Researchers scoffed when veterinarian Jean E. Sessions found what she believed to be the cause of Potomac Fever, a mysterious and deadly horse disease that has infected about 500 Maryland horses -- killing nearly one out of three stricken -- since its discovery in 1979.

But nearly a year later, scientists realized that she had been right all along when they verified her findings that the causal agent of the disease was a form of rickettsia, a tiny organism that causes Rocky Mountain Spotted Fever in humans and is not uncommon in dogs.

"Everybody at that time was thinking virus" as the cause, said Sessions, 38, a small-animal veterinary practitioner in Potomac. The seasonal aspect of the disease -- it occurs mainly during the warm months of May to October -- and its symptoms of fever, diarrhea, lethargy and colic pointed strongly to virus as the causative agent.

Last year, Potomac Fever struck 116 horses in Maryland, killing 42.

Chet Anderson, a Dickerson veterinarian who conducted some of the initial research on Potomac Fever, said that it was Sessions' rickettsia suggestion that led researchers to abandon their studies of whether the disease was linked to a virus.

"The direction we seemed to be going in 1983 with virus, we were failing," Anderson said. "Jean did come up with the rickettsia possibility, and I certainly think her efforts played a significant role in finding the cause."

And on Tuesday, the Montgomery County Council passed a resolution commending Sessions for her work.

Researchers still face the tasks of determining how the disease is spread and coming up with a cure.

Sessions discovered the causal agent of the disease while examining blood smears from affected horses under a microscope in her kitchen in August 1983. But her peers did not take the findings seriously until 10 months later, when simulated tests proved positive, she said.

Last October, researchers at the Virginia-Maryland Regional College of Veterinary Medicine at Blacksburg, Va., and at the University of Illinois announced that they had identified rickettsia as the organism that they believe causes Potomac Fever in the colons of affected horses.

For her efforts, the state's veterinarian association named Sessions Maryland veterinarian of the year for 1984. The award has not been made since 1972.

Efforts to find the cause of the disease were more than a professional concern for Sessions, whose family keeps three horses and a pony on a sprawling five-acre farm near Dickerson.

A native of rural Lawrenceville, Ill., Sessions said she had known she wanted to be a veterinarian since she was 13. But her parents balked at the idea of her pursuing a professional career, and she had to seek funds elsehwere.

"I knew that if I was going to go to school, I had to study hard to earn scholarships," Sessions said. "So I did. I had scholarships all through college."

When she graduated from the University of Illinois in 1970 with a degree in veterinary medicine -- a traditionally male-dominated field -- she was one of only five women in a graduating class of 65.

After completing her studies, she joined the Army, where she became the first woman accepted into the veterinarian corps.

It was during this time that Sessions encountered a disease similar to Potomac Fever that was killing many of the Army's guard dogs in Vietnam. Working as part of a research team, Sessions determined that the dogs were dying from ehrlichia canis, a blood disease transmitted through ticks.

Many of the dogs recovered when given antibiotic tetracycline, which historically has been used to treat rickettsia, Sessions said.

The similarities between the dog disease and that affecting horses, and the fact that the Potomac region abounds in the strain of tick carrying Rocky Mountain Spotted Fever, strengthened Sessions' hunch that rickettsia was the cause of Potomac Fever. With the identification of the rickettsia, ticks have become prime suspects as carriers of Potomac Fever, Sessions said. Researchers believe that treatment and prevention are within their grasp, she said.