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.

Rocky Mountain Spotted Fever in the United States, 2000–2007: Interpreting Contemporary Increases in Incidence

“…17 Other studies examining background seroprevalence in northern states or among geographically widespread military personnel suggest a seroprevalence of 4% to 6%. 18,19 The diagnosis of RMSF based on a single serologic test, as predominantly occurred during the current study period, may result in the erroneous diagnosis in non-RMSF cases with historical titers from past exposures or with nonspecific cross-reactive IgM class antibodies. The reported sensitivity and specificity of single IgM tests have been reported as low as 23% and 77%, respectively, when compared with testing of IgG in paired sera, for the diagnosis of rickettsial infections in endemic areas.…”

“…The reported sensitivity and specificity of single IgM tests have been reported as low as 23% and 77%, respectively, when compared with testing of IgG in paired sera, for the diagnosis of rickettsial infections in endemic areas. 20 The prevalence of anti-R. rickettsii antibodies appears to increase with age, 18,19 likely as a function of increased risk of exposure to either R. rickettsii or other rickettsial organisms over a lifetime, the effect of historical titers may be higher among adults. In the current study, children had a lower incidence rate compared with adults, but had the highest risk of fatal outcome, in contrast to prior studies that showed the highest incidence among children and placing older adults at greater risk for fatal outcome.…”

“…6,10 In contrast, incidence and case fatality among older age groups has changed dramatically. Given that background antibodies to RMSF increase with age, 18,19 this observation may suggest that single serologic tests have greater positive predictive value among children, and that single-titer data from this age group more accurately reflect true RMSF cases.…”

Human Infections by Multiple Spotted Fever Group Rickettsiae in Tennessee

“…In North Carolina, sera from probable cases of RMSF demonstrated greater titers for R. amblyommii than R. rickettsii, and R. amblyommii DNA has been sequenced from a tick that caused a rash in a patient. 15,16 Although seroprevalence studies of SFG rickettsial infection in the United States have been performed in the past, 17,18 there have been limited attempts to determine if SFG rickettsiae other than R. rickettsii are contributing to the increase incidence of SFG rickettsiosis. In this study, we investigated serologic human exposure to four SFG rickettsiae in Tennessee: R. rickettsii, R. montanensis, R. parkeri, and R. amblyommii.…”

Seroepidemiologic Study of Human Infections with Spotted Fever Group Rickettsiae in North Carolina

“…Mounting evidence suggests that infections with other species of spotted fever group rickettsiae (SFGR) may be at least partly responsible for the apparent increase in cases, which may reflect an increase in reporting or incidence or both. Seroepidemiologic surveys of adults and children in the United States show that seroprevalence to SFGR is between 6% and 12% (2,3). Because subclinical infection or mild infections with R. rickettsii are considered to be rare or even nonexistent and there is cross-reactivity among SFGR in serologic tests, exposure to other species of SFGR could account for this relatively high seroprevalence (4).…”