Defining susceptibility of broiler chicks to colibacillosis

This study aimed to define the susceptibility of broilers to colibacillosis through quantification of clinical responses and to examine the relationship between susceptibility and growth retardation. A challenge experiment was carried out twice. In each trial, 192 chicks were challenged intratracheally with Escherichia coli (E. coli) at 7 days of age and 160 chicks served as controls. Surviving chicks were euthanized at 14 or 15 days. Parameters measured were: daily mortality, lesion scores, body weight at 1, 4, 7, 10, 12 and 14 or 15 days and feeding behaviour at 6, 11 and 13 days. The results were reproducible, and increasing susceptibility to colibacillosis was defined by four categories: chicks without lesions, chicks with airsacculitis but no systemic lesions, chicks with systemic lesions, and chicks that die. Increasing susceptibility was associated with increasing growth retardation, but growth retardation was not inevitably linked to challenge with E. coli.