Outbreak of Hind Limb Paralysis in Young CFW Swiss Webster Mice

An outbreak of paralysis among 16- to 20-week-old CFW Swiss Webster sentinel mice developed in one of our barrier facilities. Two months after arrival and over a period of four weeks, six of 400 mice purchased from an approved vendor, developed progressive hind limb paralysis without other clinical signs of disease. On the basis of the histopathologic changes and negative serologic test results, lymphoblastic lymphoma causing compression of the spinal cord was diagnosed. There were two leading features to this outbreak: its unusual epidemiologic presentation, and the localization of the lesions principally in the lumbar muscles. A presumptive diagnosis of retroviral infection with Abelson's murine leukemia virus (A-MuLV) was established on the basis of histopathologic and immunohistochemical findings. Little is known about retroviral status in many commercial colonies, and few users report presence of spontaneous lymphomas. This report points out complications derived from commercially available animals that carry endogenous retroviruses. It also emphasizes the need of diagnosing and reporting clusters of hind limb paralysis or lymphomas in mice to assess the prevalence and relevance of retroviral infections in commercial colonies.