Multisystem Detection and Molecular Evidence Linking Dog-Associated Ticks to Two Fatal Human SFTSV Infections in Thailand

Abstract Severe fever with thrombocytopenia syndrome virus (SFTSV) is an emerging tick-borne pathogen responsible for severe and often fatal disease in East Asia; however, information from Southeast Asia remains limited. We report two fatal human SFTS cases in Thailand confirmed by quantitative reverse transcription polymerase chain reaction, in which viral RNA was detected in multiple specimen types from both patients, including plasma, cerebrospinal fluid, respiratory secretions, urine, and stool, indicating systemic infection. Rhipicephalus sanguineus ticks collected from pet dogs and from the patients’ households also tested positive for SFTSV RNA, with cycle threshold values ranging from 15.6 to 30.6. Phylogenetic analyses based on the S, M, and L segments showed that sequences obtained from human and tick samples clustered within genotype B-1 and were closely related to previously reported Thai and related Chinese strains. These findings provide molecular evidence of SFTSV in dog-associated R. sanguineus ticks and suggest a possible epidemiological association with human cases; however, their role in transmission and the direction of transmission remain uncertain. These findings expand the known geographic and ecological context of SFTSV and underscore the importance of heightened clinical awareness and integrated human–animal–vector surveillance in Southeast Asia, particularly in dengue-endemic regions where SFTS may be clinically overlooked.