Diagnostic dilemma in laboratory confirmation of acute scrub typhus infection: Relevance of ST IgM, IgG antibodies and molecular markers

BACKGROUND OBJECTIVES: Conventionally, the humoral immune response to scrub typhus (ST) is the early appearance of IgM during the first week of infection, followed by IgG after 2-3 weeks. Recent publications reveal different trends in Immunological response to ST. METHODS: Acute and convalescent blood from 55 (Group-I) and single samples from 152 (Group-II), both clinically suspected as ST, were subjected to ST IgM ELISA, ST IgG ELISA, and ST IgG IFA. Acute samples from Group I and single samples from Group II were subjected to Real-Time PCR (q PCR) to target three genes, 56 kDa, 47 kDa, and groEl. RESULTS: All 262 blood samples were positive for ST IgM ELISA. Positivity for IgG ELISA, IgG IFA and the presence of any one or more gene targets (56kDa/47kDa/groEL) in qPCR among the 55 acute samples was 70.9%, 36.4%, and 89.1%, respectively and 87.3%, 45.5% and 78.3% respectively in convalescent samples. In 152 unpaired samples, IgG ELISA was positive in 52.6% and 19.1% in ST IgG IFA. Among 207 samples (Group-I Acute and Group II) subjected to q PCR, groEl, 56 kDa and 47 kDa were present in 59.4%, 44.0%, and 35.7% respectively. INTERPRETATION CONCLUSION: In view of the persistence of IgM in ST over months and, in some cases, years, there is a diagnostic dilemma in confirming ST based on single ST IgM, especially in ST-endemic regions. This is compounded by the additional presence of ST IgG in the early days of acute infection. q PCR is recommended to confirm doubtful cases of ST.