Experimental infection of chickens with Salmonella enteritidis.

Following oral inoculation of newly hatched commercial broilers, four phage types of Salmonella enteritidis showed considerable variation in their virulence. The percentage mortality varied from 96%, produced by a phage type 4 strain, to 20% (phage type 13a). When high mortality was observed, chickens that died showed a polyserositis with pericarditis similar to that observed in naturally occurring cases. Strains of phage types 4, 6 and 8 were more virulent than a phage type 13a strain when inoculated by the intramuscular route. Similar variations in virulence were observed in newly hatched Rhode Island Red chickens although the overall mortality observed was much reduced. Polyserositis was again observed. Despite similarly high viable numbers of all the phage types being present in the caecal contents, the strains of phage types 4 and 6 were isolated from the spleen at 12 h post-inoculation whereas phage types 8 and 13a were not. After 24 h the viable counts of phage type 4 in the spleen were ten times higher than those of type 13a. However, it is unclear whether this is the result of enhanced invasiveness only or whether an increased ability to multiply in the spleen is also involved. All strains were very invasive in cultured Vero cells; phage type 4 strains slightly more so than types 6, 8 and 13a. Strains of phage type 4 and 13a persisted in the faeces of chickens placed in-contact with orally inoculated birds longer than did strains of types 6 or 8.