Spotted fever diagnosis: Experience from a South Indian center

Spotted fever (SF) is an important treatable cause of acute febrile illness (AFI) with rash and has reemerged in India. A prospective AFI with rash study was undertaken at a South Indian hospital to correlate specific clinical findings with laboratory confirmation of spotted fever. During the study period (December 2017 to May 2019), 175 patients with fever and rash were suspected to have spotted fever. Molecular assays for scrub typhus and spotted fever (47 kDa and ompA qPCR) and serology (IgM ELISA) was performed on the 96 individuals recruited. Laboratory confirmed SF cases (ompA qPCR positive) were 21, whereas laboratory supported SF cases (ompA negative but sero-positive by SF IgM ELISA) were 27. Among the 48 spotted fever (SF) cases, 70% of had maculopapular rash, 12.5% had macular rash, purpuric/petechial rash (severe rash) was seen in 8 patients (16.7%). Presence of rash on the palms and soles was associated with a relative risk (RR) of 4.36 (95% CI: 2.67-7.10; p < 0.001). Our study suggests that ompA qPCR though useful for confirming the diagnosis of spotted fever is not always positive. A positive SF IgM ELISA in febrile individuals with palmo-plantar rash supports the diagnosis of spotted fever especially when other causes of febrile rash have been excluded. Multi-centric prospective studies employing the serological reference standard, IFA (immunofluorescence assay) in addition to the assays used in this study are needed to validate these findings.