The polo horse named Roan Mare took sick last month. The 11-year-old fell down and developed severe diarrhea; her temperature rose six degrees above normal. For two-weeks, barn manager Iris Schoenseld tended the mare, feeding her an antacid, walking her when she developed cramps, sleeping in the barn with her at night until she recovered.

Roan Mare is one of about 40 horses in Montgomery County, where horse ownership is popular, that contracted a mysterious summertime ailment this year. The disease has baffled veterinarians and scientists for four years while alarming horse owners. Nicknamed "Potomac Fever," it has killed at least seven horses this year around Poolesville in upper Montgomery County and in lower Frederick County.

In each case, the symptoms are the same: the horses fall, contract severe diarrhea and cramps. The diarrhea dehydrates the horses; Roan Mare had to be given about 1,800 quarts of water, Schoenseld said.

"It's been here four years" in the summer months "and we still have not been able to isolate the agents that are causing this problem," said Dr. J.C. Shook, the state veterinarian. "We've checked out just about everything we can find in the textbooks."

Shook said the disease strikes between June 15 and Sept. 15. The list of suspects includes toxic plants, a disease-carrying insect, and an unknown bacterium.

No race horses have been affected, Shook said, only pleasure horses and horses bred for showing.

State agricultural experts have brought in an outside investigator, a pathologist from the National Animal Disease Laboratory in Ames, Iowa, who will spend about a week here, Shook said. They are also checking with other states, to see if there have been similar unexplained disease outbreaks.

There are estimated to be thousands of horses on farms and estates in upper Montgomery County, but Shook cautioned against an overreaction since, he said, the disease is not acutely contagious.

But some horse owners are concerned, particularly since this summer, for the first time, the disease has affected horses in lower Frederick County. "It's sort of creeping, creeping up the [Potomac] river," said Bobbie Leiberman, senior editor of Equus magazine, a horse home health care journal. "There's a lot of concern now among horse owners about going to shows in [Montgomery] county."

Leiberman estimated that "sale of Pepto-Bismol is at an all-time high in the Poolesville area. That's the thing people are giving to their horses. It [the disease] has a very acute and rapid onset."

"It hits all of a sudden," said Schoenseld, the barn manager. "There's been about 32 in our area alone that's had it this summer. It's just a summertime thing. It leaves with the first frost."

Schoenseld said three of the 11 horses she tended were killed by lightning in June. She was down to eight horses when Roan Mare took sick. "If I had lost another one, I would have had to hang it up," she said.