Kentucky veterinarians investigate mysterious foal death syndrome

Published on June 01, 2001
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mare and foal

Mycotoxins in pastures seem the most likely culprit, but veterinarians are still unsure exactly what's to blame for the loss of hundreds of foals in Kentucky.

Mares in central Kentucky are aborting early- and late-term and giving birth to stillborn and weakened foals by the hundreds—victims of a mysterious condition veterinarians are calling mare reproductive loss syndrome.

Between April 28 and May 8, 318 dead fetuses and foals of various breeds were brought in to the University of Kentucky's Livestock Disease Diagnostic Center for evaluation—an increase of almost 700 percent over the same period in 2000. The volume since then has decreased, but remains above average. At press time May 17, the laboratory had received 486 specimens, most of them Thoroughbreds.

Veterinarians and state agriculture officials at press time had yet to pinpoint the exact cause of the syndrome, although most believe it is not viral or contagious and is probably related to feed or pasture. A multidisciplinary team coordinated by Dr. David Powell, an equine epidemiologist at the University of Kentucky's Gluck Equine Research Center, continues to investigate.

"This is very much a team effort, and not just involving people at the Gluck center but in other departments of the University of Kentucky, particularly in the College of Agriculture," Dr. Powell said. "Also, we're working very closely with practicing veterinarians in the field; with private consultants, particularly in the area of pasture management; and state and federal agencies."

Signs related to mare reproductive loss syndrome

Many of the mares have no outward signs, part of what makes the cause of mare reproductive loss syndrome elusive. However, some mares that have aborted late-term fetuses have shown the following signs:

  • agalactia
  • dystocia
  • thickened and edematous placenta

Some mares that have undergone early fetal loss:

  • are determined to be pregnant at fewer than 40 days, but ultrasound at 45 to 80 days reveals the mare is no longer pregnant
  • feel normal on rectal palpation at 60 to 65 days
  • on ultrasound, have abnormal allantoic fluid, cloudy with echogenic material, surrounding the fetus
  • may have vaginal discharge or fever

Source: University of Kentucky College of Agriculture Veterinary Science Department, Kentucky Association of Equine Practitioners

Scientists at the university and private institutions have been busy analyzing pasture samples. Possible causes they are considering include endophytes in tall fescue, external fungi on the seed heads of bluegrass after a freeze, cyanide produced by white clover under stress, and phytoestrogens. But by far the most likely cause at this point is the mycotoxin zearlanone, which showed up at higher-than-average levels in testing conducted in early May.

These mycotoxins might have formed because of the unusual weather patterns this spring—in Kentucky, an early spring warm-up was followed by a frost and then drought conditions.

Whatever is causing the abortions and foal deaths is apparently beginning to cause problems in other horses as well, Dr. Powell said. Veterinarians are now seeing more pericardial perfusions than expected in horses of all ages, possibly contributed to by the immunosuppressant effects of prolonged exposure to mycotoxins. Young horses have also been developing panophthalmitis, a condition not normally seen in this area.

"Our impression is that the time frame fits so well, and because [the panophthalmitis] is a syndrome that we've never seen here before, we are strongly suspicious that they're all related," said Dr. Tom Riddle, a private equine practitioner in Kentucky who was one of the first to identify the outbreak.

So far, no problems have been reported in other livestock, but veterinarians continue to keep close watch.

Kentucky veterinary officials are recommending that veterinarians and horse owners in the state treat all horses as if they have been exposed to mycotoxins, including giving them feed with a mycotoxin binder additive. Veterinarians are also advised to consider ultrasound for mares at 60 to 65 days gestation.

"Normally, after 28 days, most people rely on manual palpations to determine pregnancy status," Dr. Riddle said. "It is necessary to ultrasound, though, because what we're finding is mares that will palpate as having a normal pregnancy, but then they'll have a dead fetus."

The Kentucky Association of Equine Practitioners recommends that veterinarians consider treating late term pregnant mares with domperidone and probiotics in addition to the mycotoxin binder additive, and mares bred in 2001 with probiotics and mycotoxin binder additive as well. The association also suggests clipping pastures to eliminate seed heads, but stopping short of scalping them. These recommendations also apply to other states that may be experiencing higher than average fetal and foal loss, especially states north of Kentucky.

Kentucky veterinarians said it appears that most of the mares that experienced early fetal loss could be rebred this season, although late-term abortions and stillborn foals were more damaging.

If a mare aborts, Kentucky owners and veterinarians should collect any fetal tissue found and a serum sample from the mare, along with one ounce of manure from the mare and one fecal cup (3 ounces) of grain concentrate fed to the mare. These should be delivered frozen to the Livestock Disease Diagnostic Center, along with the date and pertinent information about the mare, farm, and veterinarian, officials said.

Although numbers of foal losses continue to decrease, Dr. Powell said there is still much work to do in identifying the cause and that it's unclear when the crisis will be over.

There is hope for the future, though—on the basis of specific climactic data for this Kentucky spring, researchers should be able to develop guidelines to predict whether the syndrome will recur in future breeding seasons, he said. In addition, if the culprit is indeed a mycotoxin, routine tests could be developed for farm owners to identify the toxic material in their grasses, Dr. Riddle said.

Dr. Riddle also said the experience of working as a team to solve the problem will help prevent or quickly solve any future issues.

"I think that having gone through this experience, with the next crisis—whenever it may hit—we'll be better prepared for it," he said.

For more information and the most updated epidemiologic statistics, visit the mare reproductive loss syndrome Web site at