As feared earlier this year, for the fifth consecutive summer the mysterious disease known as "Potomac Fever" has been killing horses in Montgomery County. In 1982, the first year records were kept, there were 113 reported cases, 28 of which were fatal.

So far this summer, said Dr. Ralph Knowles, assistant state veterinarian, there have been at least 22 cases and six deaths, all in Montgomery.

"People are quite jittery. It's the fear of the unknown," Knowles said. "Especially at this point in the season, people are asking, 'should we load our horses up and take them away from the river?' The answer is we don't know."

On Monday, Rhea Union was in stable condition after a weekend spent fighting the disease.

The large, dark brown 3-year-old brood mare had severe colic last Thursday and began running a high fever Friday, according to Sharon Dean, manager of Liberty Stables in Dickerson, five miles east of the Potomac River.

Sunday night, with her 2-month-old foal by her side, Rhea Union had a relapse, despite the efforts of her boarders, who had built a special air-conditioned stall, and her veterinarians, who are checking the horse's condition several times a day and administering intravenous mineral water and Pepto-Bismol.

Scientists are at a loss to identify the cause or the cure for AEDS--acute equine diarrhea syndrome, the formal name for the fever.

Investigators say it strikes horses of various breeds and ages equally, and occurs in hot, humid weather, from mid-June to mid-September.

It was first documented in the Potomac-Seneca Creek area in 1979, when eight horses contracted the illness and two of them died.

Typically, the disease delivers a deathblow within three or four days. When afflicted, Knowles said, a horse will stop eating, act depressed (hanging its head, with its ears flopping and eyes dull), run a fever of 103 to 105 degrees (99.5 is normal).

Next the white blood cell count drops drastically and the horse experiences "acute explosive type diarrhea and dehydrates quite rapidly."

Then the horse goes into shock and dies.

This summer, Knowles and three others in the Maryland Department of Agriculture are working on the problem full time.

To help local efforts, a student from the University of Pennsylvania School of Veterinary Medicine, Peter Rakestraw, and a specialist from the Virginia-Maryland Regional College of Veterinary Medicine at Blacksburg, Va. also have been recruited.

Dr. Al Strating of the National Animal Disease Laboratory in Ames, Iowa, said the lab became aware of Potomac Fever last year.

Strating confirmed that the disease is localized in the Maryland and Virginia area, although there have been cases reported in the Susquehanna River Valley in Pennsylvania.

"It's still pretty much a mystery," Strating said. The laboratory sent a diagnostician here for a week last year, to perform tests in local labs, but Strating said there are no plans to send a staff member this year unless local animal health officials request help.

"Everyone is grasping at straws," Knowles said. Researchers are looking at the possibility that the disease is a virus, and studying certain types of bacteria in the digestive track of horses, as well as the possibilities that insects might be carriers and that there may be poisonous plants in pastures.

"So far a finger can't be put on one single cause," said Dr. Wayne Shipley, state field veterinarian. "Why do we have endotoxemia a syndrome implicated in equine colic, laminitis and diarrhea, the three leading causes of death in horses working in just one area? What is working in concert with this?" Shipley asks.

He noted this year's cases have involved some younger horses than in the past. But the disease strikes horses "on breeding farms, horses who have never been off the farm, those who are showing, those in pasture, those never in pasture, across the board.

"It's kind of like AIDS or Legionnaires Disease," he said. "There are no quick answers."