Molecular identification and phylogenetic analysis of spotted fever Rickettsiae in peripheral blood from suspected Rickettsiosis cases in Sri Lanka.

Rickettsioses are an emerging public health concern in Sri Lanka, particularly in the central hills, where the Spotted Fever Rickettsiosis is prevalent. This study aimed to molecularly detect and characterize Rickettsia species in peripheral blood from patients suspected of Spotted Fever Rickettsiosis. Peripheral blood samples were collected from 217 patients suspected of Rickettsiosis between January 2020 and June 2021. All samples were screened using the Rickettsia conorii immunoglobulin G (IgG) Indirect Immunofluorescence Antibody Test (IFAT). DNA was extracted from IFAT-positive samples and subjected to nested polymerase chain reaction (PCR) targeting the 17 kDa protein gene of Spotted Fever Rickettsiae. Selected PCR-positive samples were purified, sequenced, and analyzed using nBLAST and phylogenetic analysis. Of 217 clinically suspected patients, 137 (63.1%) had antibodies reactive with R. conorii antigen. DNA extracted from 113 seropositive blood samples underwent nested PCR targeting the 17 kDa protein gene, yielding 15 (13.2%) positives. Ten amplicons were sequenced, revealing three genotypes. Phylogenetic analysis showed one sequence with 100% identity to R. felis, marking the first molecular detection of this species in human blood in Sri Lanka. Other sequences matched multiple Spotted Fever Rickettsia species, including R. rickettsii, R. conorii, and R. parkeri, though species-level resolution was limited due to conserved gene regions. The majority of seropositive patients were middle-aged females from rural areas. Findings underscore the presence of diverse, potentially virulent Rickettsia species and highlight the need for enhanced molecular surveillance. This study provides critical insights into the epidemiology of rickettsioses in Sri Lanka and supports the use of molecular tools for accurate pathogen identification, crucial for diagnosis, treatment, and vector control efforts. Not applicable.