LEXINGTON, Ky. -- The death rate from the mysterious illness killing foals on Kentucky's thoroughbred farms has slowed to a trickle, and researchers and horsemen are optimistic that the worst of the crisis is past.

Only 24 deaths have been reported since Sunday and only four in the past two days. More than 500 deaths were reported between April 28 and May 19.

"It's slowed down tremendously," said Dr. Bill Bernard, internal medicine specialist at Rood and Riddle Equine Hospital. "It's near the end of the breeding season, so there's fewer mares left to foal, but it's still encouraging."

David Powell, equine epidemiologist for the University of Kentucky's Gluck Equine Research Center, said he feels safe advising owners and farmers to put their horses back out to pasture with no fear of the illness reappearing.

Scientists concede it may be too late to find the cause of what they named Mare Reproductive Loss Syndrome. They have come up with several theories about the culprit, including a fungus or toxin in pasture grasses or poisonous secretions from a caterpillar. But they have no hard evidence to back up any hypothesis.

"I think that whatever it is we're looking for has already left town," said Gus Koch, manager of Claiborne Farm.

Koch was expecting a much different atmosphere Thursday night when researchers were due to meet for the second time with breeders, farmers and owners at Keeneland.

"A couple of weeks ago, we were all very nervous, because we couldn't see what was on the horizon. It was scary," Koch said. "But things have stabilized since then and everybody's taken a slower, more reasonable approach to everything.

"I'm not saying everybody's relaxed now. I'm just saying this situation is definitely winding down its course."

Powell said even if the cause is never pinned down, the crisis has had some positive effects.

"Right now, there's just very little data about horses and the risk of exposure to these various toxins," he said. "What we're into at the present time is very pioneering. We're generating much of this information for the first time as we go along.

"We're going to continue studying this problem until we find an answer and beyond. No matter what happens from here on out, I don't think anyone will be caught off guard by something like this ever again."

Farmers don't expect another outbreak any time soon.

"This was a very rare happening. If our climate or our grasses or our methods were so fickle that we were going to have problems like this, we would've been out of business a long time ago," Koch said. "I'm not at all worried about this happening all over again."

David Switzer, executive director of the Kentucky Thoroughbred Association, said Wednesday farmers still have hopes of salvaging this foaling season, which runs through June.

"We're not even thinking about next breeding season. We're still focusing right now on getting mares in foal for the 2002 crop," Switzer said. "We'll keep proceeding forward and look for whatever the cause of this is. Once we do that, we'll all march right into 2002 with all the confidence in the world that this is not going to return."