title: Prevalence of seropositivity to spotted fever group rickettsiae and Anaplasma phagocytophilum in a large, demographically diverse US sample.
abstract: BACKGROUND
  Most epidemiologic studies of tick-borne rickettsial diseases in the United States are small and have limited demographic scope, making broader risk assessment difficult.
  
  
  METHODS
  We conducted a seroprevalence study of spotted fever group rickettsiae and Anaplasma phagocytophilum, the agent of human granulocytic anaplasmosis. Specimens were selected randomly from the Department of Defense Serum Repository for 10,000 diverse military personnel at various stages in their careers who were serving with active duty status in 1997. Antibody testing included enzyme-linked immunosorbent assay for Rickettsia rickettsii and A. phagocytophilum, and Western blot confirmation for A. phagocytophilum. Risk factors were assessed using logistic regression.
  
  
  RESULTS
  Subjects were mostly male and young and were diverse ethnically and geographically. Spotted fever group rickettsiae seropositivity was 6.0% (95% confidence interval, 5.5%-6.4%). In univariable logistic regression, seroprevalence was significantly higher among older subjects, men (6.5%, compared with 3.3% among women), black individuals (8.7%, compared with 5.6% among white individuals), subjects from states with above-average Rocky Mountain spotted fever incidence, and subjects in ground combat specialties. Associations remained significant in multivariable analysis for age, sex, black versus white race, home state with high incidence, and ground combat specialty. Among 696 subjects with serum samples obtained within 7 days after entering the military, the rate of seropositivity was 3.4% (95% confidence interval, 2.1%-4.8%). Seroprevalence was nonsignificantly lower in men (3.4%, compared with 3.7% in women ) and in black individuals (3.4%, compared with 4.1% in white individuals). A. phagocytophilum seropositivity, as determined by by enzyme-linked immunosorbent assay and Western blot, was 2.6% and 0.11% (95% confidence interval, 0.05%-0.18%), respectively. Western blot seropositivity was not significantly associated with subject characteristics in univariable analysis.
  
  
  CONCLUSIONS
  Spotted fever group rickettsiae exposure was common and A. phagocytophilum exposure was rare in a US population with broad demographic diversity.
authors.affiliation: Viral and Rickettsial Diseases Department, Naval Medical Research Center, Silver Spring, Maryland, USA.
authors.forename: Paul C F
authors.initials: PC
authors.lastname: Graf
authors.forename: Jean-Paul
authors.initials: JP
authors.lastname: Chretien
authors.forename: Lady
authors.initials: L
authors.lastname: Ung
authors.forename: Joel C
authors.initials: JC
authors.lastname: Gaydos
authors.forename: Allen L
authors.initials: AL
authors.lastname: Richards
mesh_terms: D000328:Adult; D041081:Anaplasma phagocytophilum / Q000302:isolation & purification*; D015153:Blotting, Western / Q000379:methods; D003430:Cross-Sectional Studies; D016873:Ehrlichiosis / Q000453:epidemiology* / Q000382:microbiology; D004797:Enzyme-Linked Immunosorbent Assay / Q000379:methods; D005260:Female; D006801:Humans; D008297:Male; D008889:Military Personnel; D015995:Prevalence; D012284:Rickettsia rickettsii / Q000302:isolation & purification*; D012373:Rocky Mountain Spotted Fever / Q000453:epidemiology* / Q000382:microbiology; D016036:Seroepidemiologic Studies; D014481:United States / Q000453:epidemiology