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Common Source Outbreak of Relapsing Fever -- California
In August and September 1989, six persons who had each at different times spent the night in the same cabin at Big Bear Lake, San Bernardino County, California, developed sudden onset of high fever with severe headache, prostration, nausea, and vomiting. Four patients were initially considered to have viral gastroenteritis and received no antibiotic therapy. When fevers recurred, relapsing fever was suspected and confirmed serologically in two of these patients. Two of the four patients were treated with tetracycline and recovered. Two recovered without treatment after three relapses.
The fifth patient, an elderly woman, was admitted to intensive care with septic shock. Laboratory findings included spirochetes visible in a Giemsa-stained smear of peripheral blood, neutrophilic pleocytosis of the cerebrospinal fluid, peripheral leukocytosis, thrombocytopenia, and hypophosphatemia. She was treated with intravenous doxycycline and developed a probable Jarisch-Herxheimer reaction before recovering fully.
For the sixth patient, the suspected diagnosis of relapsing fever was confirmed by the identification of spirochetes in a thick blood smear. This patient was treated promptly with tetracycline and recovered rapidly.
The Santa Barbara and San Bernardino county health departments were advised of the outbreak and conducted an epidemiologic and environmental investigation of the Big Bear Lake area and the cabin used by each of the six patients. The investigators noted a large population of California ground squirrels and chipmunks and the widespread practice by visitors of feeding these animals. Inhabited ground-squirrel burrows were found under the cabin. The cabin was fumigated to kill vectors (ticks). No other cabins were known to be associated with illness. In the spring of 1990, educational literature about relapsing fever and prevention of the disease was distributed to cabin owners in the Big Bear Lake area. Reported by: WC Koonce, MD, S Abbott, MD, S Hosea, MD, E Hunt, MD, W Morton-Smith, MD, Santa Barbara; L Harris, Santa Barbara County Dept of Health; San Bernardino County Depts of Public Health and Environmental Health, California. Bacterial Zoonoses Br, Div of Vector-Borne Infectious Diseases, Center for Infectious Diseases, CDC.
Editorial Note: In the United States, relapsing fever results from infection by the spirochetes Borrelia hermsii and B. turicatae. The disease, which occurs in the western United States, is transmitted to humans principally by the bites of the infected ticks Ornithodoros hermsi and O. turicata but possibly also by O. parkeri and O. rudis (1-6). These ticks normally inhabit the burrows and nests of rodents in which the natural infection cycle proceeds without apparent disease in the rodents.
Humans are incidental hosts when bitten by an infected tick. Ornithodoros vectors are reclusive night feeders and bites often go unnoticed. Cabins in wilderness areas are attractive nesting sites for potentially infected rodents, particularly when food is made available by cabin users. Ornithodoros spp. infestations of rodent nests in cabins have been associated with outbreaks reported in Spokane County, Washington, in 1968 (7) and in the Grand Canyon National Park in 1973 (8).
The clinical features in the patients in this report are unusual in the prominence of gastrointestinal symptoms and severity of illness. The evidence of meningeal inflammation in the fifth patient is of interest, given the known predilection for the nervous system of the related spirochete, B. burgdorferi, the causative agent of Lyme disease. The severe headaches associated with relapsing fever may reflect meningeal involvement as a relatively frequent event (9,10).
The relapsing nature of this illness is thought to be associated with the presence of antigenic variants. As an immune response develops to the predominant antigenic strain, variant strains multiply and cause a recrudescent infection. The most rapid diagnostic test is identification of spirochetes on a Giemsa- or Wright-stained thick or thin smear of peripheral blood obtained during a febrile episode. As suggested by the clinical histories in this report, tetracyclines are effective in terminating this infection (9).
Prevention of relapsing fever consists of avoiding likely tick habitats or, when this is not possible, reducing the risk of tick bites by the use of repellents or acaricides. Additional measures include fumigating nesting sites in human habitations, "rodent-proofing" buildings in endemic areas, and eliminating rodent access to unnatural food sources.
Cases of relapsing fever should be reported to local and/or state health departments. The frequency with which relapsing fever cases occur in clusters related to a single location makes an environmental evaluation of an individual case a mandatory public health preventive measure. CDC's Bacterial Zoonoses Branch, Division of Vector-Borne Infectious Diseases, Center for Infectious Diseases, requests that B. hermsii- and B. turicatae-infected ticks, B. hermsii and B. turicatae isolates, and serum samples from patients with documented infections from B. hermsii and B. turicatae be submitted for its International Borreliosis Reference Collection. These items should be sent to the Bacterial Zoonoses Branch, CDC, P.O. Box 2087, Ft. Collins, CO 80522; telephone (303) 221-6400.
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7. Thompson RS, Burgdorfer W, Russell R, et al. Outbreak of tick-borne relapsing fever in Spokane County, Washington. J Am Med Assoc 1969;210:1045-50.
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