Cytauxzoon felis cytochrome b gene mutation associated with atovaquone and azithromycin treatment

Background Atovaquone and azithromycin (A&A) with supportive care improve survival rates in cats with cytauxzoonosis. Resistance to atovaquone via parasite cytochrome b gene (cytb) mutations occurs in other Apicomplexan protozoans but is not described in Cytauxzoon felis. Objective To serially characterize the C. felis cytb sequences from a cat that remained persistently infected after A&A treatment. Animal A cat with naturally occurring C. felis infection. Methods Case report of the anemic cat persistently infected with C. felis before, during and after A&A treatment. Cytauxzoon felis cytb genes were amplified and sequenced before, during and after A&A treatment. Results Cytauxzoon felis was detected before, during and after A&A treatment including samples collected 570 days after treatment. After A&A treatment, the cat's anemia improved slightly. Cytb sequencing revealed only wild-type cytb methionine (M128) in samples collected before treatment. In samples collected after treatment, the cytb coded for isoleucine (M128I) and valine (M128I) at 2- and 4-months after treatment. These M128I and M128V mutations persisted even after a repeat treatment course with a higher dose atovaquone combined with the standard dose of azithromycin. Conclusions and Clinical Importance This report documents C. felis atovaquone resistance associated with M128 cytb mutations. This study suggests parasites with mutations of cytb M128 can be selected and impart resistance to A&A treatment even with higher atovaquone dosing.