Co-infection with severe fever with thrombocytopenia syndrome virus and Rickettsia monacensis in Korea

Severe fever with thrombocytopenia syndrome virus (SFTSV) and Rickettsia monacensis , transmitted by Haemaphysalis longicornis and Ixodes spp. ticks respectively, are endemic to East Asia and can cause overlapping febrile illnesses. Despite their shared ecological niches, co-infections remain poorly characterized. This study investigated the prevalence of R. monacensis co-infection in patients with SFTS in Korea using polymerase chain reaction (PCR) and sequencing. Clinical samples were collected from 129 patients with SFTS. One-step reverse-transcription PCR was performed to identify the SFTS virus (SFTSV), and rickettsial nested PCR testing targeted outer membrane protein A ( ompA) . Phylogenetic analysis was conducted to confirm the evolutionary relationships among the different species. Among the 129 SFTS cases, nine patients with SFTSV were positive for R. monacensis , with a prevalence rate of 7.9%. Phylogenetic tree analysis revealed that the seven sequences identified in the present study clustered closely with R. monacensis. This is the first study wherein 7.9% of patients suspected of having SFTS were also found to be co-infected with R. monacensis based on molecular testing. We believe that these findings highlight the need for the differential diagnosis of tick-borne febrile illnesses and raise awareness of the potential co-infection impacts on disease severity and management.