Overwintering Ticks as a Reservoir for Severe Fever With Thrombocytopenia Syndrome Virus in the Republic of Korea

Abstract

Severe fever with thrombocytopenia syndrome (SFTS) is a tick-borne infectious disease caused by SFTS virus (SFTSV), with reported case fatality rates of 10%–30% in Asia. In the Republic of Korea, over 170 cases are reported annually, with Haemaphysalis longicornis (Ixodida: Ixodidae) as the major vector. This tick species can overwinter as nymphs; however, its role in maintaining SFTSV during the winter remains unclear. This study aimed to identify overwintering ticks in areas adjacent to a fatal SFTS case, determine their infection rate, and evaluate the phylogenetic relationships among detected viruses. Within the residential area surrounding the site where the fatal case was reported, six heterogeneous habitat types were selected, comprising a vegetable garden, an outdoor water faucet, a dog kennel, a backyard, scrubland, and a nearby hill. To detect overwintering ticks, surveys were conducted using flagging and dry ice-baited traps, and samples of decaying wood and the ground layer comprising leaf litter, the soil surface, and the topsoil layer were collected. Overwintering ticks were detected only in soil samples from the outdoor water faucet and the nearby hill sites. A total of 58 nymphs were collected, including 5 from leaf litter, 34 from the soil surface layer, and 19 from the topsoil layer. SFTSV was detected in three ticks collected from the outdoor water faucet, including two specimens from the soil surface layer and one from the topsoil layer, resulting in an infection rate of 5.2%. Phylogenetic analysis identified the detected viruses as genotype B-2, with 99.6%–100% nucleotide identity to sequences from the patient and local ticks collected during the active season. These findings suggest that overwintering ticks in soil environments may contribute to viral persistence and early-season transmission risk.