Antibodies to tick salivary antigens AV422 and calreticulin in patients with spotted fever group rickettsioses and other febrile illnesses in northern Tanzania

Background Timely and accurate diagnosis of spotted fever rickettsioses (SFR) is difficult due to non-specific symptoms and testing challenges. We evaluated serum antibodies to tick salivary proteins, AV422 and calreticulin, as potential markers of recent tick exposure in febrile patients from northern Tanzania. Methods We recruited febrile patients at two hospitals in Moshi, Tanzania from February 2012 through May 2014. Acute and convalescent sera from enrolled participants were tested against Rickettsia africae antigen using indirect immunofluorescence assays (IFA). Participants with ≥four-fold rise in R. africae IFA immunoglobulin G reciprocal titer between sera were classified as cases (n = 70), those with paired sera with reciprocal titer ≥64, but no ≥four-fold rise as exposed (n = 40), and participants with reciprocal titer <32 in either sera as unexposed (n = 38). Acute and convalescent sera were tested for calreticulin and AV422 antibodies using enzyme-linked immunosorbent assay. Results We found significantly higher calreticulin antibody levels in acute sera of cases compared to exposed, unexposed, and the combined comparator of exposed and unexposed (adjusted mean difference 17.49, 95% confidence interval (CI): 6.37–28.61; 17.61, 6.55–28.67; 18.11, 9.13–27.10, respectively). Calreticulin distinguished cases from exposed and unexposed among 0-15 years old with positive predictive value of 0.77 (95% CI: 0.61-0.88) and sensitivity of 0.94 (95% CI: 0.81-0.99). Conclusions In the acute phase of febrile illness, measurement of calreticulin antibody, a proposed marker for tick exposure, was higher in patients with confirmed tick-borne illness and had reasonable diagnostic test characteristics.