A 2002 outbreak of tick-borne relapsing fever in Montana — the first confirmation of relapsing fever in the state — has led to the discovery of a bacterium and species of tick not known previously to exist in Montana.

In the September 2003 issue of the journal Emerging Infectious Diseases, investigators at Rocky Mountain Laboratories (RML) in Hamilton, MT, and their collaborators describe this outbreak and the results of laboratory and clinical investigations.

“Given the 100-year history of research into ticks and tick-borne diseases in Montana, we are gratified to have helped solve the mystery of this outbreak and to have identified a focus of this disease as well as the bacterium and tick in our back yard,” states Tom G. Schwan, Ph.D., lead investigator of the study. RML, which began by conducting pioneering studies in Montana on Rocky Mountain spotted fever, is part of the National Institute of Allergy and Infectious Diseases (NIAID), a component of the National Institutes of Health in Bethesda, MD.

“This investigation demonstrates once again the potential for diseases to emerge unpredictably in areas where they have not been recognized previously,” notes Marshall Bloom, M.D., associate director of RML. “These findings will also alert health care providers in this region that relapsing fever bacteria may cause a recurrent febrile illness, which is curable when recognized and treated promptly with antibiotics.”

The mystery presented itself between July and August 2002, when five of 20 people became ill during or after visiting a cabin on an island in Flathead Lake in western Montana. RML investigators and tick experts Dr. Schwan and Paul Policastro, Ph.D., worked with physicians in Seattle; Montana State Epidemiologist Todd Damrow, Ph.D.; the Lake County Health Department; and the curator of the U.S. National Tick Collection to determine the cause of the outbreak.

The patients, between 5 and 54 years old, experienced a variety of signs and symptoms including high fever, headache, joint and muscle pain, vomiting, diarrhea and rash. One individual had a second onset of illness, or relapse, which gives this illness its name. Large numbers of the bacteria circulate in the blood, giving rise to recurrent episodes of illness interspersed with periods of feeling well. All the patients eventually recovered.

Blood samples from four patients who had returned home to Seattle were sent to RML. There Dr. Schwan and his group isolated and identified the cause of the illness as Borrelia hermsii, a spiral-shaped bacterium or spirochete. Drs. Schwan and Damrow then visited the cabin where the individuals had become infected. In the attic, they discovered the species of tick that is known to transmit this bacterium in other regions of the western United States.

The bacteria that cause relapsing fever are related to those bacteria that cause Lyme disease. The illnesses, however, are quite different, and the ticks that transmit relapsing fever spirochetes have a strikingly different lifestyle compared with the ticks that transmit Lyme disease spirochetes.

For example, while Lyme disease is usually acquired from ticks in an outdoor setting, most people who develop relapsing fever become infected while sleeping in rodent- and tick-infested cabins. Because the ticks feed quickly and only at night, most people are not aware of having been bitten. In nature, the bacteria — which infect rodents such as pine squirrels and chipmunks — are transmitted among these animals by the ticks that live in or near their nests.

NIAID is a component of the National Institutes of Health (NIH). NIAID supports basic and applied research to prevent, diagnose, and treat infectious and immune-mediated illnesses, including HIV/AIDS and other sexually transmitted diseases, illness from potential agents of bioterrorism, tuberculosis, malaria, autoimmune disorders, asthma and allergies.