The comparative virulence for chicks of Salmonella enteritidis phage type 4 isolates and isolates of phage types commonly found in poultry in the United States.

Phage type 4 Salmonella enteritidis has been associated with morbidity and mortality in broiler chickens in the United Kingdom. The recent isolation of this phage type from poultry in the United States has raised concerns about whether the current regulatory approach to S. enteritidis should be modified to consider phage type 4 differently from other phage types. The present study assessed and compared the virulence of phage type 4 S. enteritidis isolates, S. enteritidis isolates of other phage types, and an S. pullorum isolate in both single-comb white leghorn and white Plymouth Rock chicks. The mean incidence of severe illness or death following oral inoculation with phage type 4 S. enteritidis was significantly lower than the incidence associated with S. pullorum inoculation in both lines of chicks. Nevertheless, some individual phage type 4 S. enteritidis isolates caused severe effects at a frequency similar to that of S. pullorum in single-comb white leghorn chicks. In general, severe morbidity or mortality following infection with S. enteritidis isolates of all phage types tested occurred more often in single-comb white leghorn chicks than in white Plymouth Rock chicks. The mean frequency at which chicks were severely affected following inoculation with phage type 4 isolates was significantly higher than the mean for isolates of other phage types. However, in both lines of chicks, some significant differences in virulence were apparent within the set of phage type 4 strains tested. The observed virulence for chicks of recent U.S. poultry isolates of phage type 4 S. enteritidis was similar to that of earlier isolates from various sources, including poultry isolates from the United Kingdom.