Prevention of immunologic stress contributes to the growth-permitting ability of dietary antibiotics in chicks.

The growth-permitting ability of antibiotics fed to broiler chicks was studied as it relates to the state of activation of the immune system. In Experiment 1, chicks were fed two levels of antibiotics (0 or 100 mg streptomycin + 100 mg penicillin/kg diet) and were raised either in an environment with poor sanitation to create a chronic immune stress or in a clean environment. Chicks raised in the unsanitary environment and not fed antibiotics had significantly lower (P < 0.05) rates of weight gain and efficiencies of feed utilization, and higher levels of plasma interleukin-1, compared with chicks raised in the clean environment or chicks raised in the unsanitary environment and fed antibiotics. Adding antibiotics to the diet of birds in the clean environment did not affect any variable. In Experiment 2, chicks were raised in a conventional environment and fed two levels of an antibiotic (0 or 100 mg tetracycline/kg diet). After a 15-d feeding period, half of the chicks were injected with Salmonella typhimurium lipopolysaccharide to create an acute immunologic stress. Feeding antibiotic resulted in improved weight gain, feed consumption and efficiency of feed utilization. Lipopolysaccharide-injected birds developed heavier livers, spleens and intestines relative to body weights and higher rectal temperatures and hepatic metallothionein concentrations, presumably due to an immunologic stress. Omitting antibiotic from the diet resulted in similar changes. These results indicate that feeding antibiotics may permit growth by preventing immunologic stress and associated metabolic changes brought about by monokines including interleukin-1.