Ornithobacterium rhinotracheale (ORT) is an acute highly contagious disease of chickens and turkeys. ORT infection manifests in relatively mild respiratory signs in young birds, especially in turkeys. The disease is usually associated with respiratory symptoms, growth retardation, reduction of egg production, and mortality, with economic losses to the poultry industry. The respiratory symptoms include tracheitis, air-sacculitis, pericarditis, sinusitis, and exudative pneumonia, with fibrin purulent lesions and often unilateral pneumonia. Inactivated vaccines are used to protect birds. The major challenge in vaccine development against ORT infection is the existence of different serotypes.