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/growl) 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.