Research Note: Internal organ colonization by Salmonella Enteritidis in experimentally infected layer pullets reared at different stocking densities in indoor cage-free housing

Contamination of eggs by Salmonella has often been identified as a source of food-borne human illness. S. Enteritidis is deposited inside developing eggs when invasive infections of laying hens reach the reproductive organs. The susceptibility of hens in cage-based housing systems to S. Enteritidis has been associated with their stocking density, but the applicability of this information to extensive (cage-free) systems is uncertain. The present study assessed internal organ colonization by S. Enteritidis in egg-type pullets reared at 2 different stocking densities in cage-free housing. Pullets were reared at either 374 cm(2) or 929 cm(2) of floor space per bird. At 16 wk of age, 4 groups of 72 pullets were moved into isolation rooms simulating commercial cage-free barns; 1/3 of the pullets in 2 rooms were orally inoculated with S. Enteritidis immediately after transfer and pullets in 2 rooms were similarly infected at 19 wk. At 6 and 12 d postinoculation, the pullets were euthanized and samples of liver, spleen, and intestinal tract were removed for bacteriologic culturing. No significant differences (P > 0.05) in S. Enteritidis isolation frequencies from any tissue were observed between high and low density rearing groups following infection at either age. However, S. Enteritidis was found significantly (P < 0.05) more frequently among pullets infected orally at 19 wk than at 16 wk in spleens and intestines. Likewise, the frequency of S. Enteritidis isolation from all birds (inoculated plus contact-exposed) at 19 wk was significantly higher than at 16 wk in livers and spleens. This increased susceptibility to invasive S. Enteritidis infection at reproductive maturity emphasizes the importance of risk reduction at a critical stage in the egg production cycle.