Experimental study of some factors limiting 'competitive exclusion' of salmonella in chickens

Some factors affecting the efficiency of competitive exclusion of Salmonella typhimurium var copenhagen in chicks were studied experimentally under gnotobiotic conditions. Axenic chickens were given dilute suspensions of adult faeces and exposed to the salmonellae. Prevention of colonisation of the gut by salmonellae ('competitive exclusion') was variable and depended possibly in part on the source of the adult faeces used to protect the chicks. Exclusion was also dose-dependent, a large inoculum of the salmonellae (10(7) viable organisms per animal) leading to colonisation in treated chicks. An inapparent carrier state was sometimes produced by lower doses of the salmonellae (10(2) viable organisms per animal), but bursts of excretion still appeared after inoculation of the salmonellae. In inapparent healthy carriers, the inoculation of a dose of Eimeria tenella oocysts that was known to produce subclinical caecal coccidiosis led to the shedding of large numbers of salmonellae for over two weeks, and the use of 'competitive exclusion' in poultry as a preventive measure for salmonella infections might thus be limited by the frequent occurrence of subclinical coccidiosis in the field.