RESUMO
Yolk sac infection (YSI) is a major cause of mortality of broilers during the first week post-hatching. The aim of the present-study was to analyze the possible sources of fertile egg contamination and to establish the etiology of YSI. Sixty fertile eggs, sixty sawdust samples from the nest, sixty nonfertile 19 to 21 day old incubation eggs and liver and yolk sac samples from 216 dead, 1 to 7 day old chicks, were cultured. Five hundred and eighty eight colonies were isolated and further characterized using biochemical tests. Escherichia coli was the most common bacterium recovered from all samples except the sawdust and fertile eggs collected from the nest. Fertile egg contamination at breeder farm level was found to be minimal. In broilers, both mortality and the rate of E. coli isolation were increased with the time. These results suggest that egg contamination does not occur at the breeders farm, as previously has been reported. Bacterial contamination causing YSI in vertically integrated operations can occur at a latter stage. It can be considered that the main etiologic agent of YSI is E. coli, since YSI mortality was highly correlated with E. coli isolation.