Clinical and Pathological Findings in Fatal Cases of Severe Fever With Thrombocytopenia Syndrome With High Viremia in Cats

Severe fever with thrombocytopenia syndrome (SFTS) is an emerging zoonotic disease caused by the SFTS virus (SFTSV). SFTSV causes severe symptoms both in humans and cats. In this study, we report the clinical and pathological findings of 4 fatal cases of cats with high SFTS viremia levels. These cats showed an acute onset of fever, leukopenia, thrombocytopenia, and increased serum amyloid A and pro-inflammatory cytokine levels. A high viral copy number was detected in the blood, oral swabs, rectal swabs, conjunctiva swabs, and urine. Histopathologically, necrotizing lymphadenitis, splenitis with lymphoblastoid cell proliferation, and hemophagocytosis were observed in all 4 cats. Immunohistochemistry revealed the presence of SFTSV antigen on lymphoblastoid B cells. SFTSV-RNA was detected in systemic tissues, including the brain. The present findings provide useful information for understanding the features of fatal SFTS in cats. To elucidate the mechanisms of severe progress of SFTS cats, as well as its role as a source of human infection, further research is needed.