Spontaneous ophthalmic lesions in young Swiss mice.

Background and purpose: Outbred mice are frequently used in toxicity evaluation. Due to their small size, ophthalmologic examination of such animals is difficult with regard to restraint and use of instruments designed for human medicine. The clinical appearance and incidence of spontaneous ophthalmic lesions should be helpful in selecting mice for toxicity studies and allow distinction between intercurrent spontaneous ocular changes and those attributable to drugs or chemicals.

Methods: Pretest ophthalmologic examinations of about 3,000 4- to 5-week-old Swiss mice, Crl:CD1 (ICR)BR, conducted in 1995 and 1996, provided information about spontaneous ocular changes and their incidence. Eye evaluations were performed after pupil dilatation (0.5% tropicamide instillation), using indirect ophthalmoscopy, and when indicated, a portable slit lamp.

Results: Lenticular opacities and heterogeneity/prominence were the most common findings (up to 19%) in the anterior segment. Abnormalities of the cornea 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 the vitreous of 12 to 17% of mice. Approximately 2 to 4% of mice had colobomatous fundus, retinal fold, or retinal atrophy. A few mice had chorioretinal atrophy, hemorrhage, or abnormal pattern of the retinal vasculature. Remaining findings consisted of incomplete palpebral fissure, microphthalmia, exophthalmia, ophthalmic hemorrhage, and scleral mass.

Conclusions: Due to severity of the condition or interference with ocular examination, affected mice should be eliminated from experimental studies. Hence, pretest ocular examinations of mice are indicated in safety-assessment toxicity studies.