Viral disease with rapid onset (2-12 days after infection with infectious aerosol). Young birds are susceptible. Clinical signs depend on the form of the disease:
1/ velogenic viscerotropic form: poultry may die without symptoms or show lethargy, respiratory distress, comb cyanosis, head oedema and watery green diarrhoea
2/ velogenic neurotropic form: neurological signs, acute to diphtheroid laryngitis and tracheitis, high morbidity and mortality
3/ pneumotropic mesogenic form: respiratory disease, sometimes mortality in young poultry, decrease in egg production
4/ lentogenic form: mild respiratory signs, 5/ enterotropic form: mild intestinal infection