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Over the past 3 years the frequency of Salmonella hadar infections has increased in Belgium in both poultry and humans. Therefore, the course of infection with S. hadar in poultry was investigated. One day-old and 4 week-old specific pathogen-free chickens were orally infected with one of two S. hadar strains, SH1 or SH2. Mortality was 6% (SH1) and 17% (SH2) in birds infected at 1 day-old. Chickens infected at 1 day-old with SH2 showed a mild diarrhoea. The S. hadar faecal excretion in birds infected at 1 day-old remained high throughout the experiment until 12 weeks post-inoculation (pi). Faecal excretion was lower in older birds. Antibodies to S. hadar were observed from 2 weeks pi (SH2, infected at 1 day-old) or 4 weeks pi (SH1, both groups; SH2, chickens infected at 4 weeks of age). The percentage of chickens with antibodies was higher after infection at 1 day-old than after infection at 4 weeks of age. In a second experiment 1 day-old chicks were infected with SH1 and autopsied at regular intervals until 42 days pi. SH1 was isolated from the caeca from 3 h pi onwards and from the liver and spleen from 18 h until 14 days pi. Serous typhlitis and omphalitis were the main lesions. The number of macrophages in the lamina propria of the caecal tonsils was slightly increased from 18 h until 2 weeks pi. In the liver, inflammation was observed in the portal triads and in the sinusoids. This study indicates that infections with S. hadar lead to intense colonisation of the gut and extensive faecal shedding. It may also cause invasive infections in 1 day-old chickens.

White leghorn specific-pathogen-free chickens were inoculated orally with Salmonella enteritidis phage type 4 at the age of one day (group 1) and four weeks (group 2). From 3 h until 4 weeks post inoculation (pi), birds were sacrificed. Gross lesions were recorded and different sites of the intestine and visceral organs were collected for bacteriological and histopathological examination. Clinical disease and mortality were only observed in group 1. Mortality was 8%. The birds were depressed, had diarrhoea and an indurated yolk sac. Infection of the liver and the heart was present within 12 h pi in both groups. The percentage of infected organs was very high and similar in both groups during the first week pi. Thereafter the isolation rate of Salmonella was declining faster in group 2. The crop, the proventriculus, the lower intestinal tract and the bursa of Fabricius were the predilective sites of isolation in both groups. Most prevalent lesions were serous typhlitis, omphalitis and polyserositis. Histopathology revealed inflammation in the intestines and visceral organs. In some birds granulomatous nodules were present in the caeca. Antibodies were detected from 18 and 5 days pi in group 1 and 2, respectively. Granulomatous nodules were typical of infection with this strain of S. enteritidis phage type 4. These granulomatous nodules together with the retained yolk sac possibly are a source of Salmonella organisms that may account for intermittent faecal shedding by carrier birds.