Mare Reproductive Loss Syndrome Average Cost

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What is Mare Reproductive Loss Syndrome?

During spring 2001 several hundreds of horses were affected in Central Kentucky and the surrounding states with an unusually large number of aborted foals and early fetal loss, causing panic in the area. Some foals born alive did not survive as they were sickly and had severe neurological signs and respiratory problems. Veterinarians and scientists were called on to find the possible causes of the problem. The common factor turned out to be a large increase in the number of the eastern tent caterpillars in the area, and the presence of black cherry trees (which the caterpillar like to nest in). Later, in 2004 Scone, Australia suffered a loss of a substantial number of aborted foals from a similar cause.

Mare reproductive loss syndrome (MRLS) results in equine abortions. With this condition, the unborn foal has been affected by nonspecific bacterial infections of the placenta of pregnant mares.

Symptoms of Mare Reproductive Loss Syndrome in Horses

A few affected mares show signs of colic, high fever and vulvar discharge but most show no signs at all. Documentation shows, however that these signs can be seen:

  • Colic
  • Fever
  • Lethargy
  • Apathy
  • Vulvar discharge
  • Loss of foal
  • Foal loss through early abortions in the first 40 - 80 days or as late as 140 days
  • Some mares displayed skin reactions to contact with the caterpillar 
  • Ultrasound results show the dead fetus


  • The eastern tent caterpillar is the culprit for the Kentucky episode, exploding into life and causing catastrophic losses of late term foals 
  • These caterpillars leave the trees to search for a site to build their silken cocoons and pupate 
  • In Australia, it was the hairy processionary caterpillars that cause huge losses 
  • It is possible other hairy caterpillar types also cause MRLS

Causes of Mare Reproductive Loss Syndrome in Horses

  • Hairy caterpillars in pastures have been identified as the leading cause of MRLS 
  • The caterpillar hairs or setae, are picked up by your horse while grazing and find their way into your horse’s system
  • They pierce the mouth, nose, esophagus, stomach, and bowel after mares have inhaled or unconsciously eaten them 
  • Opportunist bacteria enter the bloodstream causing infection of the placenta
  • They cause enormous health risks for pregnant mares

Diagnosis of Mare Reproductive Loss Syndrome in Horses

Physical examination, clinical signs and laboratory tests are evaluated by the veterinarian. Usually though, by the time it is discovered it is already too late to save the unborn foal. The ultrasound scan typically finds the first signs of this condition, and often there is nothing one can do to reverse the condition. If the foals do make it to birth they are usually poorly and suffer decline quickly. 

The early spring and summer, with some seasons showing rapid outbreaks of caterpillar infestation, is the danger time and as the caterpillar likes to breed in nests in nearby trees, this can present a real problem to a horse owner. The many thousands of wild cherry trees that grow a prolific leafy cyanide laden leaf that the caterpillars love to eat, were deemed the cause of the earlier outbreaks of MRLS. Close observation of your horse, especially if it is a pregnant mare, is imperative. 

Most of the cases of MRLS are diagnosed after the fetal loss has occurred. Attending veterinarians may observe a thickened placenta and umbilical cord. If further testing is done after the fact, bacteria exposure in relation to the foal is often noted.  Foals born alive may often have serious health problems such as pneumonia or neurological conditions.

Treatment of Mare Reproductive Loss Syndrome in Horses

Supportive care is the only treatment available at present although ongoing research is looking for ways to handle this condition. Most mares get through without suffering, but the unborn young are the ones who are affected the most. Prevention is the best course of management for MRLS. Eliminating wild black cherry trees from the premises and avoiding exposure to your horses (especially a pregnant mare) to any caterpillar are recommended measures. 

Removal of the silken nests and using insecticides to cut the numbers of caterpillars may help a little but large dispersing caterpillars are not feeding much so it has limited use. Pregnant mares may be removed from suspected areas and stabled until birth occurs which, may be feasible and help if you only have one or two horses. Check fence posts and trees as caterpillars like vertical objects. The caterpillars also move along their own trails on the ground, for days on end until nature signals them to stop and pupate.

Recovery of Mare Reproductive Loss Syndrome in Horses

Your mare will probably come through this experience without harm, but it may take until the next season to breed again. A check up with the veterinarian to ensure she is alright after the abortion is advisable. Giving her time to recover with plenty of rest and good food to build up her immune system is important. Next year, when any of your horses are confirmed pregnant, be very observant and move them away from any affected areas as much as possible. Prevention is the best possible alternative for this distressing condition. Research is ongoing as scientists battle to find some sort of prevention that will halt the caterpillar invasion.