In FVR , the conjunctivae and nasal raucous membranes are reddened, swollen, and covered with a serous to purulent exudate. In severe cases, focal necrosis of these membranes may occur. The larynx and trachea may be mildly inflamed. The characteristic histologic lesion of FVR is the acidophilic intranuclear inclusion body. During the early stage of the illness, inclusions may be present in sites of epithelial necrosis on the tongue, nasal membranes, tonsils, epiglottis, trachea, and nictitating membranes.
Inclusion bodies are transitory.
Inclusions do not occur in calicivirus infections. The characteristic lesion caused by FCV is ulceration of the oral mucosa. Lesions on the tongue or hard palate initially may appear as vesicles, which subsequently rupture. Ulcerations are occasionally found on the epithelium covering the median nasal septum. Early in the clinical course of feline pneumonitis, the causative organism may be identified in Giernsa-stained conjunctival smears or scrapings.