Pathologic and bacteriologic findings in 27-week-old commercial laying hens experimentally infected with Salmonella enteritidis, phage type 4

Two strains of 27-wk-old commercial laying chickens (strain A, brown-egglaying type and strain B, white-egg-laying type) were inoculated either orally (PO) or intravenously (IV) with a field isolate of Salmonella enteritidis phage type 4. Chickens were sequentially necropsied at regular intervals throughout the 17-wk observation period. Gross and microscopic lesions were most evident between 1 and 14 days postinoculation (DPI). Gross lesions consisted of enlarged livers with white foci, enlarged and mottled white spleens, fibrinous exudate in the peritoneum, and atretic, misshapen ovarian follicles. Microscopic lesions included multifocal coagulative necrosis of hepatocytes and inflammation, fibrinous exudation in vascular sinuses of the spleen, and fibrinosuppurative inflammation of the peritoneum and ovarian follicles. The proportion of reproductive organ infections (ovary and oviduct) in the IV group, 83% (20/24, P = 0.007; 50% and 33% for strains A and strain B birds, respectively), was higher than that of the PO group, 46% (11/24; 29% and 17% for strains A and B, respectively), for the first 16 days of observation postinoculation. The proportion of fecal shedding for the IV group of birds was significantly (P = 0.009) lower, 29% (7/24; 33% and 25% respectively fur strain A and strain B birds, respectively), than the PO group, 67% (16/24; 75% and 58% for strain A and strain B birds, respectively). Three (2.6%) of 234 egg pools were culture-positive for group D Salmonella from strain A chickens (1 of 119 pools from the IV group and 2 of 115 pools from the PO group of birds). Chickens infected with the field strain of S. enteritidis phage type 4 harbored the organism in tissues only for a brief time, most clearing within 8 DPI and nearly all within 16 DPI. Overall the percentage of culture-positive birds did not differ significantly (P > 0.05) between birds with and without lesions, but isolation of S. enteritidis tended to be more frequent when lesions were evident. This experiment also demonstrated that brown-egg-laying-type chickens were more susceptible than white-egg-laying-type chickens to S. enteritidis phage type 4 isolated from California based on gross and microscopic lesions and bacteriologic findings.