Seroprevalence & seroincidence of Orientia tsutsugamushi infection in Gorakhpur, Uttar Pradesh, India: A community-based serosurvey during lean (April-May) & epidemic (October-November) periods for acute encephalitis syndrome

BACKGROUND & OBJECTIVES: In India, acute encephalitis syndrome (AES) cases are frequently reported from Gorakhpur district in Uttar Pradesh. Scrub typhus is one of the predominant aetiological agents for these cases. In order to delineate the extent of the background of scrub typhus seroprevalence and the associated risk factors at community level, serosurveys during both lean and epidemic periods (phase 1 and phase 2, respectively) of AES outbreaks were conducted in this region. METHODS: Two community-based serosurveys were conducted during lean (April-May 2016) and epidemic AES (October-November 2016) periods. A total of 1085 and 906 individuals were enrolled during lean and epidemic AES periods, respectively, from different villages reporting recent AES cases. Scrub typhus-seronegative individuals (n=254) during the lean period were tested again during the epidemic period to estimate the incidence of scrub typhus. RESULTS: The seroprevalence of Orientia tsutsugamushi during AES epidemic period [immunoglobulin (Ig) IgG: 70.8%, IgM: 4.4%] was high as compared to that of lean AES period (IgG: 50.6%, P<0.001; IgM: 3.4%). The factors independently associated with O. tsutsugamushi positivity during lean AES period were female gender, illiteracy, not wearing footwear, not taking bath after work whereas increasing age, close contact with animals, source of drinking water and open-air defecation emerged as additional risk factors during the epidemic AES season. IgM positivity was significantly higher among febrile individuals compared to those without fever (7.7 vs. 3.5%, P=0.006). The seroincidence for O. tsutsugamushi was 19.7 per cent, and the subclinical infection rate was 54 per cent. INTERPRETATION & CONCLUSIONS: The community-based surveys identified endemicity of O. tsutsugamushi and the associated risk factors in Gorakhpur region. The findings will be helpful for planning appropriate interventional strategies to control scrub typhus.