Although infection of domestic cats with Cytauxzoon felis is usual fatal, some cats survive. The piroplasms stage of the protozoal pathogen may persist in the surviving cats indefinitely, as it does in the bobcat reservoir host. The purpose of this prospective, randomized clinical trial was to compare the efficacy of two treatment protocols for the eradication of parasites in persistently infected cats. A colony of cats with multiple survivors of C. felis was identified. Nine cats positive for C. felis by PCR were randomly divided into two groups. All cats were treated topically with fipronil q 3 weeks throughout the study period. Imidocarb dipropionate was administered to the first group (4 mg/kg IM twice, 2 weeks apart) while atovaquone (15 mg/kg PO q 8 hr) and azithromycin (10 mg/kg PO q 24 hours) were administered to the second group for 10 days. Blood was obtained from each cat immediately before treatment and again at 1, 6, and 8 weeks after treatment. Stained smears were examined microscopically, and samples underwent PCR testing for C. felis DNA using previously described methods. Cats that failed to clear the infection at either 6 or 8 weeks were "crossed over" and the alternative treatment administered with identical follow-up.
Four cats were initially treated with imidocarb and 5 with atovaquone/azithromycin. All cats treated with imidocarb remained persistently infected at all 3 time periods afterwards. Of the cats treated with atovaquone/azithromycin, 2 cats tested negative at 1, 6 and 8 weeks; 1 cat was positive at 1 week only, another at 6 weeks only, and yet another at 8 weeks only. After the 4 cats initially treated with imidocarb were then treated with atovaquone/azithromycin, 1 cat tested negative at 1, 6 and 8 weeks, 1 cat tested negative at 1 week and was lost to follow-up and 2 cats tested positive at 1 week but negative at weeks 6 and 8. The two that cats initially "failed" treatment with atovaquone/azithromycin went on to receive imidocarb, and one cat tested positive at 1, 6 and 8 weeks while the other tested negative on all 3 post-treatment dates.
Our results demonstrate that although neither treatment completely eliminates parasitemia due to C. felis infection, the parasite burden is greatly decreased by treatment with atovaquone/azithromycin as compared to imidocarb dipropionate. It is unclear if persistently infected cats can serve as competent reservoirs for infection when bitten by a tick. If so, a marked reduction in the circulating parasite load might reduce the likelihood that surviving cats infect naïve ticks. Certainly, imidocarb dipropionate does not effectively reduce parasitemia in persistently infected cats.