Respiratory signs in poultry have many causes and overlapping presentations. Treatment started before a working diagnosis wastes money and drives antimicrobial resistance.
Begin with history: age, vaccination record, ventilation, litter condition, ammonia levels, stocking density and the shape of the mortality curve. The pattern of spread across the shed narrows the list before any bird is examined.
Postmortem a representative sample, including recently affected birds rather than only long-dead carcasses. Record lesions systematically along the respiratory tract, then match the lesion pattern with the flock history.
Confirm with laboratory support where available, and use the result to revise the vaccination and biosecurity plan. Diagnosis is only useful if it changes the management of the next cycle.