The mechanism of antibiotic-promoting effect in body weight gain of chicks.

A series of 3 experiments was conducted on 3-day-old, single combed, white Leghorn cockerels for the purpose of studying the mechanism of aureomycin-promoting effects in body weight gain of chicks. In each experiment, 3 groups of chicks were fed for 4 weeks respectively on 1) basal ration, 2) basal ration supplemented with 25 ppm aureomycin and 3) basal ration with different amounts of alkali-decomposed aureomycin. At the end of each experimentation, 131I was injected intraperitoneally into the chicks. One hour later, the chicks were sacrificed by decapitation. The thyroid glands were removed immediately and weighed. The glands were prepared for radioactivity determination by the TraceLab multi/matic 501 counting system. The percentage of 131I uptake of the injected dose per mg thyroid gland was ascertained. The results indicated a significant increase of thyroid indices (mg thyroid gland per 100 gm body weight) after feeding aureomycin or decomposed aureomycin. The radioactivity determinations showed that the thyroid 131I uptake of the experimentals, especially those fed aureomycin, was significantly smaller than that of the controls. The present results, therefore, are in accord with the hypothesis of antithyroid effect of aureomycin propsed by Calesnick et al. in 1954. Furthermore, it suggests that such goitrogenic action might have resulted from aureomycin degraded under the influences of alkaline medium of the intestines and high body temperature of the chick. In addition to the functional change of the thyroid gland, aureomycin or decomposed aureomycin also promoted body weight gain and an increase in fat content in the chicks. These results suggest that body weight-promoting effect of aureomycin in animals may be attributable to an alteration of thyroid function which, in some way, is related to the products resulting from the degradation of the antibiotic in vivo.