Research Communications of the 27(th) ECVIM-CA Congress: Intercontinental, Saint Julian's, Malta, 14th to 16th September 2017

reatment with atovaquone and azithromycin. An approximately nine-year female spayed domestic shorthair was presented for adoption assessment; the cat was anemic which persisted despite fenbendazole and doxycycline treatment. Comprehensive infectious disease testing detected C. felis via microscopy and polymerase chain reaction (PCR). Atovaquone (15 mg/kg PO q8 h 9 10 days) and azithromycin (10 mg/kg PO q24 h 9 10 days) treatment were administered and the PCV improved, but the cat remained persistently infected with C. felis. Sequencing of the atovaquone binding region of cytb over the course of infection (pre and post-treatment samples) would reveal a transition from wild-type (M128) to isoleucine and valine (M128I and M128V) at two and four months post treatment. A repeat treatment course with A&A using an increased atovaquone dose (25 mg/kg PO q8 h 9 10 days) would fail to clear the C. felis infection and M128I and M128V mutations persisted. The wild-type M128 cytb was not detected in post-treatment samples up to 210 days. This is the first documentation of resistance of C. felis to atovaquone associated with M128 cytb mutations. Additionally, this carrier cat appears to be the first chronically C. felis-infected felid with persistent anemia. This case suggests that parasites with mutations of cytb M128 can be selected and impart resistance to A&A treatment even in the face of increased atovaquone dosing.