Because of an editing error, an article on Potomac Horse Fever in the Jan. 25 Metro section indicated that all the information came from an article in the journal Science. Some, however, came from interviews with the researchers studying the disease.

Potomac horse fever, the often fatal mystery disease that has become an increasingly serious epidemic every summer in Maryland and Virginia since its discovery in 1979, may soon be a thing of the past.

Veterinary researchers at the University of Illinois, who isolated and identified the germ that causes the disease last year, said in an article to be published today that they have developed a fast and reliable diagnostic test for the disease and identified an existing antibiotic, tetracycline, that appears to be an effective treatment.

The researchers also said a vaccine to prevent Potomac fever could be produced within two years if the demand is great enough.

Potomac fever, which is a blood disease, was first recognized in 1979 among horses in the Potomac area. It causes fever, severe diarrhea, loss of appetite and listlessness. About a third of the afflicted animals die.

Although Maryland horses have been the hardest hit -- last year there were 109 cases and 18 deaths -- the disease has also turned up in Virginia, Pennsylvania and a scattering of other states across the country and in Europe. About 600 animals have been affected, including some valuable racehorses.

Identification of the cause -- a microorganism called a rickettsia that is probably carried by ticks -- has revealed that the microbe is related to another rickettsia that causes disease among humans in the Orient. The researchers say the horse germ might be capable of infecting people.

Rickettsiae are microbes that resemble bacteria but are much smaller although they are still larger than viruses. Rickettsiae, which are usually transmitted by the bites of fleas, ticks and mites, are the cause of Rocky Mountain spotted fever and typhus.

Although word of the rickettsia finding emerged in October, a formal report by the scientists is being published only today, in the Feb. 1 issue of the journal Science.

The discovery was made by Cynthia J. Holland, a doctoral student at the University of Illinois College of Veterinary Medicine at Urbana. She worked in collaboration with Miodrag Ristic, a specialist in the blood diseases of animals at Illinois.

Proof that the microbe causes the disease was established by fulfilling the rigorous requirements known as Koch's postulates, which are that the suspect microbe must be found in all cases of the disease, that the microbe must be grown in a pure culture in the laboratory and that the lab-grown organism must cause the disease when given to a healthy animal or human subject.

Although a Potomac veterinarian, Jean Sessions, had suspected a rickettsia was the cause, it was not until the Illinois scientists grew the organism in the laboratory and used it to induce the disease in ponies that her suspicion was confirmed.

Holland said she has found that the Potomac fever rickettsia can also infect dogs but that it causes no symptoms.

"It's possible, though," Holland said, "that dogs can be carriers. The ticks bite them, pick up the rickettsia and then give it to horses. Now, ticks bite people, too, and we think it's worth checking whether people might be getting sick."

Holland said that a very close relative of the Potomac fever rickettsia is a known cause of human disease in Japan and Southeast Asia. Although it is believed to be seldom fatal, the disease causes severe symptoms that resemble those of the flu and mononucleosis.