QUANTIFICATION OF EXPERIMENTAL SALMONELLA-ENTERITIDIS CARRIER STATE IN B13 LEGHORN CHICKS

Quantification of the carrier state of Salmonella enteritidis in chick s (i.e., persistent asymptomatic association of S. enteritidis with th e host), should provide an opti mized means for further investigations into this problem. We therefore developed an experimental carrier sta te model by oral inoculation of low doses (10(2)-10(4)) of S. enteriti dis in B13 chicks at different ages. Liver, spleen, and ceca colonizat ions by the challenge strains were measured weekly by enumeration of S . enteritidis colony forming units (CFU) for 7-12 weeks. High mortalit y rates, incompatible with the carrier state, were observed in chicks inoculated with 10(2) organisms of either a parental strain of S. ente ritidis (5556) or a mutant resistant to streptomycin (Sm-r) and nalidi xic acid (Nal(r)) (strain 1009) at 1 day old. Both strains colonized o rgans similarly, allowing us to use subsequently the Sm(r)Nal(r) mutan t strain. The selected low doses of S. enteritidis induced no deaths i n chicks inoculated at 1 or 3 weeks of age. However, inoculation of 3- week-old chicks did not induce a satisfactory carrier state; organ col onization by S. enteritidis was weak and transient, even after inocula tion of 10(8) SE. In contrast, some birds infected at 1 week of age pr esented the challenge strain in the liver and spleen for 3 weeks after inoculation and in the ceca for 12 weeks postchallenge. Most of these birds were colonized by S. enteritidis in the liver and in the ceca f or 3 weeks and 10 weeks, respectively, following inoculation. Generall y, CFU levels were highest during the first week(s) after inoculation and then decreased progressively. Levels of S. enteritidis were lower in the liver and spleen than in the ceca. Oral inoculation of 1-week-o ld birds with 5 x 10(4) S. enteritidis provided the required model, al lowing quantification of the carrier state of S, enteritidis in chicks .