Abstract
Respiratory viral diseases infecting poultry lead to variable lesions in the respiratory organs, including nasal sinuses, trachea, lungs, and air sacs. Additional involvement of eyelids/conjunctiva was reported. The distribution and the intensity of lesions depend on multiple factors, including virulence, the host's immunity, and secondary or concurrent infections. It may be challenging to detect remarkable lesions during experimental infections conducted in a controlled environment because some viruses fail to produce the intense lesions seen in field cases. This creates a challenge in developing a reliable model to study pathogenicity or vaccine efficacy experimentally. The development of the proposed histologic respiratory index (HRI) aims to help monitor the least microscopic changes that can be scored, thereby creating an objective and accurate grading of lesions in experimentally infected birds. HRI scores the changes in eyelids/conjunctiva and respiratory mucosa, including hyperplasia, metaplasia, inflammatory cellular infiltration in the submucosa, including lymphocytes and heterophils, and vascular changes (vasculitis) in nasal sinuses, trachea, and lungs. The score was validated in birds infected experimentally with avian metapneumovirus (aMPV) and low pathogenic avian influenza (LPAI-H4N6). The HRI reliably graded higher scores in the respiratory organs of experimentally infected birds compared with non-infected control ones. The HRI is the first of its type with poultry viral respiratory pathogens and it was initially proven to be a reliable in pathogenicity and vaccine trials of certain poultry respiratory viral diseases.