Documentation of in vivo and in vitro aerobic resistance of feline Tritrichomonas foetus isolates to ronidazole.

The mainstays of treatment for clinically important trichomonad infections are the 5-nitroimidazoles. Metronidazole resistance of feline Tritrichomonas foetus is presumed because of common treatment failure, and tinidazole does not consistently eradicate infection. To date, ronidazole is the only drug demonstrated as effective for treatment of cats infected with T. foetus. To document in vivo treatment failure and identify underlying causes and in vitro conditions of resistance of feline T. foetus to ronidazole. Two intact male Abyssinians failing>or=5 courses of treatment with increasing doses of 5-nitroimidazole drugs. An intact male Abyssinian documented to clear infection after treatment with a single course of ronidazole. T. foetus isolates were cultured from feces and tested in vitro for susceptibility to ronidazole under aerobic and anaerobic culture conditions. A urogenital nidus of T. foetus infection was investigated by culture, polymerase chain reaction, or immunohistochemical testing of urogenital specimens. Resistance to ronidazole under aerobic conditions was uniquely identified in T. foetus isolated from cats with well-documented treatment failure. Treatment failure could not be attributed to reinfection, inappropriate treatment protocol, or presence of a urogenital nidus of infection. Clinical resistance to metronidazole, low efficacy of tinidazole, and present documentation of in vivo and in vitro resistance to ronidazole in some cats are consistent with a high level of cross resistance of feline T. foetus to 5-nitroimidazole drugs. Current lack of alternative drugs with clinical efficacy against feline T. foetus suggests that active investigation of other treatment approaches is warranted.