Effect of substitution of medium-chain organic acids for zinc bacitracin in a diet containing colistin on performance and development of intestinal lymphoid tissues in growing rabbits experimentally infected with Escherichia coli O103 and Clostridium perf

This work tested the effect of replacing zinc bacitracin with a blend of caproic and caprylic acids in a diet including colistin on growth performance, mortality, gut histology and development of intestinal lymphoid tissues in rabbits experimentally infected with Escherichia coli and Clostridium perfringens. Three experimental diets were formulated: a control non-medicated diet (diet C), diet C medicated with 150 mg zinc bacitracin and 120 mg colistin/kg (diet B) and diet C supplemented with a 4-g mixture of the acids and 120 mg colistin/kg (diet A). Forty rabbits weaned at 28 days of age and housed individually were assigned to each diet. At 35 days of age, rabbits were orally infected with 109 cfu of E. coli and 109 cfu of C. perfringens. At 35 and 56 days of age, five rabbits per diet and age were slaughtered to assess caecal traits, jejunal histology and follicular development in the caudal ileal Peyer's patch and the appendix. No significant differences were reported on growth performance. Mortality rate reached 60.0% for control rabbits whereas no mortality occurred in rabbits fed the supplemented diets. Caecal pH was not influenced by diet but was lower at 56 than at 35 days (6.20 vs. 6.49, P<0.001). No differences were detected on villus height of control rabbits between 35 and 56 days of age (average height of 597 μm) but villi of rabbits fed diets B and A were, respectively, 19 and 34% (P<0.001) longer at 56 days than at 35 days. The average follicle area in the Peyer's patch (81,900 μm2vs. 56,800 μm2, P<0.001) and in the appendix (116,000 μm2vs. 56,900 μm2, P<0.001) increased after the infection. Besides, average follicle area of the Peyer's patch was influenced by diet (78,983 μm2vs. 69,632 μm2vs. 59,457 μm2, P=0.011, for diets C, B and A, respectively). No significant effect of treatments was detected on the number of follicles of the appendix (37.2 follicles on average). To conclude, the substitution of caproic and caprylic acids for zinc bacitracin in a diet including colistin increased jejunal villus height and reduced follicle hypertrophy of the caudal ileal Peyer's patch in growing rabbits experimentally infected with C. perfringens A and E. coli O103. The dietary combination of colistin with both zinc bacitracin and the blend of esterified caproic and caprylic acids prevented the rise of fattening mortality, even after the infection. However, the effect of including medium-chain organic acids in a diet free of medication on challenged rabbit health is yet to be elucidated.