Contaminated eggs have been implicated more often than any other food vehicle in the transmission of human illness caused by Salmonella enteritidis (Centers for Disease Control, 1996; Hogue et al., 1998). The implementation of a program to assure the microbial quality of shell eggs has been associated with a reduction in the frequency of isolation of S. enteritidis from poultry houses in Pennsylvania (White et al., 1997), but the overall national incidence of S. enteritidis in poultry flocks does not appear to have declined significantly in recent years (Hogue et al., 1997). The development of effective strategies for lowering the incidence of S. enteritidis in commercial laying flocks is contingent on accurate identification of the principal
sources of infection.
Environmental reservoirs, including poultry house dust and rodents, have been shown to play important roles in the introduction and perpetuation of S. enteritidis infection in laying flocks (Henzler and Opitz, 1992; van de Giessen et al., 1994; Davies and Wray, 1995, 1996). The possible significance of vertical transmission (or horizontal transmission during the first days of life) is far less certain because the ability of S. enteritidis infection to persist in chickens from the hatchery to the laying house has not been thoroughly documented. Egg contamination by S. enteritidis can lead to vertical transmission of the organism to chicks (Methner et al., 1995) and the hatching of Salmonella-contaminated eggs can result in horizontal transmission of infection in the hatchery (Bailey et al., 1994; Cason et al., 1994). Newly hatched chicks are highly susceptible to Salmonella infection through diverse orifices (Cox et al., 1996) andairborne movement of bacteria can result in rapid and extensive dissemination of infection (Lever and Williams, 1996; Gast et al., 1998). In the context of efforts to prevent foodborne disease in humans, the significance of vertically transmitted or hatchery-acquired S. enteritidis infections is directly related to the potential for these infections to persist until the affected chicks mature and begin to lay eggs. Without documentation that such persistence is possible, the most cost-effective strategy for preventing egg contamination would involve focusing almost entirely on environmental sources of S. enteritidis in pullet and laying houses. The objective of the present study was to determine whether intestinal colonization and organ invasion by S. enteritidis can persist into adulthood following the oral inoculation of newly hatched chick.