Detection of Immunoglobulin M and Immunoglobulin G Antibodies Against Orientia tsutsugamushi for Scrub Typhus Diagnosis and Serosurvey in Endemic Regions

Objectives To estimate the regional cutoff of optical density (OD) values for immunoglobulin M (IgM) antibodies against Orientia tsutsugamushi in serum and cerebrospinal fluid (CSF) for clinical diagnosis of scrub typhus and immunoglobulin G (IgG) antibodies in serum for sero-epidemiology in Gorakhpur, Uttar Pradesh, India. Methods We used data from a serological investigation of acute encephalitis syndrome patients ( n =407) during the 2016 outbreak in Gorakhpur, India to determine the cutoff for OD values for IgM antibodies, and from community-based serosurveys ( n =1991) to estimate the cutoff for OD values for IgG antibodies. Results We determined regionally relevant cutoff for OD values of 0.76 for IgM antibodies in serum and 0.22 in cerebrospinal fluid for scrub typhus diagnosis. For serosurveys, IgG antibody cutoff was 1.5. Conclusions We have proposed locally relevant cutoffs for scrub typhus endemic regions, which may be useful for correctly classifying infected population.