Use of antibiotics in livestock and human health concerns.

Animal feed additives have for many years constituted the largest and most controversial category of antimicrobial use. The primary concerns addressed have been the generation of antibiotic resistance in animal bacterial and the influence of such resistance on human health. Studies designed to shed light on the controversy generally have yielded information leading to ambiguous conclusions. A 1989 report from the National Academy of Sciences was the latest of a long collection of assessments that have been meant to provide expert guidance to regulatory authorities on this matter. The FDA has been trying for some time to determine whether a regulatory decision on existing products is warranted. After several decades of research directed at this question, most qualified study groups have reported a paucity of appropriate information necessary to judge the reality of a public health risk.

Comparison of therapeutic versus subtherapeutic levels of chlortetracycline in the diet for selection of resistant salmonella in experimentally challenged chickens.

Sixteen broiler chicks per group were fed chlortetracycline (CTC) at 0 (control) and 55, 110, and 220 ppm (subtherapeutic levels) continuously for 44 days or 550 ppm (therapeutic level) for three 5-day periods from 1 to 19 days of age. All birds were challenged at 4 days of age with a 10:1 mixture of CTC-sensitive and resistant (CTCr) Salmonella typhimurium. Chicks were sampled periodically through postchallenge day (PCD) 41, half in each pen by cloacal swabbing and the remainder by collection of droppings. Escherichia coli was monitored at PCD 6 and 34. Salmonella recovery from cloacal swabs indicated increased (P less than .05) prevalence of CTCr salmonella-positive birds in the 550 ppm treatment at PCD 6 and 13 compared to all other treatments, and at PCD 27 compared to 0, 55, and 110 ppm CTC. Mean recovery scores followed a similar pattern. Area under the curve analysis of CTCr salmonella scores from cloacal swabs for PCD 3 to 41 confirmed increased (P less than .05) selection for CTCr salmonella by 550 ppm CTC. Isolations from droppings showed increased (P less than .05) CTCr salmonella prevalence at PCD 20 for the 100 ppm treatment group compared to control, and at PCD 34 for the 110 ppm groups compared to all other groups and for 220 and 550 ppm groups compared to 0 and 55 ppm CTC birds. The CTCr salmonella counts in droppings were higher (P less than .05) at PCD 34 for the 110, 220, and 550 ppm groups compared to the control.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors influencing salmonella shedding in broiler chickens.

Three variables were included in a study to determine their effect on the incidence of Salmonella typhimurium in broilers challenged at four days of age. Variables included the presence or absence of a feed additive, avoparcin; the use of new or used litter and the initiating dose of salmonella. Cloacal swabs were taken from approximately 600 chicks at weekly intervals for 45 days. At 10(4), 10(6) and 10(8) c.f.u./chick there was a direct association of challenge dose and the incidence of positive chicks for the first several weeks. Chicks raised on used litter showed an appreciable reduction in susceptibility to salmonella when compared to control animals on fresh litter. As the birds approached slaughter age, the influence of litter hygiene and challenge dose diminished under the conditions of this study. Avoparcin in the diet at 10 p.p.m. had no enhancing effect on salmonella shedding at any time during the 45-day sampling period. The implications of competitive exclusion are discussed.

Enhancement of immunity against murine syngeneic tumors by a fraction extracted from non-pathogenic mycobacteria.

The data reported here demonstrate that a preparation extracted from nonpathogenic mycobacteria such as Mycobacterium smegmatis and hereafter referred to as interphase material protected mice against Ehrlich ascitic carcinoma, L-1210 leukemia, and another syngeneic lymphoid leukemia. Furthermore, mice treated by this preparation were much less susceptible to endotoxins than when stimulated by BCG (bacillus Calmette-Guerin) or M. smegmatis cells. Moreover, guinea pigs treated by interphase material administered in Freund's incomplete adjuvant showed an increased immune response, yet their sensitivity to tuberculin was much weaker than that of controls sensitized with Freund's complete adjuvant. Finally, resistance to Columbia SK virus infection could be demonstrated when interphase material was administered to mice prior to virus challenge.

Biological study of a nontoxic, water-soluble immunoadjuvant from mycobacterial cell walls.

Whole mycobacterial cells, which are used in Freund's complete adjuvant, besides inducing hypersensitivity to tuberculoprotein, also can elicit hyperreactivity to endotoxins, lymphoid hyperplasia, and allergic polyarthritis in rats. The data reported here demonstrate that a potent water-soluble adjuvant obtained from mycobacterial cell walls is also effective in increasing the immune response to viruses, and that it is free of the toxic effects observed with whole mycobacterial cells.