Toxicologic pathology involves microscopic examination of organ and tissue specimens from laboratory animals that have been exposed to candidate drugs, devices, or various chemical or biological agents. The goals are to identify organ system toxicity, dose levels that produce toxicity, and biomarkers of toxicity. A variety of investigative techniques are employed in the detection of histomorphologic alterations, most commonly light microscopic examination of histologic tissue sections and preparation of reports containing subjectively based diagnostic terms and interpretations that convey the identity and anticipated significance of observations. Contemporary clinical pathology evaluations are performed to help identify changes in bodily fluids which may precede and/or accompany histological alterations and further characterize these changes and their adversity. Clinical pathology evaluations also help identify potential clinical biomarkers of xenobiotic-associated tissue damage.

Study objectives: Idiopathic pulmonary fibrosis (IPF) is a poorly understood chronic respiratory disease of humans, which has no correlate in other animals. Understanding the role that inflammation, alveolar epithelial cells, and myofibroblasts play in the progression of the disease is controversial, and hampered by the lack of an animal model. We have identified spontaneous IPF in domestic cats and hypothesized that this newly identified disease shares the pathology of human IPF; further, this work provides data suggesting that the disease is related to a defect in type II pneumocyte biology.Setting and subjects: Chronic respiratory disease with pathology consistent with usual interstitial pneumonia (UIP) spontaneously developed in 16 domestic cats.Results: The histopathology of feline IPF consisted of the following: (1) interstitial fibrosis with fibroblast/myofibroblast foci, (2) honeycombing with alveolar epithelial metaplasia and type II pneumocyte hyperplasia, and (3) alveolar interstitial smooth-muscle metaplasia. Interstitial inflammation was not a prominent feature of the disease. alpha-Smooth muscle actin-positive myofibroblasts were prominent in myofibroblast foci, beneath honeycomb and hyperplastic epithelium, and in alveolar septa away from the remodeling. Feline IPF type II pneumocyte, ultrastructure is similar to a heritable form of human IPF, with abnormal cytoplasmic lamellar body-like inclusions.Conclusions: We conclude the following: (1) chronic respiratory disease with clinical and pathology features of UIP/IPF occurs in the domestic cat; (2) as in human IPF, the type II pneumocyte and myofibroblasts are important cellular constituents of feline IPF; and (3) type II cell ultrastructure suggests feline IPF is a defect in the type II pneumocyte.