Influence of continuous challenge via the feed on competitive exclusion of salmonellas from broiler chicks

In three replicate trials, chicks were treated orally with a culture of caecal contents from an adult hen and then exposed to continuous challenge from a strain of Salmonella kedougou incorporated in feed at a level of ca 10(2)/g. The contaminated feed was introduced at intervals from 0 to 48 h after treatment. Under these conditions, only one of 119 chicks in 12 separate groups was salmonella-positive at 7 d, compared with 36 of 119 controls, but thereafter protection was less consistent and in some cases the subsequent incidence of infected birds increased during the 3-week rearing period. At 21 d, 14 of 60 treated chicks were positive and 32 of 59 controls. Protection was more evident in chicks challenged 24 or 48 h after treatment than in those groups which received the salmonella-contaminated feed at 0 or 5 h and even untreated, control chicks showed greater resistance to salmonella infection from 24 h onwards. In some control and treated groups containing salmonella-positive birds, contamination of the litter reached ca 10(6) salmonellas/g, thus providing a considerable secondary challenge. Introduction of salmonellas via contaminated feed probably reduces the efficacy of protective treatment under field conditions and helps to explain the more limited success in commercial use.