Published in the Oct. 5 isse of The Blood-Horse
Most of the recent mare reproductive loss syndrome news has focused on the Eastern tent caterpillar. However, two researchers--Dr. Tom Swerczek of the University of Kentucky's Department of Veterinary Science, and Jennifer Taylor of the Equine Sports Medicine Laboratory in Melbourne, Australia--believe pastures are at the root of the problem. Both reported on their work in a recent edition of the Journal of Equine Veterinary Science.

According to Swerczek, MRLS is the result of a series of events that begins when a freeze or frost damages lush, fast-growing grass and causes acute changes in the plant's electrolytes and minerals. Chloride, potassium, and sugars increase; calcium decreases. These responses make the plant more nutritious but also turn the grass into an ideal medium for fungi to grow. The fungi break down the grass even further, enhancing the level of its nutrients.

When a horse eats the damaged grass, "it's like he's broken into a grain bin," Swerczek said. "There is a massive change in the dietary factors."

The change causes a population explosion among microorganisms found naturally in the horse's gut. These microorganisms get in the circulatory system, causing abortion in mares, pericarditis, and other problems associated with MRLS. Swerczek believes Streptococcus bacteria play an important role in the disease's development.

Swerczek developed his explanation from necropsy examinations and numerous field observations. His work dates back to the early 1980s, when symptoms similar to MRLS were seen in Central Kentucky.

In addition, Swerczek believes he knows how to prevent MRLS. His protocol involves feeding mares loose salt on a free-choice basis to counteract the elevated potassium and chloride. He also recommends reducing the plane of the mares' nutrition by decreasing protein and carbohydrates in their diet.

Dr. Alan Dorton of the Woodford Veterinary Clinic in Versailles, Ky., said he has successfully used Swerczek's suggestions to prevent MRLS. Dorton's program involves three steps. He feeds free choice loose salt to mares. Then, in the last month of their pregnancies, he reduces the amount of grain they are fed drastically. The ration for each mare, he estimated, is decreased from "six to eight pounds a day to a half-pound." He also limits the exposure to pasture after a frost to three or four hours a day in the afternoon.

In 2002, Dorton said clients at three farms followed his prevention program, and their rate of MRLS "was less than 3%." One of the farms in the group did some spraying to kill caterpillars after they began to migrate, but the other two did not take any measures to control the insects. Another farm experienced no problems with the disease after working hard to eradicate caterpillars and implementing the veterinarian's MRLS prevention program. This led Dorton to believe caterpillars may play a role in the disease, but are not its primary cause. He also reported that one farm did nothing to control caterpillars and did not implement his prevention program. The nursery had an MRLS rate of 30%.

In her article that appeared in the Journal of Equine Veterinary Science, Taylor described a cascade of events leading to MRLS that was much like Swerczek's explanation. However, she took her investigation one step further and tried to identify the actual toxin that is produced from all the changes that occur in a mare's body after she eats freeze- or frost-damaged grass. Her studies indicated the likely culprit is ammonia.

According to Taylor, improved pastures and high-protein diets provide horses with a large nitrogen load. Normally the horse's body is able to cope with this, resulting in maximum growth of the fetus in mares. However, frost that affects pastures at a time of maximal spring growth causes "chemical changes in the grass from the lack of photosynthesis. This in turn changes the distribution of the normal bacterial gut flora and results in the formation of excessive nitrate and ammonia levels in the hindgut, which are absorbed into the horse and result in a severe nitrogen overload. When nitrogen intake or turnover in the body exceeds the ability to cope with excretion through urea formation in the liver, the excess nitrogen will form ammonia." She reported cattle and sheep studies have shown ammonia is responsible for embryonic losses.

In the spring of 2001, Taylor examined about 80 mares in Kentucky. Most that had lost their pregnancies were clinically normal in other aspects, so a variety of tests were performed. The only consistent abnormal finding involved high levels of blood urea nitrogen (BUN). This January, Taylor studied 24 pregnant mares, measuring the ammonia levels in their blood. The mares with higher levels of ammonia experienced abnormalities consistent with MRLS when they foaled. Mares with lower levels of ammonia had normal deliveries.

The University of Kentucky's Maxwell H. Gluck Equine Research Center coordinates much of the research on MRLS and has emphasized the caterpillar in the studies it has made public. However, Gluck director Peter Timoney said the center's scientists recognize that multiple factors are probably involved. Asked about Swerczek's work, Timoney responded: "He has advanced some very interesting ideas and hypotheses; they bear further review, and they need to be proved with controlled studies."

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