Research and Development Corporation (RIRDC) support disease prevention, diagnosis and treatment as well as  animal breeding & genetics. If you are interested in on going research into the control of caterpillars on farm, please contact Dr Judy Cawdell-Smith on or .



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