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A six-week trial was conducted to determine if therapeutic use of chlortetracycline (CTC) would be affected by previous use of the same antibiotics at subtherapeutic levels in the feed. Results indicated therapeutic effect of CTC on mortality was not compromised by the previous use of the low level antibiotic.

Twenty broiler chickens were fed bambermycins (Flavomycin; an antibiotic produced by Streptomyces) at the rate of 3 g/ton (US) for 63 days, and 20 control birds were fed nonmedicated feed. The birds were inoculated (dosed) on the 10th and 11th feeding day with Salmonella typhimurium. The study evaluated the effects of bambermycins on Salmonella incidence, shedding, and antimicrobial resistance. Bambermycins had no effect on body weights, duration of shedding of salmonellae, number of salmonellae shed on postdosing day 3, tissue recoverability of salmonellae, and total number of resistance patterns. Bambermycins resulted in the decrease of salmonellae to be more gradual; however, both treatments were comparable at the end of the study. The majority of S typhimurium from bambermycins-treated birds maintained the original antibiogram of streptomycin, sulfadiazine, and nalidixic acid. The salmonellae isolated from the control birds were more resistant to 2 drugs (varying antibiograms). Bambermycins as a feed additive in broiler diets given at the dose level of 3 g/ton had no detrimental effects based on salmonellae shedding and antimicrobial resistance.

Twenty broiler chickens were fed 80 g/T salinomycin, an antibiotic produced by Streptomyces albus, and 20 birds were fed a control, unmedicated feed. The birds were experimentally infected with Salmonella typhimurium. The study evaluated the effects of salinomycin on Salmonella incidence, shedding, and antimicrobial resistance. Salinomycin had no effect on body weights, length of time salmonellae were shed, number of salmonellae shed on postdosing day 3, salmonellae tissue recoverability, or on the total number of resistance patterns. Salinomycin caused the decline of salmonellae to be more gradual; however, both treatments were comparable at the end of the study. The majority of isolated from birds receiving salinomycin maintained the original S. typhimurium antibiogram of streptomycin, sulfadiazine, and nalidixic acid. The salinomycin salmonellae were more susceptible to tetracycline, amikacin, carbenicillin, gentamicin, and cephalothin. The multiple resistance patterns of eight and nine drugs tended to be more prevalent among salmonellae from control birds than salinomycin treated birds. The antibiotic salinomycin appears to be an acceptable feed additive in broilers at the level of 80 g/T based on these results of its effects on salmonellae shedding and antimicrobial resistance.

Duplicate trials were conducted with male broiler chickens to evaluate virginiamycin as treatment against experimentally induced necrotic enteritis infection. Each trial consisted of seven treatments, each replicated four times, with 10 birds per replicate. Two treatments were fed control ration (noninfected control and infected control) and the five remaining treatments were fed virginiamycin at 5, 10, 15, 20, or 40 g/ton. Birds were orally dosed with 10 ml of Clostridium perfringens culture at 14 days of age. At 5 weeks of age, surviving birds were killed and necropsied to obtain lesion scores. Birds fed virginiamycin had significantly less mortality and lower intestinal lesion scores than nonmedicated birds when experimentally infected with necrotic enteritis.

Eighty broiler chickens were randomly assigned to each of 12 chambers in a controlled environment poultry facility. Ammonia gas was injected at 0, 50, or 100 ppm from 28 to 53 days of age. All chicks, except the control pens, were innoculated with coccidia at 36 days of age. No significant differences were found in mortality, but weight gains were significantly reduced in chambers that received ammonia gas. This was probably due to the increase in severity of air sac and intestinal lesions in the ammonia pens.

Procedures are described for determining the effects of low level antibiotic feed additives in broiler chick rations when chicks are artificially infected with Salmonella typhimurium var. copenhagen and the effectiveness of the same antibiotics used therapeutically for salmonellosis. These procedures also permit study of development of antibiotic resistant enteric flora related to antibiotic feeding and the transferability of the resistance. Measures should be taken to monitor and assure minimal extraneous bacterial contamination of eggs, chicks, feed and facilities and minimize baseline antibiotics resistance. Facilities and equipment should be fumigated extensively, chicks should be from primary breeder flocks off antibiotics for two generations, handled minimally, and fed all-vegetable protein rations. Optimum procedures for oral induction of salmonellosis with a chick-virulent strain of Salmonella typhimurium var. copenhagen are reported. Effects of low level antibiotic feeding and subsequent therapeutic level feeding of infected and control birds are measured by mortality, feed efficiency and weight gain, and microbiological analyses of fecal samples, intestinal samples and necropsied dead chicks. Gathering all data possible from all facets of the trials is imperative for ascertaining antibiotic effects.

Three replicate trials were conducted with broiler male chicks to test the therapeutic efficacy of doxycycline, chlortetracycline and lincomycin-spectinomycin in water against an artifically induced Escherichia coli infection. Mortality, lesion scores (heart, liver and air sac), and performance data were the criteria in evaluating therapeutic efficacy of these drugs. Results indicated the therapeutic efficacy of doxycycline was greater than chlortetracycline and lincomycin-spectinomycin.