Environmental contamination and horizontal transmission of Salmonella Enteritidis among experimentally infected layer pullets in indoor cage-free housing

The persistence and transmission of the egg-associated pathogen Salmonella Enteritidis in laying flocks are significantly influenced by the poultry housing environment. The present study assessed environmental contamination and horizontal transmission of S. Enteritidis within groups of layer pullets in cage-free housing after infection just before the age of sexual maturity. In each of 3 trials, 144 pullets were transferred from a rearing facility at 15 wk of age and randomly distributed between 2 isolation rooms simulating commercial cage-free barns with perches and nest boxes (72 birds/room). One wk after placement in the containment facility, a proportion of the 72 pullets in each room were orally inoculated with approximately 6 × 10⁷ cfu of S. Enteritidis: 1/3 in trial 1, 1/6 in trial 2, and 1/12 in trial 3. At 2 wk post-inoculation in each trial, samples of liver, spleen, and intestinal tract were collected from 40 uninoculated (contact-exposed) birds in each room for bacteriologic culturing to detect horizontal transmission of S. Enteritidis. At 6 intervals between inoculation and necropsy, 5 types of environmental samples (wall dust swab, nest box swab, perch swab, flooring substrate drag swab, and flooring substrate composite) were collected and cultured for S. Enteritidis. The overall frequencies of S. Enteritidis recovery from both environmental samples and internal organs from contact-exposed pullets after initial oral inoculation of 1/3 of the birds in each room (97 % and 75 %, respectively) were significantly greater than after initial infection of 1/6 of the birds (78 % and 58 %), and S. Enteritidis recovery from birds inoculated at a 1/12 proportion (10 % of environmental samples and 18 % of organs) was significantly lower than from the 1/6 inoculation group. Flooring substrate composites were the most efficient environmental sample type for all 3 trials combined (72 % positive), providing significantly better S. Enteritidis recovery than the least efficient samples (flooring substrate drag swabs; 53 % positive). These data suggest that a high frequency of environmental contamination may be an important contributor to horizontal transmission of S. Enteritidis infections among pullets in cage-free housing.