Characterization of the invasiveness of monophasic and aphasic Salmonella Typhimurium strains in 1-day-old and point-of-lay chickens.

Egg-related outbreaks of salmonellosis are a significant health concern. Although Salmonella Enteritidis (SE) is the major egg-associated serotype, Salmonella Typhimurium (ST) can also infect the hen's reproductive tract and contaminate eggs. Recently, monophasic and aphasic variants of ST have been reported with increased frequency in Europe, and the isolation of these variants from laying flocks triggers the same legislative restrictions associated with biphasic ST strains. However, little is known about the colonization, invasiveness and persistence of monophasic and aphasic ST strains in laying hens. In this study, seven groups of 1-day-old and point-of-lay commercial Hy-line chicken layers were separately challenged with four different strains of monophasic ST, one aphasic ST, one biphasic ST and one egg-invasive SE strain. Tissue samples and cloacal swabs (point-of-lay chickens only) were collected at regular intervals post challenge in order to recover the Salmonella challenge strains. In 1-day-old chicks, only the aphasic ST strain and the SE strain were recovered after direct plating, suggesting that the number of salmonellas colonizing the tissues of the chicks infected with the other strains was likely to be low. Interestingly, all of the strains colonized well in the point-of-lay chickens, and there was no statistical difference in the overall number of positive samples or Salmonella counts between the seven strains. Salmonella was recovered from the point-of-lay birds to the end of the study (20 days after challenge). Monophasic and aphasic ST strains colonized point-of-lay birds as efficiently as biphasic ST and SE strains. Further studies are necessary to estimate the invasiveness of these strains in naturally-infected vaccinated laying hens, and to assess the impact of natural infection on egg contamination.