In chickens, outbreaks may be so severe that almost all of the affected flock may dies within 72 hours without showing clinical signs. Canaries generally show a mild disease with low mortality. Most infections are in laboratory personnel. Pathogenic strains of ND cause sudden onset of depresssion, rapid breathing and anorexia in all ages of birds, and a sharp drop in egg production in laying hens. The viscerotropic pathogenic strain may cause edema of the head and bright green or bloody diarrhea as prominent clinical features.
Pathogenic strains are exotic in commercial poultry operations in Canada.