St. Louis, July 15, 1999 – Researchers have discovered a new form of a tick-borne disease that can be fatal in humans. This finding provides more insight into ehrlichiosis, first identified in humans in the United States in 1986.
A bacterium called Ehrlichia ewingii causes ehrlichiosis in dogs, cattle and other animals. But in the past few years, four people exposed to ticks in Missouri have contracted the disease from this bacterium.
"We want people to know that ehrlichiosis is in Missouri, Arkansas and southern Illinois," said Gregory A. Storch, M.D., professor of pediatrics at Washington University School of Medicine in St. Louis. "If someone is bitten by a tick and feels sick afterward, he or she should contact a physician as soon as possible and make sure the physician knows about the exposure."
The researchers report their findings in the July 15 issue of The New England Journal of Medicine. The lead authors are Storch and Richard S. Buller, Ph.D., research instructor of pediatrics.
The symptoms of ehrlichiosis resemble the flu, and they include fever, malaise, headache and muscle and joint pain. They usually appear 7 to 10 days after a tick bite. Dog ticks, deer ticks and the Lone Star tick are known to spread the disease.
Ehrlichiosis can be successfully treated with antibiotics. But someone who does not receive treatment can develop serious liver and lung problems that can cause organ failure.
Two other Ehrlichia species previously have been identified in the United States. E. chaffeensis, first detected in 1986, also is endemic in Missouri. Infecting a special white blood cell called a mononuclear cell, it causes human monocytic ehrlichiosis. In 1994, a new form of human ehrlichiosis called human granulocytic ehrlichiosis surfaced. This form involves a white blood cell called a granulocyte.
Storch’s laboratory, in the early 1990s, began using a molecular technique called polymerase chain reaction (PCR) for rapid confirmation of E. chaffeensis. After the agent of human granulocytic ehrlichiosis was discovered in Minnesota in 1993, the laboratory redesigned the PCR assay to detect all known Ehrlichia species. From 1994 to 1998, the researchers used a broad-range PCR assay designed to detect all known Ehrlichia in blood samples from patients suspected of having ehrlichiosis.
Altogether, they tested samples from 413 patients, and a total of 60 samples were positive for Ehrlichia. Although the broad-range assays for the disease were positive, the two assays specific for E. chaffeensis and the bacterium that causes human granulocytic ehrlichiosis were negative in four patients.
They pondered this puzzle and eventually realized they needed to determine the nucelotide sequence of the bacterial DNA they were detecting with PCR. Consulting a database of DNA sequences, Max Arens, Ph.D., research assistant professor of pediatrics, discovered that the mysterious sequence matched DNA from E. ewingii, which was known to infect dogs.
"We realized right away that we were on to something very interesting – that this bacterium previously known to cause illness in dogs had been discovered in humans," Storch said.
All four patients with E. ewingii were male and had an illness with fever during the summer months of various years. They ranged in age from 11 to 65 years, and all reported having been exposed to ticks and having had contact with dogs before their illness. Three of the four patients were receiving immunosuppresive therapy for other conditions, and all four had low platelet and white blood cell counts. After receiving antibiotic treatment for ehrlichiosis, they recovered. In two of the patients, Ehrlichia organisms were visible in granulocytes, as in dogs with E. ewingii.
The investigators do not know whether E. ewingii infection in humans, like many emerging infections, is a new phenomenon or merely a newly recognized one. Because E. ewingii is closely related to E. chaffeensis and both are found in Missouri, it's possible that previous cases of E. ewingii have been misdiagnosed as E. chaffeensis.
Regardless, Storch and colleagues believe it's important for the public and physicians to be more aware of ehrlichiosis. He said some physicians still do not recognize the disease when their patients have classic symptoms. "It's important for physicians to be thinking about ehrlichiosis and to be suspicious so they can start the appropriate treatment," Storch said. "It can make the difference between life and death."
In addition to giving the public more information about ehrlichiosis, this finding will help researchers learn more about the disease, its relationships to animals and ticks and how people get infected. "The whole Ehrlichia story is rapidly developing, and this is one new chapter about a very important emerging infection," Storch said.
The full-time and volunteer faculty of Washington University School of Medicine are the physicians and surgeons of Barnes-Jewish and St. Louis Children's hospitals. The School of Medicine is one of the leading medical research, teaching and patient care institutions in the nation. Through its affiliations with Barnes-Jewish and St. Louis Children's hospitals, the School of Medicine is linked to BJC Health System.
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