With MRLS Outbreak, 2000 Was Year of Loss For Breeding Industry

From the Dec. 22 issue of The Blood-Horse
The worst health problem to hit the Thoroughbred industry since CEM and EVA occurred during the spring of 2001. While there isn't any news to report on mare reproductive loss syndrome (MRLS) and the eye and heart syndromes thought to be associated with environmental conditions that set off a chain of events in Kentucky, Ohio, and other places, a new retrospective study was just released Dec. 14.

What was grouped by the industry under the heading of MRLS actually was two reproductive situations. The first was the loss/abortion of late-term or at-term gestations. Some foals were born compromised and later died. The second situation was early fetal loss.

The common denominator was environmental--below normal temperatures in March and drought conditions in early spring, followed by higher temperatures in early April and a burst of plant growth, followed by severe frosts on April 17 and 18, followed by uncommonly high temperatures. This was the same weather pattern seen in 1980 and 1981, when the same types of losses were reported, but on a smaller scale. However, these syndromes were discovered early in 2001 because of technology. Dr. Tom Riddle of Rood and Riddle Equine Hospital was performing fetal sexing examinations early in gestation and finding an unusually high number of dead or dying fetuses. He reported his findings to the state diagnostic lab on May 1.

At one point during the MRLS crisis, the number of dead foals taken to the Kentucky Livestock Disease Diagnostic Center was reported to be 700% higher than normal. Mares were having "red bag" deliveries, which means the placenta was becoming prematurely detached and being delivered before the foal, thus suffocating the already compromised foal. With early fetal loss, managers seldom find the tiny fetus, but are faced with an empty mare when reproductive examinations are performed.

In the face of MRLS, mares were undergoing ultrasound examinations on a very frequent basis.

No one knew what was causing the tremendous fetal/foal losses, although researchers at the diagnostic lab and the Gluck Equine Research Center ruled out all known contagious/infectious pathogens early in the scenario. Then there was the question of why a vast majority of farms suffered losses, but some more than others. And there were a few farms that lost no foals/fetuses at all. Rumors and theories of the cause abounded, some including the massive crop of Eastern tent caterpillars. Other theories included mycotoxins in pastures, endophytes from grasses, and cyanide from the leaves of black cherry trees (possibly associated with the caterpillars).

Due to tremendous effort and cooperation, an epidemiological survey was conducted and quickly analyzed. It found four factors associated with increased MRLS on farms surveyed:

* Breeding date in February 2001;

* Moderate to high concentrations of caterpillars in mare areas;

* Presence of cherry trees around pastures;

* Having more than 50 mares on the farm.

The last mentioned is thought to be a risk factor only because farms with more mares probably had more mares in foal early in the season during the time of the environmental impact.

The survey found only two factors associated with little or no incidence of MRLS: absence of caterpillars, and feeding hay to mares on pasture.

An association between the following factors and MRLS was not detected in this survey:

* Pasture composition (fescue, clover, orchardgrass, bluegrass, or other grass types);

* Mowing before, after, or during the frosts of April 17 and 18, 2001;

* Fall 2000 or spring 2001 fertilization with ammonium nitrate, urea, or any other fertilizer;

* Treatment with lime in 1999, 2000, or 2001;

* Manure spread on pastures;

* Presence of surface water or source of drinking water;

* Chain harrowing of fields;

* Feeding grain to mares, type of grain fed to mares, source of grain (bags, bulk source, other), grain/hay contamination by domestic or wild animals;

* Bedding type used;

* Evidence in the mares' environment of mice, rats, raccoons, skunks, coyotes, fox, deer, or opossums;

* Mares' contact with cattle, sheep, goats, poultry, pigs, deer/elk, dogs, or cats; and

* Use of various dewormers.

The researchers said one factor warranted further investigation: While nearly all farms observed waterfowl at least sometimes, a higher percent of farms with early fetal loss reported observing waterfowl "often."

Attempts to recreate the syndrome proved unsuccessful by year's end. Cyanide, thought to be the lead culprit after it was found in tissues from three aborted fetuses, proved difficult to test, and no conclusions had been reached at the end of 2001 whether it was the cause or not.

What researchers have concluded is that this syndrome was probably caused by a combination of factors, which will make it tough to nearly impossible to reproduce in the laboratory. The good news is that the combination of factors also is unlikely to be replicated in nature.

The University of Kentucky College of Agriculture has hired someone to be in charge of next year's pasture monitoring. As one researcher said, "Kentucky will have the best-monitored fields in the world in 2002!"