ND is a notifiable disease to the World Animal Health Organization (OIE), a global risk analysis can be done. It enables to distinguish high challenge (where ND outbreaks are frequent) from low challenge countries.
In high challenge countries, when industrial chickens are experiencing a ND outbreak, they would display high mortality (up to 100%), high morbidity, listlessness, dyspnea, diarrhea, sometimes nervous signs (eg, torticollis, ataxia). At necropsy, internal organs are usually heavily haemorrhagic, especially the proventriculus, the cecal tonsils, the duodenum, the trachea, the brain.
In low challenge countries, industrial chickens are often facing the uncontrolled circulation of lentogenic (vaccine) strains, especially in high densely populated poultry areas. As a result, they may show subtle to overt respiratory signs, because of the inflammation of the trachea. It may worsen in case of suboptimal husbandry conditions (too high stocking density, high ammonia level, wet litter, poor ventilation). As a result, flock uniformity will decrease, and secondary opportunistic respiratory infections (eg, E.coli) may arise with the need to apply antibiotic medication. Ultimately, slaughterhouse condemnations may increase due to excessive airsacculitis.