Tent Caterpillar Toxicity (Pregnant Mares) Average Cost

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What is Tent Caterpillar Toxicity (Pregnant Mares)?

Premature delivery due to the tent caterpillar is known as mare reproductive loss syndrome (MRLS). In 2001, it is documented that Kentucky lost more than a third of the state’s foals to mare reproductive loss syndrome.  The loss was estimated at 500 million dollars. Researchers were able to determine that the tent caterpillar was the cause of all the premature births.

Tent caterpillars are seen in spring and usually make a nest in cherry, apple and crabapple trees.  Full grown caterpillars are about 2 inches long.  They are black in color with blue spots on their sides.  

If your pregnant mare shows signs that she is going into to labor prematurely (before 320 days of gestation), please contact an equine veterinarian as soon as possible.

The ingestion of the Eastern tent caterpillar (Malacosoma americanum) by pregnant mares will cause premature delivery of her foal.  When the pregnant mare ingests the Eastern tent caterpillar, their setae (stiff bristles) penetrate the lining of digestive tract.  Once the setae are embedded in the intestinal walls, bacteria can enter the reproductive organs and the amniotic fluid.

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Symptoms of Tent Caterpillar Toxicity (Pregnant Mares) in Horses

When the mare goes into premature labor the symptoms may include:

  • Vaginal discharge
  • Change of the shape of the mare’s belly
  • Infected uterus
  • Weakness
  • Low grade fever
  • Restless
  • Mare appears to be in distress
  • Circling her stall
  • Sweat on her neck and flanks
  • Passing small amount of manure or urine

The pregnant mare may also have:

  • Unilateral uveitis
  • Inflammation around a single eye
  • Hemorrhaging around the iris

Causes of Tent Caterpillar Toxicity (Pregnant Mares) in Horses

The cause of mare reproductive loss syndrome (MRLS) is the ingestions of the tent caterpillar.

  • The pregnant mare ingests tent caterpillar while foraging
  • The mare drinks from a water bucket that has tent caterpillars
  • Old skin of the tent caterpillar is ingested by the mare while grazing
  • Migrating caterpillars make their way into the horse’s feeding trays

Diagnosis of Tent Caterpillar Toxicity (Pregnant Mares) in Horses

The veterinarian will give the pregnant mare a physical exam.  The physical exam may include taking the mare’s temperature, pulse, and blood pressure. He may choose to examine the  eyes and palpate the abdomen. A complete blood count could be done to determine if there is a bacterial infection.  Once the mare goes into labor, she may lay down.  Foaling begins when the placenta breaks open and the amniotic fluid is released. Usually, the front hooves of the foal are delivered first.  Premature foals may be stillborn or in weak condition.  It is imperative that premature foals are seen and treated immediately to ensure survival.

Treatment of Tent Caterpillar Toxicity (Pregnant Mares) in Horses

There is no treatment for tent caterpillar toxicity and the possible loss of foal.  The main goals are to care for the mare and to save her newborn foal. The veterinarian will treat the mare with antibiotic medicines. Mares with uveitis will also be given anti-inflammatory drugs.  The veterinarian will clean the foal’s navel with an iodine solution.  The foal will need to be kept warm. Some premature foals can stand and nurse on their own, though if the foal is too weak he may need to be given oxygen, IV fluids and antibiotics.  If the foal is not nursing, he will need to get nutrition through a feeding tube placed in his nostril. The veterinarian may recommend that the foal be hospitalized and have 24 hour care.

In other cases, the veterinarian may suggest that the foal have ongoing treatment care at home, for at least 3-4 weeks.  Usually, the veterinarian team will need to make daily visits.  Exercise and activities will be limited until the premature foal has x-rays taken. The veterinarian will want to make sure that there are no skeletal problems in the limbs. If it is determined the foal has any deformities or if the bones are not fully developed, the veterinarian may need to cast or splint his legs.

Recovery of Tent Caterpillar Toxicity (Pregnant Mares) in Horses

The veterinarian will need to make follow up visits to check on the mare.  He will want to take a second complete blood count to ensure that there is no longer a bacterial infection.  If the mare had uveitis the veterinarian will need to examine her eyes for any inflammation. The premature foal must be closely monitored. The foals temperature and respiratory rate must be checked daily. The first 24-48 hours of the foal’s life are crucial to his survival rate. It is very important to follow veterinarian treatment plan. X-rays will need to be retaken to check on the bone development of the foal. Please contact your veterinarian if the foal has:

  • Swollen joints
  • Milk coming out of his nose
  • Sunken eyes
  • Low or high temperature
  • Lameness
  • Diarrhea
  • Irregular respiration
  • Colic
  • Weakness

Tent Caterpillar Toxicity (Pregnant Mares) Questions and Advice from Veterinary Professionals

13 Years
Fair condition
0 found helpful
Fair condition

Has Symptoms



My and my horse went on a trail today and I caught him on the action of eating some leaves where I saw tent caterpillar on it. So I assume he has eaten one or maybe more. He's a 13 years old gelding, not a mare. Now knowing that there is a risk of toxicity for pregnant mare, is there one for male horses too ?

Thank you !

Dr. Michele King, DVM
Dr. Michele King, DVM
1611 Recommendations
It would seem that there is the potential for infection and uveitis in horses in general, but it would be best to follow up with your veterinarian to see if Tosca is showing any signs of toxicity, as they can examine him.

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