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The perfect storm
Sequence of environmental conditions set the stage for foal abortions, research says
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 northeaststorm, and a hurricane combined to produce inverted wind patterns, 100-footwaves 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 timeand location sequence led to the "perfect storm."
Recent events in the heart of Lexington's Blue Grass region have mimickedthe above pattern leading to a perfect storm of a different kind - the perfectabortion storm.
Just following the Kentucky Derby, the equine industry in Kentucky andparts of Ohio were devastated by a series of abortions occurring on someof the most well-known and best-managed farms in the area.
Mares were aborting late-gestation foals, some foals were being bornweak and struggling and mares being examined via ultrasound as part of routineearly pregnancy screening were being found to be "open" at analarming rate. (See related coverage beginning on page 1 of this issue.)
Concerned horsebreeders began bringing aborted fetuses to the Universityof Kentucky Disease and Diagnostic Lab on April 28. The first few days broughta few fetuses from several farms but quickly the numbers started to escalate.Over the next week that lab received nearly 375 aborted fetuses and stillbornfoals. At that point the horse community in those areas was in a justifiablepanic and though there were many postulated causes, no definitive answerswere available.
Adult horses and horses in training were not affected. More than 18 breedssuffered from these fetal and young foal losses and the name Mare ReproductiveLoss Syndrome (MRLS) was applied to the problem. Seventy percent of thehorses affected were Thoroughbreds.
Mares that aborted or lost their foals were not clinically ill themselvesthough some exhibited some vaginal discharge and slight fevers. Early gestationlosses seemed to affect mares that were checked in foal at 30 days of gestationand then were found to still be in foal on palpation at 60 to 65 days.
During ultrasound examination of these mares however, uterine fluid appearedto be abnormal. The fluid was cloudy and flocculent around the fetus. Fetaldeath and expulsion usually followed this ultrasound finding.
Late gestation mares aborted fetuses with slightly darkened lungs andsome foals had blood in their eyes.
Dr. Lenn Harrison, director of the University of Kentucky lab, reportedthat of 247 completed necropsies, 127 have shown strains of streptococcusbacterial species. This percentage is certainly higher than would normallybe expected, but the exact significance of this bacteria's presence is unknown.
Harrison would later comment, "Bacteria probably plays an importantrole in the syndrome we're dealing with, but it might not be the primaryor initiating agent."
Because there were no easy early answers to the MRLS problem, a teamof researchers was quickly assembled to tackle this puzzle.
Veterinarians, toxicologists, bacteriologists, serologists, virologists,feed and pasture analysis specialists and others all began the process ofinvestigation, speculation and postulation. Field teams visited numerousfarms and collected soil, water, pasture, hay and other samples that werethen taken to the UK lab and evaluated.
The search for answers began with the usual abortion suspects. Abortionstorms, situations where higher than normal numbers of mares abort aroundthe same time, have been traditionally caused by a number of agents.
* Equine Herpes Virus Type I (EHVI) is the single most importantinfectious cause of equine abortions. This virus causes initial respiratorydisease and then, abortion between seven months and term. The abortion occurswith typically rapid separation of the placenta from the endometrium causingrespiratory distress and eventual death of the fetus.
Many lesions are occasionally found in the aborted fetus and the placentamay sometimes be abnormal as well. Fluorescent antibody staining of fetaltissues and viral isolation can often be helpful in establishing a diagnosis.
* Equine Viral Arteritis (EVA) is another prominent viral causeof abortions. Inapparent or mild clinical signs of fever, conjunctivitisand nasal discharge can occur with abortions following in a few days. Manycases are difficult to diagnose because aborted fetuses may show no clinicalsigns and virus may not always be isolated from EVA cases.
* There are many types of bacteria that can cause abortions inmares. The most common type are Streptococcus species, but E. coli, Pseudomonas,Klebsiella, and staphylococcus have all been isolated. Most of these bacteriaget into the uterus via the cervix and the majority of bacterial abortionsshow characteristic placental lesions. The placenta is thickened with abrown fibrous exudate. Placental fluid may be cloudy and thick. Abortedfetuses usually show non-specific lesions.
* Various species of fungi can cause abortions as can exposureto many different toxins. Toxins can be man-made or environmental and thestage of gestation seems to contribute greatly to the specific sensitivityto some toxins.
With so many possible causes, the MRLS research team had its work cutout for it. Fortunately many of these typical causes were rapidly discardedbecause the necropsy findings and viral isolation data cleared most of theseusual suspects. By the second week of the crisis toxins were being seriouslyconsidered and environmental factors were pointing toward endotoxins andcaterpillars.
Building the storm
Once-a-century weather conditions seemed to be a part of this abortionstorm as well.
Central Kentucky experienced a very warm early spring. Temperatures werein the 60's. A cold air mass moved through the area soon thereafter andtemperatures dropped below 30 degrees for two days.
Warmer weather soon followed again but that cold snap was thought tohave damaged many plants and grasses. Researchers began to investigate thepossibility of various types of fungal mycotoxins.
It is well known that fescue grasses support a fungus that produces atoxin that can cause prolonged gestation and agalactia in pregnant mares.Many other toxins are produced by other fungi and veterinary researchersbegan to suspect zearalonone as a possible cause for MRLS. The theory wasthat weather stress caused ideal conditions for fungal growth in Kentuckyfields and that some mycotoxin caused the abortions.
Field teams began collecting samples and doing the analysis. Even asthe veterinarians stressed that nothing was certain yet, many horse ownersturned to feed additives that were known mycotoxin binders.
These products bind the toxins in the horse's system and make those toxinsunable to cause problems. While this is an interesting idea and more researchcertainly needs to be done, it may have been premature.
While some feed companies were talking about producing mycotoxin binderadded feeds, the lab results came back showing no definitive correlationbetween the abortions and zearalonone or other mycotoxins. It would be beneficialto have such binders readily available in the future and this tragedy mayprevent others because of the exposure that mycotoxins have received, butthe cause of MRLS was still not known.
Attention next turned to the high numbers of tent caterpillars that werepresent in the region during this time of year.
Kentucky is home to the eastern tent caterpillar, which builds nestsoften 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 causesome plants and trees to produce or concentrate toxins.
Cherry trees are very toxic to horses in normal situations. When severeweather occurs and cold, high winds or drought damages these trees, theytend 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 inearly April further stressed the large amount of black cherry trees foundalong pastures in central Kentucky. Researchers then began postulating thatthe large number of caterpillars rapidly defoliated many stressed cherrytrees just around the time that the abortions began occurring.
These caterpillars produced large amounts of cyanogenic glycocides thatwere then ingested by the broodmares that were eventually affected. Whilecyanide can be difficult to recover from aborted tissue, samples of caterpillarsfrom affected pastures and water troughs have given researchers some reasonto hope that they are close to an answer.
Yet, caterpillars and black cherry trees have been a fixture of Kentuckysprings for many years. Researchers must still explain the specifics asto how and why these seemingly unrelated events all combined to produceMRLS.
As in "The Perfect Storm", normal weather events all came togetherat the exact time and location. In MRLS, stressed cherry trees, large numbersof caterpillars and some yet unknown method of transmission of toxin tothe mares resulted in the abortion storm. It may be that a cold snap threeweeks later may have had little effect since the caterpillars would havealready eaten the trees. In other years lower numbers of caterpillars mayhave been too few to produce enough toxin to cause more than a scatteredfew problems.
Cyanide toxins given to mares at different stages of their gestationmay not have had such severe effects either. It seems that all of the factorspresent in this situation occurred exactly as they needed to, in locationand time sequence, to cause this devastating problem.
Researchers are still not being definite, and the cyanide theory is beingcalled a "working hypothesis" but the fetal losses and abortionshave subsided and the problem seems over.
The exact extent of the economic impact is now being debated, but theentire episode has taught veterinarians and breeders a number of things.
Horses and cherry trees do not mix. Mycotoxin binders are potentiallyhelpful items that need to be investigated further and climatic conditionscan sometimes conspire to turn normal situations into deadly mysteries.