Scientists at the University of Illinois say research concluded this week confirms that bacteria known as an ehrlichia causes Potomac Fever, the disease that has killed horses in the Washington area and baffled scientists for the past six years.

Dr. Jean Sessions, a Potomac veterinarian who has coordinated some of the research on Potomac Fever for the University of Illinois, said that Illinois scientists identified and cultured a kind of rickettsiae known as an ehrlichia that caused Potomac Fever when injected into a susceptible pony. Rickettsiae are very small bacteria that cause diseases such as typhus and Rocky Mountain spotted fever.

Researchers at the Virginia-Maryland Regional College of Veterinary Medicine at Blacksburg, Va., announced Tuesday that they had identified ehrlichiae as the organisms that they believed cause Potomac Fever in the colons of affected horses. A spokesman there said scientists "are in the process right now of trying to isolate, purify and culture" the bacteria.

According to Sessions, the Illinois researchers already have taken that step. "We got the organism out of the horse, grew it in a cell culture . . . and injected it back into a horse, to get the same clinical syndrome" shown by horses stricken with Potomac Fever," she explained.

"It was like seeing a handprint on a door, but not knowing how it got there or where it came from," Sessions said. "Now we've seen the hand that put it there."

Horses stricken with Potomac Fever develop high fevers and severe diarrhea, lose their appetites and become listless. Thirty percent of them die, Sessions said.

The disease has triggered anxiety among many horse owners in the Potomac area, where the illness was first recognized. Since last year, worried owners have moved 300 horses, one-third of the equine population, from the River Road section of Potomac, according to Sandie Cafritz, equine state chairman for the Morris Animal Foundation, which funds research on the disease.

Relaxing yesterday in her Rockville home, where framed photographs of her horses are scattered about the living room, Cafritz said she was happy about the news.

"It's a major breakthrough. This is what we've all been waiting for. We are excited, and we are celebrating."

Austin Kiplinger, horse owner and publisher of the Kiplinger Report who lives in Poolesville, said he is encouraged by the new research. "It certainly is a very exhilarating and encouraging development," he said.

Last year, 116 horses in Maryland and 32 in Virginia contracted the disease. More than 50 died from it.

This year, Cafritz said, 94 cases and 16 deaths have been reported in Maryland; in Virginia, six horses have contracted the disease and one has died from it.

Cafritz cautioned that while finding the cause of the disease is good news, finding a cure will demand more research and more money. Two of her own horses have contracted and recovered from the disease, but "I keep watching the one that didn't have it," she said. "I'm still taking their temperatures twice a day."

The declining number of cases and the latest research, however, may calm some owners' fears, she said. "I think next summer I'll rest easier," Cafritz said.