The Establishment of a Novel In Vitro System for Culturing Cytauxzoon felis

Cytauxzoonosis, a highly fatal tick-borne disease in domestic cats caused by Cytauxzoon felis, poses diagnostic and therapeutic challenges due to the inability to culture the parasite in vitro. This study aimed to artificially replicate C. felis infection and characterize in vitro replication kinetics. Concanavalin A-activated feline embryonal macrophages (Fcwf-4) were plated at 3-5 × 10⁵ cells/mL and incubated with C. felis-positive blood samples from either a (1) chronically infected bobcat (Lynx rufus), (2) chronically infected domestic cat, or (3) acutely infected domestic cat with clinical signs of cytauxzoonosis. Temporal changes in parasite load were quantified by droplet digital PCR (ddPCR), and the inhibition of infection/replication was assessed using atovaquone, imidocarb dipropionate (ID), artemisinin, ponazuril, and neutralizing antibodies. Tick cell lines AAE2 and ISE6 were also tested for infection. In vitro inoculation with chronic infection led to transient replication, while acute infection resulted in sustained replication beyond 10 days post-inoculation. Atovaquone, ID, and artemisinin inhibited replication, and neutralizing antibodies prevented infection. The inoculation of tick cells in vitro indicated infection; however, parasite replication was not observed. The results of this study established an in vitro model for studying infection dynamics, assessing therapy efficacy, and testing vaccination strategies in cytauxzoonosis-infected cats.