A team of scientists today are reporting the discovery of a tick-borne disease -- neither Rocky Mountain spotted fever nor Lyme disease -- that killed two people and afflicted 10 others in northern Wisconsin and Minnesota between 1990 and 1993. The disease is caused by bacteria, called ehrlichia, that also infect a variety of animals including dogs, sheep, cows and goats. The veterinary diseases may be caused by different strains.
Although 12 cases are being reported in today's Journal of the American Medical Association, researchers say they believe they have discovered about 20 other cases, including a fatality in Connecticut and a non-fatal case in Florida. The remaining patients, none of whom died, were from the two Midwestern states.
Patients developed a high fever, severe headache, muscle aches and frequently suffered from nausea and diarrhea -- making the disease easy to confuse with influenza and common viral illnesses.
"You could be sick with it and not even realize you have it," said Dr. J. Stephen Dumler, a pathologist with the University of Maryland Medical Center and one of the scientists reporting the discovery.
"Unless you know you've been bitten by a tick, you may suspect that all you have is the flu."
Doctors with the Duluth Clinic in northern Minnesota became suspicious when they saw a succession of patients who had been sick longer and suffered more serious symptoms than do most flu patients. Most striking was that many patients recalled being bitten by ticks days or weeks before falling ill.
"Our consciousness was raised by the fact that they resided in places with lots of tick activity," said Dr. Mark R. Eckman of Duluth. Counties of northeastern Minnesota and northwestern Wisconsin have long been known for their ticks and have had a fairly high incidence of Lyme disease.
In 1990, the Minnesota doctors began sending blood samples to the University of Texas Medical Branch in Galveston, where Dr. Dumler worked before moving to the University of Maryland. He has continued his tick studies in Baltimore.
Scientists identified the tick-borne ehrlichia as the culprit by examining the genetic material of bacteria found in the patients' blood. They used a revolutionary technique called PCR (polymerase chain reaction), which has been available only in the last few years.
"It's not at all clear how significant a threat this is," Dr. Dumler said. "We don't know if we're scratching the tip of the iceberg or if we've seen all the cases we're likely to see."
Scientists are also unsure if the human form of ehrlichiosis is truly a new disease or if it has been striking people for many years without anyone recognizing it as a distinct illness. Also, they don't know what type of tick spreads it or whether more than one variety may be responsible.
The investigators, however, say they strongly suspect that the tick responsible for Lyme disease -- the deer tick -- is at least partly to blame for transmitting ehrlichia to humans. This is because the "new" disease has appeared in an area where Lyme disease is prevalent. Also, many people diagnosed with ehrlichiosis also have antibodies to Lyme disease.
People with ehrlichiosis do not develop a rash. In contrast, people with Lyme disease develop a red dot that gradually expands into a reddened area several inches across. Most people with Rocky Mountain spotted fever develop small pink spots on the wrists and ankles that later spread over the body.
Ehrlichia infects white blood cells, hampering the body's ability to fight other infections. The patients who died succumbed to infections that do not often cause severe illness in people with healthy immune systems.
These secondary infections -- yeast in one case, and both herpes and cryptococcus in another -- are often seen in AIDS patients. Unlike AIDS, which is believed to be universally fatal, the tick-borne disease is not usually fatal.
Dr. Dumler said ehrlichiosis responds readily to the antibiotic tetracycline. The same drug is used often to cure Rocky Mountain spotted fever. And while it is not the first line of defense against Lyme disease, it is an effective weapon against it.