Sensitivity of Escherichia coli O157:H7 strain 932 to selected anticoccidial drugs in broiler chicks

The ability of selected anticoccidial drugs to inhibit the colonization of day-old male broiler chicks (Cornish Rocks) by Escherichia coli O157:H7, strain 932 was examined. Chicks were challenged with 1.8 x 10(9) E. coli O157:H7 on Day 1, and fed diets supplemented with three selected anticoccidial drugs; monensin, nicarbazin, or robenidine. The cecal and colon fecal contents of the chicks were removed on Day 7, 14, and 21 postinoculation and examined for the concentration of E. coli O157: H7 per gram of contents. Monensin effectively reduced cecal and colon colonization of the chicks by E. coli O157:H7. By Day 7, there were significant reductions in the bacterial population of the cecal contents of chicks receiving the monensin-medicated feed, and by Day 21 no E. coli O157:H7 was recovered from the cecal and colon contents. The bacterial counts in the colon contents of the nicarbazin- and robenidine-medicated and unmedicated chicks were significantly higher than the monensin-treated chicks. Bacterial populations in the colon contents were high only when there were high bacterial concentrations in the cecal contents.

The influence of nutrient density of broiler rations on the effective dose of anticoccidial drugs

Anticoccidial drugs were added to diets formulated with a nutrient density of 1479 kcal/lb and 24% protein or with 80% of those energy and protein levels (low density) to determine whether rate of feed intake influences efficacy of drugs in broilers infected with coccidiosis. In four experiments with monensin, broilers receiving 60 or 100 ppm of monensin in the lower-density ration consumed more feed per unit of gain but gained less weight than those on the control diet. Coccidiosis (Eimeria tenella) was better controlled by 60 ppm of monensin in the low-density diet than in the high-density diet. There was some evidence of a similar efficacy-enhancing effect with nicarbazin, salinomycin, and lasalocid but not with robenidine. When drug intake was calculated as mg/kg of weight gain, there was a strikingly higher rate of intake of most drugs in the low-density diet, accounting for the apparent increase in activity. These results suggest that in countries where nutrient density of feeds varies significantly from those used in the United States, some compensation in levels of certain feed additives might be considered.