Determination of the in vitro susceptibility of feline tritrichomonas foetus to 5 antimicrobial agents

Background: The nitroimidazole, ronidazole, has been demonstrated to have in vitro and in vivo activity against the protozoan Tritrichomonas foetus in cats. The purpose of this study was to evaluate the in vitro susceptibility of feline T foetus isolates obtained from naturally infected cats to 5 antimicrobial agents and to compare the in vitro time kill of ronidazole and metronidazole. Hypothesis: We hypothesized that nitroimidazoles have in vitro activity against T foetus, whereas furazolidone, omeprazole, and paromomycin do not. Animals: Fecal specimens were cultured from 4 naturally infected Bengal cats with a history of T foetus-associated diarrhea. Methods: A 24-hour susceptibility assay was performed on all 4 isolates for the 5 antimicrobial agents. A time-kill microdilution method was performed on 2 isolates for metronidazole and ronidazole. Results: Paromomycin and omeprazole showed no in vitro effect at concentrations ≤80 μ/mL. There was no significant difference in 24-hour susceptibilities among metronidazole, ronidazole, and furazolidone. In addition, only the results of the highest concentration tested (80 μg/mL) and concentrations of 1.25 and 2.5 μg/mL revealed significant differences in the rate of trophozoite killing, with ronidazole having a faster reduction in trophozoite survival. Conclusions and Clinical Importance: Time-kill assays demonstrated ronidazole had a higher lethal activity compared with metronidazole. These findings contrast with a previously published report and may reflect strain variation, different methodologies, or both. The lack of clinical response seen with metronidazole administration to treat feline trichomoniasis may not reflect inherent resistance but rather in vivo events involving drug distribution and pharmacokinetics.