1. The characteristic lesions of infectious laryngotracheitis are ordinarily restricted to the respiratory tract and are most pronounced in the larynx and trachea. Sometimes the eyelids are affected. A certain percentage of the cases are associated with bronchitis and peribronchitis, pneumonic areas and hemorrhages in the lung, while the involvement of the nasal passages, communicating sinuses, and eyes seems to be dependent upon the mode of infection and the course of the disease.
2. The virus affects the epithelial cells primarily, but soon inflammation develops in the submucosa and underlying parts. Edema is often extremely pronounced in the submucosa. The destruction taking place at later stages is due to edema, cellular infiltration, and hemorrhages, and in some instances to secondarily invading bacteria.
3. Characteristic intranuclear inclusions in the epithelial cells of the trachea are present in many cases. They bear a close resemblance to the inclusions occurring in herpes, varicella, virus III of rabbits, and submaxillary gland disease of guinea pigs.