The perfect storm

The perfect storm

Jul 01, 2001

Last year's weather movie thriller, "The Perfect Storm," introduced us to the concept of multiple occurrences that, collectively, build to a situation of devastating proportions.

In that movie, based on real events occurring in 1991, a severe northeast storm, and a hurricane combined to produce inverted wind patterns, 100-foot waves and the eventual destruction of numerous vessels caught in this "once­a-century" storm.

"Nor'easters" and hurricanes are individually severe enough, but the specific combination of these individual events in an exact time and location sequence led to the "perfect storm."

Recent events in the heart of Lexington's Blue Grass region have mimicked the above pattern leading to a perfect storm of a different kind - the perfect abortion storm.

Just following the Kentucky Derby, the equine industry in Kentucky and parts of Ohio were devastated by a series of abortions occurring on some of the most well-known and best-managed farms in the area.

Mares were aborting late-gestation foals, some foals were being born weak and struggling and mares being examined via ultrasound as part of routine early pregnancy screening were being found to be "open" at an alarming rate. (See related coverage beginning on page 1 of this issue.)

Concerned horsebreeders began bringing aborted fetuses to the University of Kentucky Disease and Diagnostic Lab on April 28. The first few days brought a few fetuses from several farms but quickly the numbers started to escalate. Over the next week that lab received nearly 375 aborted fetuses and stillborn foals. At that point the horse community in those areas was in a justifiable panic and though there were many postulated causes, no definitive answers were available.


Adult horses and horses in training were not affected. More than 18 breeds suffered from these fetal and young foal losses and the name Mare Reproductive Loss Syndrome (MRLS) was applied to the problem. Seventy percent of the horses affected were Thoroughbreds.

Mares that aborted or lost their foals were not clinically ill themselves though some exhibited some vaginal discharge and slight fevers. Early gestation losses seemed to affect mares that were checked in foal at 30 days of gestation and then were found to still be in foal on palpation at 60 to 65 days.

During ultrasound examination of these mares however, uterine fluid appeared to be abnormal. The fluid was cloudy and flocculent around the fetus. Fetal death and expulsion usually followed this ultrasound finding.

Late gestation mares aborted fetuses with slightly darkened lungs and some foals had blood in their eyes.

Dr. Lenn Harrison, director of the University of Kentucky lab, reported that of 247 completed necropsies, 127 have shown strains of streptococcus bacterial species. This percentage is certainly higher than would normally be expected, but the exact significance of this bacteria's presence is unknown.

Harrison would later comment, "Bacteria probably plays an important role in the syndrome we're dealing with, but it might not be the primary or initiating agent."

Because there were no easy early answers to the MRLS problem, a team of researchers was quickly assembled to tackle this puzzle.

Veterinarians, toxicologists, bacteriologists, serologists, virologists, feed and pasture analysis specialists and others all began the process of investigation, speculation and postulation. Field teams visited numerous farms and collected soil, water, pasture, hay and other samples that were then taken to the UK lab and evaluated.

Abortion suspects

The search for answers began with the usual abortion suspects. Abortion storms, situations where higher than normal numbers of mares abort around the same time, have been traditionally caused by a number of agents.

* Equine Herpes Virus Type I (EHVI) is the single most important infectious cause of equine abortions. This virus causes initial respiratory disease and then, abortion between seven months and term. The abortion occurs with typically rapid separation of the placenta from the endometrium causing respiratory distress and eventual death of the fetus.

Many lesions are occasionally found in the aborted fetus and the placenta may sometimes be abnormal as well. Fluorescent antibody staining of fetal tissues and viral isolation can often be helpful in establishing a diagnosis.

* Equine Viral Arteritis (EVA) is another prominent viral cause of abortions. Inapparent or mild clinical signs of fever, conjunctivitis and nasal discharge can occur with abortions following in a few days. Many cases are difficult to diagnose because aborted fetuses may show no clinical signs and virus may not always be isolated from EVA cases.

* There are many types of bacteria that can cause abortions in mares. The most common type are Streptococcus species, but E. coli, Pseudomonas, Klebsiella, and staphylococcus have all been isolated. Most of these bacteria get into the uterus via the cervix and the majority of bacterial abortions show characteristic placental lesions. The placenta is thickened with a brown fibrous exudate. Placental fluid may be cloudy and thick. Aborted fetuses usually show non-specific lesions.

* Various species of fungi can cause abortions as can exposure to many different toxins. Toxins can be man-made or environmental and the stage of gestation seems to contribute greatly to the specific sensitivity to some toxins.

With so many possible causes, the MRLS research team had its work cut out for it. Fortunately many of these typical causes were rapidly discarded because the necropsy findings and viral isolation data cleared most of these usual suspects. By the second week of the crisis toxins were being seriously considered and environmental factors were pointing toward endotoxins and caterpillars.

Building the storm

Once-a-century weather conditions seemed to be a part of this abortion storm as well.

Central Kentucky experienced a very warm early spring. Temperatures were in the 60's. A cold air mass moved through the area soon thereafter and temperatures dropped below 30 degrees for two days.

Warmer weather soon followed again but that cold snap was thought to have damaged many plants and grasses. Researchers began to investigate the possibility of various types of fungal mycotoxins.

It is well known that fescue grasses support a fungus that produces a toxin that can cause prolonged gestation and agalactia in pregnant mares. Many other toxins are produced by other fungi and veterinary researchers began to suspect zearalonone as a possible cause for MRLS. The theory was that weather stress caused ideal conditions for fungal growth in Kentucky fields and that some mycotoxin caused the abortions.

Field teams began collecting samples and doing the analysis. Even as the veterinarians stressed that nothing was certain yet, many horse owners turned to feed additives that were known mycotoxin binders.

These products bind the toxins in the horse's system and make those toxins unable to cause problems. While this is an interesting idea and more research certainly needs to be done, it may have been premature.

While some feed companies were talking about producing mycotoxin binder added feeds, the lab results came back showing no definitive correlation between the abortions and zearalonone or other mycotoxins. It would be beneficial to have such binders readily available in the future and this tragedy may prevent others because of the exposure that mycotoxins have received, but the cause of MRLS was still not known.

Tent caterpillars

Attention next turned to the high numbers of tent caterpillars that were present in the region during this time of year.

Kentucky is home to the eastern tent caterpillar, which builds nests often in the forks of tree branches.

These caterpillars feed on many types of trees but tend to prefer apple, peach and cherry trees. It is well known that conditions of stress cause some plants and trees to produce or concentrate toxins.

Cherry trees are very toxic to horses in normal situations. When severe weather occurs and cold, high winds or drought damages these trees, they tend to produce higher than normal levels of a cyanogenic glycocide.

This toxin can be processed by animals to produce cyanide in their systems.

Kentucky experienced drought situations last year and the cold snap in early April further stressed the large amount of black cherry trees found along pastures in central Kentucky. Researchers then began postulating that the large number of caterpillars rapidly defoliated many stressed cherry trees just around the time that the abortions began occurring.

These caterpillars produced large amounts of cyanogenic glycocides that were then ingested by the broodmares that were eventually affected. While cyanide can be difficult to recover from aborted tissue, samples of caterpillars from affected pastures and water troughs have given researchers some reason to hope that they are close to an answer.

Yet, caterpillars and black cherry trees have been a fixture of Kentucky springs for many years. Researchers must still explain the specifics as to how and why these seemingly unrelated events all combined to produce MRLS.

Coming together

As in "The Perfect Storm", normal weather events all came together at the exact time and location. In MRLS, stressed cherry trees, large numbers of caterpillars and some yet unknown method of transmission of toxin to the mares resulted in the abortion storm. It may be that a cold snap three weeks later may have had little effect since the caterpillars would have already eaten the trees. In other years lower numbers of caterpillars may have been too few to produce enough toxin to cause more than a scattered few problems.

Cyanide toxins given to mares at different stages of their gestation may not have had such severe effects either. It seems that all of the factors present in this situation occurred exactly as they needed to, in location and time sequence, to cause this devastating problem.

Researchers are still not being definite, and the cyanide theory is being called a "working hypothesis" but the fetal losses and abortions have subsided and the problem seems over.

The exact extent of the economic impact is now being debated, but the entire episode has taught veterinarians and breeders a number of things.

Horses and cherry trees do not mix. Mycotoxin binders are potentially helpful items that need to be investigated further and climatic conditions can sometimes conspire to turn normal situations into deadly mysteries.