Effect of various oral dose levels of a trimethoprim/sulphadiazine mixture on Bordetella bronchiseptica infection and on the proliferation of trimethoprim-resistant faecal coliforms in pigs.

When a 1:5 mixture of trimethoprim (TMP) and sulphadiazine was fed to pigs intra-nasally infected with bordetella bronchiseptica, 10 mg/kg/day was shown to be highly effective in suppressing the organism. This dose level had little effect on numbers of TMP-resistant coliforms in faeces, but oral doses of 30 mg/kg/day eventually selected a resistant population. It is suggested that the proliferation of resistant coliforms would be minimized by administration of the lowest oral dose rates of antibacterial drugs compatible with efficacy.