Spontaneous ophthalmic lesions in young Swiss mice

Background and Purpose: Outbred mice are frequently used in toxicity evalua tion. Due to their small size, ophthalmologic examination of such animals i s difficult with regard to restraint and use of instruments designed for hu man medicine. The clinical appearance and incidence of spontaneous ophthalm ic lesions should be helpful in selecting mice for toxicity studies and all ow distinction between intercurrent spontaneous ocular changes and those at tributable to drags or chemicals. Methods: Pretest ophthalmologic examinations of about 3,000 4- to 5-week-ol d Swiss mice, Cr1:CD1(R)(ICR)BR, conducted in 1995 and 1996, provided infor mation about spontaneous ocular changes and their incidence, Eye evaluation s were performed after pupil dilatation (0.5% tropicamide instillation), us ing indirect ophthalmoscopy, and when indicated, a portable slit lamp, Results: Lenticular opacities and heterogeneity/prominence were the most co mmon findings (up to 19%) in the anterior segment. Abnormalities of the cor nea and iris were detected in up to 4% of mice. Hyaloid artery remnant, as well as isolated cases of floating bodies or hemorrhage, was observed in th e vitreous of 12 to 17% of mice. Approximately 2 to 4% of mice had coloboma tous fundus, retinal fold, or retinal atrophy. A few mice had chorioretinal atrophy, hemorrhage, or abnormal pattern of the retinal vasculature. Remai ning findings consisted of incomplete palpebral fissure, microphthalmia, ex ophthalmia, ophthalmic hemorrhage, and scleral mass. Conclusions: Due to severity of the condition or interference with ocular e xamination, affected mice should be eliminated from experimental studies. H ence, pretest ocular examinations of mice are indicated in safety-assessmen t toxicity studies.