Dr. Richard Gast of the USDA-ARS National Poultry Research Center recently presented data on experiments to ascertain the transmissibility of Salmonella Enteritidis (SE) in pullets reared as cage-free. Subjects, presumably specific pathogen-free and unvaccinated were infected with two strains of S. Enteritidis at a level of 7.8 x 107 CFU. Pullets were infected at either nine or fifteen weeks of age and sampled one to two weeks post inoculation and again at 21 weeks of age.
Rate of recovery of SE from organs in infected pullets at 21 weeks of age was higher in the group infected at 15 weeks compared to the nine-week treatment. The experimental design incorporated non-infected contact birds to determine rate of transmission under cage-free rearing. SE was recovered at a higher rate in contacts with pullets infected at 15 weeks compared to the nine-week treatment.
A second trial evaluated the rates of horizontal transmission according to the proportion of pullets infected with a range of 8 to 33 percent of a batch of 144 birds. It was determined that the rate of isolation of SE from organs of contact exposed birds at the age of transfer was proportional to the number of pullets infected. The proportion of pullets infected also influenced recovery of SE from the environment.
Sampling disclosed that a composite sample of litter was the most sensitive to detect Salmonella Enteritidis two weeks after infection. with consistent recovery from wall dust in the experimental rooms housing the infected and contact pullets. It is noteworthy that drag swabs from litter were the least sensitive among environmental samples.
The two experiments would have been more relevant to commercial production had the infective dose been lower, corresponding to the possible level of infection under practical conditions. The second question relates to the protective effect of vaccination. Industry practice for over 20 years has included live attenuated mutant Salmonella Typhimurium vaccine administered orally at day-old or during the early brooding period followed by an inactivated SE oil emulsion vaccine at 12 to 14 weeks. Results relating to infection of organs, contamination of the flock environment and horizontal transmission may have been different from the data obtained in the USDA trial had the experimental design approximated commercial practice.
The results of this trial are essentially intuitive and confirm the ability of SE to spread among flocks with access to litter and to result in contamination of their environment. Investigating the transmission of SE among floor reared pullet is commendable but additional studies should be conducted reflecting current practice by determining the response to a range of infective doses with and without vaccination in a factorial design.
Given that drag swabs showed lower sensitivity of detection of SE compared to other environmental samples, this method of determining the SE status of a flock as mandated by FDA is questioned. Literature shows that swabs from fan blades, paralleling the high recovery rate from wall dust, would probably be more appropriate and certainly less trouble than for the FDA protocol.