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Background Ehrlichiosis due to Ehrlichia chaffeensis usually occurs sporadically or in small clusters, with an annual incidence estimated at 3 to 5 cases per 100,000 population in areas of endemic disease. The putative principal vector is the Lone Star tick (Amblyomma americanum). We investigated an outbreak of ehrlichiosis that occurred in June 1993 among members of a golf-oriented retirement community (community A) in Tennessee. The community is densely wooded and borders a wildlife-management area where deer are numerous.
Methods We conducted a case–control study, using medical-history reviews, serologic testing, and testing with the polymerase chain reaction for E. chaffeensis infection. We also surveyed a sample of 10 percent of the households in community A and in another golf-oriented community (community B) more than 20 miles (32 km) from the wildlife-management area. Survey participants completed a questionnaire and provided specimens for serologic testing. In both communities, searches for ticks were undertaken.
Results Eleven cases of symptomatic ehrlichiosis were identified in the case–control study, 10 of which were in community A (attack rate, 330 per 100,000). Of 311 surveyed residents of community A, 12.5 percent had serologic evidence of past E. chaffeensis infection, as compared with 3.3 percent of 92 in community B (relative risk in community A as compared with community B, 3.9; 95 percent confidence interval, 1.2 to 12.2). The risk of infection was associated with tick bites, exposure to wildlife, golfing, and among golfers, retrieving lost golf balls from the rough. Persons who never used insect repellent were more likely to have had infection than persons who did. In community A, thousands of Lone Star ticks were found; in community B, only three ticks were found.
Conclusions The high rate of E. chaffeensis infection in community A resulted from its proximity to a wildlife reserve. When outdoor recreational activities are common and concentrations of ticks are high, outbreaks of arthropod-borne zoonoses can be anticipated.
Source Information
From the Epidemic Intelligence Service, Division of Field Epidemiology, Epidemiology Program Office (S.M.S.), and the Division of Viral and Rickettsial Diseases, National Center for Infectious Diseases (J.E.D., J.E.C., K.L.B., J.S.), Centers for Disease Control and Prevention, Atlanta; Vanderbilt University School of Medicine (W.S.) and the Tennessee Department of Health (R.H.H.), both in Nashville; the Department of Entomology and Plant Pathology, University of Tennessee, Knoxville (R.R.G.); and the Tennessee Valley Authority, Land between the Lakes, Golden Pond, Ky. (M.L.K.). Presented in part at the 43rd Annual Epidemic Intelligence Service Conference, Centers for Disease Control and Prevention, Atlanta, April 18, 1994, and the 32nd Annual Meeting of the Infectious Diseases Society of America, Orlando, Fla., October 9, 1994.
Address reprint requests to Dr. Schaffner at the Department of Preventive Medicine, A-1124 MCN, Vanderbilt University School of Medicine, Nashville, TN 37232-2637.
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