Twelve cats with pathologic diagnoses of feline infectious peritonitis were studied. The lesions were primarily extraserosal. This pathologic syndrome was characterized by granulomatous inflammation in a variety of organs, but principally affected the kidneys, visceral lymph nodes, lungs, liver, eyes, and leptomeninges. Histologically, the granulomas were composed of mixtures of neutrophils, lymphocytes, plasma cells, and large spindleshaped histiocytes, and contained areas of necrosis. Inflamed superficial and deep pulmonary and renal cortical veins, the latter often thrombosed, were consistently detected. Phlebitis was also present in other organs, but to a lesser degree. Cell-free tissue extracts from one cat produced clinical and pathologic changes compatible with those originally described for feline infectious peritonitis. Observations of these natural and induced cases show that this form of the disease with disseminated granulomas and minimal serosal lesions may progress by vascular routes.
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