For 50 plus years, the subtherapeutic uses of antibacterial agents have played a significant role in the efficiency of animal production, particularly for swine and poultry. The major discovery of antibiotic use for this purpose came as a by-product of the evaluation of fermentation products as sources of APF (animal protein factor or antipernicious anemia factor). Following Fleming's report of the bacterial inhibitory effects of a certain mold, it was a decade later before Chain and coworkers identified the active substance, penicillin. The miraculous effects of penicillin stimulated extensive searches for other antimicrobial agents that might have similar uses. At the same time, extensive research was in progress to find alternative sources and to identify the active substance in APF, a factor associated with animal protein. Researchers at Lederle Laboratories (The American Cyanimid Co.) had two independent teams, one involved in searching for antibacterial agents and the other searching for APF sources. The antibiotic team discovered the highly effective antibiotic Aureomycin (chlortetracycline) produced by Streptomyces aureofaciens. Some time passed before the APF team tested the same organism. When Stockstad and coresearchers tested the S. aureofaciens, they found a growth-stimulating effect greater than could be accredited to APF. Subsequent tests using crys talline chlortetracycline and APF (by this time determined to be vitamin B12) demonstrated that the additional growth stimulation was an effect of the antibacterial agent. This naturally led others to confirm these results and to determine if other antibiotics available at that time resulted in similar improvements in performance.
These studies on antibiotics were taking place at a time when other major changes were taking place in swine production. Pigs were being weaned at a younger age, major changes were being made in type and capacity of housing, more pigs were being reared in close confinement, herd sizes were increasing, and soybean meal was becoming even more economically competitive with milk, meat by-products, and fish meal as a supplemental protein. Antibiotics and vitamin B12 allowed greater application of these changes.
These observations were the forerunners of the widespread testing and subsequent use of numerous antimicrobial agents in swine and poultry production. Rapers reported that more than 300 antibacterial substances had been identified. No doubt some of these were duplications as the reports were often published before complete identification of the active principle had been established and verified. To date, the number of antibiotics would far exceed this; however, a relatively small number has been adequately tested and approved for use as feed additives for the purposes of improving growth and feed conversion in pigs.
The effective antibiotics vary in chemical structure and in the relative amount absorbed. Some are readily absorbed and others are hardly absorbed at all. The absorptive capacity certainly influences their effectiveness against systemic infections; however, the absorption pattern is less readily associated with their effectiveness in improving growth rate and efficiency of feed conversion in apparently healthy animals. At least three modes of action have been postulated and have varying degrees of support: 1) a metabolic effect, in which the chemical constituency of the antibiotic, in some way, alters the rate or the pattern of the metabolic processes; 2) a nutrient-sparing effect, which reduces the dietary requirement for certain nutrients, either by allowing the growth of desirable organisms that synthesize essential nutrients, by depressing organisms that compete with the host animal for nutrients, or by improving the absorption of nutrients by the host animal; and 3) a disease-control effect through suppression of organism causing clinical or subclinical manifestations of disease.
Due consideration should be given to the first two or even other ways antibacterial agents may be affecting improved performance in pigs and chicks. However, the evidence for the first two modes of action would indicate they are of relatively minor importance, highly variable, and of questionable significance. Most diets can be adequately fortified with appropriate levels of all nutrients, though there may be some localities or extenuating circumstances that would limit availability of an optimum diet. There is evidence that the intestinal wall is thinner and interpreted to be more healthy with antibiotics and some experiments suggest this results in improved absorption. The greatest benefits are from limiting the effects of harmful organisms or preventing adverse effects of organisms that may or may not result in identifiable disease situations. At subtherapeutic or feed additive levels, antibiotics improve performance in the absence of clinical signs of harmful organisms.
There are numerous feeding, housing, and management programs that will affect the observed response to antibiotics. Also, the response is greater (percentage wise) in younger animals than in animals that are more mature or older. If it were economically and physically practical to house animals in a germ-free environment or in an environment free of any harmful organisms, there would be no need for antibiotics as feed additives or for therapeutic purposes. There are numerous reports that demonstrate that cleanliness in the environment improves performance and reduces the relative response to antibacterial agents. Wacholz and Heidenriech reported the results of an experiment in which pigs were housed in previously used dirt lots or in a clean barn. The observed responses to antibiotics were much greater in the dirt lots; however, the performance was much higher with the combination of a clean barn plus antibiotics. Hays and Speer reported the results of trials in facilities that differed in cleanliness at the start of the experiments. In one, the building was completely emptied, thoroughly cleaned, and all pigs moved in the same day. In the other experiment, pens were emptied and cleaned for one replication at a time, but the building was not completely emptied and thoroughly cleaned. The response to antibiotics was less (33%) in the cleaner environment than in the unclean barn (75%), but the overall performance was greater for the clean environment plus the antibiotic.
Mixing pigs of different ages, mixing pigs from different farms, or even mixing pigs from other buildings on the same farm can expose them to harmful organisms and higher incidents of clinical and subclinical disease. When such exposure is necessary, the adverse effects can be lessened with the appropriate use of antibiotics as feed additives.
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