SUMMARYThree 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 104, 106 and 108 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.

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)

Summary Three slightly different models were used to examine the influence of avoparcin on salmonella in broilers. All chicks were raised on litter and were infected with low levels of a nalidixic acid resistant strain of Salmonella typhimurium. In the first study, chicks were infected by gavage with approximately 106cfu at 4 days of age. The incidence of chicks positive for salmonella fell in both groups from more than 50% on day 6 to below 10% on day 56. The salmonella disappeared at a slightly faster rate in the controls. In the second experiment, chicks were infected with slightly less than 103 cfu at 4, 7, 11 and 14 days of age. The higher initial incidence of salmonella shedding in the chicks given virginiamycin fell sharply to levels below the controls by day 33. The avoparcin chicks showed slightly higher shedding rates than the controls on day 19, but levels slightly lower than the controls on days 5, 12, 26, 33, 40 and 47. In caecal samples at slaughter the incidence of salmonella was between 9 and 16% in the three groups. In these two experiments feed additives were started in the diet on day 1, soon after the chicks arrived from the hatchery. In the third experiment, avoparcin was not added to the diet until 16 days of age and birds were challenged with 2.2 × 106 cfu at 21 days. The peak incidence, of over 50%, 7 days after challenge, fell to low levels in both groups within 28 days. The rate of restoration from salmonella positive to negative was slightly greater in avoparcin chicks than in controls but the difference was not significant. Mean salmonella numbers per gram of excrement at each sampling interval were also slightly lower in the avoparcin group. The results with these experimental models suggest that the use of avoparcin at 10 ppm. in poultry diets does not increase the incidence of salmonella in broilers approaching market age. Zusammenfassung Einfluß von Avoparcin auf Salmonellen‐Infektion in Broilern Um den Einfluß von Avoparcin auf Salmonellen in Broilern zu prüfen, wurden drei etwas unterschiedliche Methoden angewandt. Bei allen drei Methoden wurden die Küken auf Lattenrosten gehalten und mit geringen Dosen eines Nalidixinsäure‐resistenten Stammes von Salmonella typhimurium infiziert. Im ersten Versuch wurden die Küken mittels Magensonde mit etwa 106 cfu am 4. Lebenstag infiziert. Das Vorkommen von Salmonella‐positiven Küken wurde in beiden Gruppen von mehr als 50% am 6. Tag auf weniger als 10% am 56. Tag reduziert. Bei den Kontrolltieren wurden die Salmonellen etwas schneller abgebaut. Im zweiten Versuch wurden die Küken am 4., 7., 11. und 14. Lebenstag mit etwas weniger als 103 cfu infiziert. Das höhere anfängliche Vorkommen von Salmonellenausscheidung bei den Küken, die mit Virginiamycin behandelt worden waren, wurde stark reduziert auf Werte, die am 33. Tag unter denen der Kontrolle lagen. Die Küken, die Avoparcin bekommen hatten, zeigten am 19. Tag eine höhere Salmonellenausscheidung, aber die Werte waren am 5., 12., 26., 33., 40. und 47. Tag etwas niedriger als die der Kontroll...