The number of stillborn foals and spontaneously aborted fetuses from Kentucky mares has increased to a staggering rate this spring, in a mystery plague that has shot fear throughout the state?s $1.2 million thoroughbred horse industry. In April, 318 deaths were reported, in contrast to just 46 during the same period last year. Some farms have not reported any unusual miscarriages, while others have reported death rates of 10 to 75 percent.

?It?s got a lot of people spooked, no doubt about it,? says Steven Johnson, president of the Kentucky Thoroughbred Farm Managers Club. ?I?ve talked to a lot of farm owners who aren?t going to sleep very much until they find out what is going on with their mares.? So far, tests for toxins or viruses have come back negative.

?So many farms having problems over such a narrow period of time indicates that there is a common source,? says David Powell, of the University of Kentucky?s Gluck Equine Research Center. He doesn?t believe the problem is spread from one horse to another, unlike the outbreak of foot-and-mouth disease in Europe. Biopsies show that about half of the dead foals had streptococcus bacteria in their blood, but it?s believed this may be due to a secondary infection and was not the cause of death.

One theory is that the famous Kentucky bluegrass may be causing the problem. Dr. Tom Riddle, of the Rood and Riddle Equine Hospital in Lexington, says, ?It?s reasonable to assume pastures, so remove horses from pastures.? One farmer took this recommendation so seriously that he plowed under his pasture, while others are mowing them shorter than usual.

The problem may be traced to changes in the weather. Experts say that when last month?s unusually dry weather suddenly turned from cold to warm, a deadly fungus could have thrived in pasture grass. Other possible theories involve bacterial or chemical agents. Scientists from the University of Kentucky are analyzing samples of pasture and hay from affected farms. ?I can tell you I?ve seen contradictions in every theory I?ve heard,? says Roger Ollman, who has been visiting farms to collect pasture samples for study.

Scientists are looking for elevated levels of mycotoxins, that are produced by molds that grow on the surface of plants. So far, they haven?t found evidence of the most common mycotoxins, but there have been indications that a toxin called zearalenone may be the culprit. This toxin mimics the effects of estrogen and is known to cause similar problems in pigs.

A similar outbreak occurred in 1981, but affected only a handful of mares. The cause was never found.

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