A mysterious disease that first began killing horses in Montgomery County four years ago has been reported in nearby counties, a group of county horse owners was told last week.

The disease--veterinarians call it "acute equine diarrhea syndrome," the media have dubbed it "Potomac Fever," and horse owners call it "The Crud"--has not been identified.

It strikes during hot, humid weather, from mid-June to mid-September, and was first documented in the Potomac-Seneca Creek area of Montgomery County in 1979, when eight horses contracted the disease and two of them died. From that beginning, the disease has spread rapidly, both in the number of horses it has stricken and in geographic distribution, veternarians say.

Several cases were reported in the past year in Loudoun, Frederick and Howard counties. Outbreaks of a seemingly similar disease were reported at a race track in southern Ohio last summer and in the Susquehanna River valley of Pennsylvania, according to Wayne Shipley, Maryland state field representative for the department of agriculture.

Beallsville veterinarian Chet Anderson, who has identified and treated most of the cases in Montgomery, last week told about 250 horse owners about the outbreaks in neighboring counties at a meeting sponsored by the Potomac Valley Dressage Association.

In 1980, the second year of outbreaks in Montgomery, about 20 cases were reported, including five that resulted in death. Anderson said that by the summer of 1981, when 12 horses died among 36 known cases, "we started getting nervous." Last summer, the disease came to the attention of the National Animal Disease Laboratory in Ames, Iowa, as efforts were intensified to identify the cause.

Last summer, 113 horses came down with the disease in Montgomery, and 36 died. Over a four-year period, Anderson said, 55 of the 175 horses afflicted with the disease have died from it--a mortality rate of 31 percent, he said.

At the onset of the disease, a horse usually goes sharply off its feed and appears lethargic. Its temperature shoots up quickly to between 102 and 105 degrees (100 is normal for adult horses) and it has "angry" gut sounds, said Damascus veterinarian Peter Radue. Within two days, profuse diarrhea strikes and the animal either "gets well, or goes downhill in days," Radue noted. "The course of the disease is extremely rapid."

Because a horse can be dead almost overnight, playing a waiting game with a horse who backs off his feed even a little can be fatal, veterinarians say. They said the horse should be examined immediately, and the veterinarian should:

* Perform a blood test. AEDS victims consistently exhibit a low white-cell count at the onset of the disease.

* Check the horse's gums. A marked blue line along the edge of the gums is evidence that the disease is about to break.

Because no one knows what is causing Potomac Fever, veterinarians can only nurse the sick animal through the acute phase of the disease with intravenous fluids to combat the debilitating dehydration that often accompanies diarrhea. "There is no universally successful treatment," Anderson said.

Investigators at both the facility in Iowa and the state agricultural office in Beltsville have been stymied in their efforts to pinpoint the disease-causing agent. They have suspected everything from sludge contamination from a plant near Dickerson on the Potomac River, moldy feed and worm medicine to salmonella and insect-borne viruses.

Thus far, they've run into nothing but epidemiological dead ends. The disease does not appear to be contagious; it usually strikes only one or two horses on a given farm.

Asked if victims shared any common denominator of age, sex, breed or level of fitness, Anderson said that of the 113 cases reported in 1982, there has been a "magnificent cross section" of the horse population at large. The disease does not appear to discriminate between "backyard" horses and those stabled at more elegant facilities. Still, fear of contagion could put a significant damper on area horse activities this summer in Montgomery County, owners indicated at last week's meeting.

Last summer, for instance, attendance was down at an East Coast championship meet at the Potomac Horse Center, a large horse show facility, because owners were afraid to bring horses into this area, owners here said.

State field veterinarian Shipley said he suspects that endotoxemia--a syndrome recently implicated in equine colic, laminitis and diarrhea, three leading causes of death in horses--may also be present in animals with Potomac Fever. Endotoxemia, literally "inner poison," is the result of runaway bacteria production in the gut, he said. When the bacteria start to die off in large numbers, they become toxic to the horse's system.

When Shipley told the owners that he knew of no way to test for endotoxemia, horse owner Noel Buterbaugh of Ijamsville, Md., said that his company, M.A. Bioproducts of Walkersville, Md., manufactures a product used to test for contamination by endotoxins.

The product, called LAL, is produced from the blood of horseshoe crabs, which has a unique clotting property in the presence of endotoxins. Thousands of adult crabs are pulled from the waters off the Eastern Shore, trucked in saltwater tanks to Walkersville where they "donate" blood, and are returned to the ocean unharmed.

Buterbaugh, whose family owns several pleasure horses, has offered to donate enough LAL to the state of Maryland to test 100 horses.

Shipley plans to collect blood samples from all horses that get the disease to see if there is any common link among them, and he is urging horse owners to "set up a plan" to deliver horses to the University of Maryland lab in College Park for post-mortem exams within a few hours of death.