Evaluation of virginiamycin in feed for treatment and retreatment of swine dysentery.

Virginiamycin, fed at a concentration of 110 mg/kg of feed for 2 weeks followed by concentrations of either 27.5 or 55 mg/kg for 3 weeks, was effective in treatment and control of experimentally induced swine dysentery. However, diarrhea recurred 4 days after withdrawal of medicated feed. Subsequently, the frequency of diarrhea decreased in 3 of 4 groups retreated with virginiamycin (110 mg/kg of feed) for 5 days. Feeding of virginiamycin (110 mg/kg) for 1 week after onset of diarrhea was of little value because of the development of a more severe (augmented) form of the disease after withdrawal of medicated feed. Feeding of virginiamycin (55 mg/kg) at the time of exposure and continuing for 3 weeks (followed by intermittent retreatments of 55 mg/kg) aided in treatment and control of swine dysentery. With this regimen, a few swine developed diarrhea during initial medication, but all developed diarrhea after withdrawal fo medicated feed. Frequency of diarrhea was less after each retreatment. This was attributed to the development of immunity from the recurring diarrhea.