Practical pathology of aging mice

Pettan-Brewer, Christina; Treuting, Piper M.

Aging mice housed at the UW have four common clinical presentations, which are readily observable at the cage level and thus are frequently reported. These include rectal prolapse, alopecia and dermatitis, ocular lesions, and palpable masses. Whereas mice are reported and sometimes euthanized due to one of the above disease processes, there are often unsuspected comorbidities that may be more severe than the initial presenting clinical diagnosis. Many pathophysiological processes associated with aging, such as tissue atrophy, chronic inflammation, dysplasia, and occult neoplasia may not result in robust clinical signs (18) . Additionally, moribund animals may be euthanized due to relatively non-specific signs such as hunched, cold to the touch with loss of body condition, and increased respiratory effort. The morbid state is often a result of the combination of diseases, especially in older mice (21, 23) . To characterize the full spectrum of lesions present at the EOL, and the definitive diagnosis of diseases, necropsies and histological examinations are required. Whereas clinical pathology including complete blood counts with differentials and chemistry panels are widely used antemortem diagnostic tests in most species, the limited amount of blood that can be humanely obtained from live mice restricts the number of tests that can be performed with any one sample. However, terminal blood collection usually provides an adequate volume for full panels. Table 3 provides clinical pathology data and histopathological diagnoses for two of the mice illustrated in this manuscript. In the mice presented here, renal, vascular, inflammatory, and degenerative diseases were the common lesions diagnosed by histology. In the following section, the common age-associated diseases and lesions in the C57BL/6J colony at the UW are reviewed and illustrated.