Abortions and premature births can happen when pregnant mares are accidentally exposed to some types of hairy caterpillar.

In this article, Equine Reproduction Specialist Dr John Chopin shares a case that could have ended in tragedy but resulted in a healthy foal thanks to their early intervention.

Take home message:

  • Equine Amnionitis & Foetal Loss (EAFL) is the veterinary term used to describe abortions and the birth of compromised foals when pregnant mares are inadvertently exposed to some types of hairy caterpillar.
  • A less severe form of EAFL is now recognised as Focal Mucoid Placentitis can cause premature birth.
  • These conditions are very common and a significant cause of reproductive loss in Australia.
  • Early intervention and targeted treatment can protect the pregnancy and result in a healthy foal.
  • Having your mare under the care of an experienced veterinarian or a reproduction specialist who can identify the condition early can save lives.

A pregnancy in crisis

One of our mares presented with signs of precocious udder development six weeks before she was due. Over the next few days she developed premature lactation. As you can see in the photo below, her udder was swollen with milk dripping and splashing on her hind legs.

Hairy Catterpillars: Chirazz bagged up and dripping milk
The mare was bagged-up and milking six weeks before the foal was due

Clinical evaluation did not show any abnormalities in her vital signs (heart rate, temperature, etc.), but when we examined in more detail the events leading up to her presentation, we determined there was weight loss about 3-4 weeks before development of these clinical signs.

We performed a high risk pregnancy evaluation using an ultrasound machine. A rectal examination with the ultrasound machine did not reveal any abnormalities but the abdominal ultrasound revealed a focal area of placentitis  (inflammation of the placenta) in the ventral body of the uterus with a thickened amnion (the protective membrane enveloping the foetus) and a slight increase in echogenicity of the amniotic fluid.

(Echogenicity refers to the ability to bounce an echo, that is, return the signal in ultrasound examinations. The higher the echogenicity, the lighter the colours on the ultrasound image.)

Below is a transabdominal ultrasonogram of a different and more severe case of Focal Mucoid Placentitis where the measured area is an accumulation of pus between the endometrium and separated chorion. The foetus is on the lower left.

Placentitis in the mare
Transabdominal ultrasonogram of a severe case of Focal Mucoid Placentitis

In order to protect the mare and foal, we initiated therapy with broad spectrum antibiotics and anti-inflammatories.

Chirazz's foal was born 5 weeks early
The filly was born 5-weeks early, in lean body condition, with meconium staining of the hooves and amnion along with diarrhea from the anus.
Chirazz's foal survived exposure to hairy caterpillars
After 72 hours of treatment, she was totally independent and willing to take on life! She is still thriving today.

The mare responded with cessation of milk loss and a reduction in the size of the udder. After a week of treatment, she birthed a filly foal that was 5 weeks early, had evidence of meconium staining, poor body condition but was initially bright and responding to stimulus normally.

Dr John Chopin, Equine Reproduction Specialist

Antibiotics were started immediately and the filly was fed colostrum via a nasogastric tube. Over the next few hours, she could not stand without assistance. She was set up in a stable and nursed over the next 48 hours. She was given intravenous plasma within 6 hours and a follow up IgG test of the foal’s serum at 24 hours revealed adequate levels of antibodies.

The filly responded to treatment and improved in strength, eventually she was able to rise and stand, and started searching for the udder.

By 72 hours she was sucking well, was no longer on intubated feeds and antibiotics were stopped. She continued to progress well in the days and weeks after.

In the images above, the first picture is in the first 24 hours with an indwelling nasogastric tube for feeding. By the end of 48 hours she was strong enough to stand and eventually found the udder to suck.

By 72 hours she was totally independent and willing to take on life. She continues to thrive.

The pathology

Examination of the foetal membranes after birth revealed signs consistent with EAFL (Equine Amnionitis and Foetal Loss). The amnion was thickened and discoloured (yellow), the amniotic portion of the umbilical cord was thickened and discoloured.

There was a focal area of chorionitis (a bacterial infection) in the body of the uterus with loss of chorionic villi, a pale exudate with granular bodies coating the area of pathology.

(The chorion is the outermost membrane between the developing foetus and the mother. Together with the amnion, they make the amniotic sac. The chorionic villi that were reduced in this case, are the tiny finger-like projections that sprout from the chorion to provide maximal contact area with maternal blood.)

Below is a picture of the chorionic surface of the foetal membranes showing the area of focal inflammation and infection (mucoid placentitis).

foetal membranes after birth revealed signs consistent with EAFL
The foetal membranes show the infected area and point to Equine Amnionitis & Foetal Loss.

EAFL was initially characterized in the Hunter Valley in 2004 and shows similar characteristics to MRLS (Mare Reproductive Loss Syndrome) described in Kentucky, USA, in the 2001/2002 foaling season.

The disease is caused by exposure to the hairs of the Processionary Caterpillar (Ochrogaster lunifer) when horses accidentally ingest them while grazing pasture or eating contaminated feeds.

The process of the disease

The caterpillars grow in nests that resemble huge tents in the branches, or at the base, of trees.

Each caterpillar moults (sheds its outer covering, or exoskeleton), up to eight times within the nest. Once the larvae leave the nest, the nest falls apart and the discarded exoskeletons (hairs) are dispersed by the wind into the environment.

Hairy caterpillars nest
Processionary Caterpillars in a cocoon shelter.
Hairy caterpillar nest in tree branches
An early cocoon nest with small larvae exiting to feed.
Hairy caterpillar nests breaking up
An old cocoon bag that is breaking up and falling to the ground.

Horses accidentally ingest the hairs while grazing or feeding, the hairs then migrate through the lining of the gastrointestinal tract and invade the deeper tissues.

A large dose of the ingested hairs can cause illness and endotoxaemia. Smaller doses might go unnoticed or have mild symptoms like low grade colic, weight loss or urticaria might be noticed.

The body takes about six weeks to break down the hairs and there is quite an inflammatory focus around these hairs.

The threat to pregnant mares

In pregnant mares, the uterus lies above the large colon and the caterpillar hairs can leave the colon and enter the pregnant uterus.

The hairs migrate through the tissues and are commonly coated with environmental bacteria. Focal areas of infection (mucoid placentitis) occur when the hairs travel through the uteroplacental unit.

Hairs that travel further into the allantoic fluid can lodge in the amnion and/or umbilicus and cause inflammation +/- infection.

This condition has varying severities, from mild changes in health to severe illness in the adult to severe infection of the foetus, foetal death and abortion.

The infection can be mild to severe

An important variation of this condition is focal mucoid placentitis, which refers to a small area of infection of placental tissues and is an early stage or a mild form of EAFL.

In these cases you may not realise there’s been a problem until you examine the foetal membranes post-partum, but they still require investigation, particularly when there are other expectant mares on the property.

Focal mucoid placentitis used to be called nocardioform placentitis but there was never a sound pathogenesis for nocardioform placentitis. Through work I performed, I produced a causative link and association with the processionary caterpillars and other hairy caterpillar species around the world.

Focal Mucoid Placentitis is now recognised as a mild form of EAFL and is recognized in areas where there are other species of hairy caterpillars. While it may be mild, it can be the cause of premature birth and become severe when the infection spreads to other spaces.

Protecting your horses from caterpillars

The aim is to prevent exposure and provide safe pasture environments by removing nests from trees and disposing of them.

While prevention is always the best medicine, it is also important to know that pregnancies can be protected if the condition is identified early and treated appropriately.

Today, we know much more about these conditions and how to treat them to ensure the situation doesn’t end in tragedy, but early intervention is paramount.

Having your mare under the care of an experienced veterinarian or a reproduction specialist who can identify the condition early can save lives.

The website eafl.net.au contains excellent advice and information about this condition and the life cycle of caterpillars.

On the eafl.net.au website you can download a print version of this poster:

Broodmare owners should become familiar with the caterpillar life cycle, seasonality and timing which will vary depending on the species and the region’s climate.

Processionary caterpillar moths in Australia emerge in late spring, lay eggs, and then die. The eggs hatch, and by late fall or early winter the caterpillars are mature and ready to pupate (they form a cocoon and turn into a moth).

In Australia, mares are most likely to be exposed to caterpillars while grazing around trees where nests are present or when caterpillars form long lines (“processions”) as they move between trees. In the Southern Hemisphere (including Australia), mares appear most at risk of exposure to these caterpillars from about early fall to early winter (March to June).

Take care when removing nests: Caterpillar nest material can cause irritation to humans in the form of allergic skin reactions and eye, nose, and mouth irritation. Individuals with asthma should be hypervigilant and avoid such material. Further, personal protective garb—including disposable coveralls, gloves, head cover, etc.—should be worn when working with nests.

For disposal, deep burial is better than burning, as nest material appears to be dispersed by burning. Nest material is very light, so avoid working in windy conditions that could spread it further.

It is imperative to thoroughly investigate every case of placentitis, premature birth and abortion on a property.