Observations on experimental infection of chicks with Escherichia coli.

A study was made of the pathogenesis of colibacillosis in chicks in order to develop a reproducible model for the induction of the disease by inoculation. In experimental infections with Escherichia coli (E. coli), serotype 026K60 was pathogenic only during the first week of age, whereas serotype 078K80 caused lesions during the first 3 weeks. Natural resistance against E. coli occurred after 1 and 3 weeks respectively. Chicks were much more susceptible to intratracheal than to oral inoculation. The disease could be produced in newly hatched chicks only with large oral doses. It is considered likely that natural infection occurs by the airborne route, and that most field infections are secondary to infections of the respiratory tract by virulent viruses. Adequate ventilation, especially during the last half of the fattening period, is therefore recommended as a possible prophylactic measure. The infection model in which E. coli is inoculated 4 days after vaccination with infectious bronchitis virus (strain H52) can be used for experimental purposes, such as challenge experiments, but only with chicks younger than 4 weeks old.