Preliminary molecular characterisation of Orientia tsutsugamushi genotypes in scrub typhus patients from Thiruvarur district, Tamil Nadu.

BACKGROUND OBJECTIVES: Scrub typhus is a re-emerging vector-borne zoonotic disease in India, with increasing case incidences and outbreaks reported across several states. The bacteria, Orientia tsutsugamushi , shows substantial genetic diversity, with reported evidence of more than 70 different strains globally. However, there are limited molecular studies on exploring the circulating genotypes of this bacterium in disease endemic regions. Therefore, the objective of the study was to identify the antigenic diversity of Orientia tsutsugamushi strains circulating in the delta district of Thiruvarur, using nested PCR, targeting the partial 56 kDa type-specific antigen and GroEL genes. METHODS: This descriptive cross-sectional study, conducted from January to December 2024, screened 235 clinically suspected scrub typhus cases serologically. IgM positive samples were subjected to nested PCR targeting the partial 56 kDa and GroEL genes, followed by sequencing and phylogenetic analysis. RESULTS: Fourteen of 235 clinically suspected cases (6.0%) were positive for IgM antibodies by both serological assays. Of these, seven samples (50.0%) yielded PCR amplicons, and five produced high-quality sequences suitable for phylogenetic analysis (GroEL, n = 4; 56 kDa, n = 1). Phylogenetic analysis of the study revealed that the sequences clustered closely with Karp and Kato-related genotypes, suggesting the circulation of these genotypes in the region. The sequences were deposited in GenBank under accession numbers PX781774, PX804529, PX804530, PX804531, and PX804532. INTERPRETATION CONCLUSION: This study provides preliminary evidence that the strains of Orientia tsutsugamushi circulating in the Thiruvarur district are closely related to Karp and Kato related genotypes. Although sequenceable amplicons were obtained from only a limited number of samples, the findings add to the emerging molecular epidemiological data on scrub typhus in Tamil Nadu. Larger studies integrating molecular findings with clinical and epidemiological data are needed to better define strain diversity in endemic settings.