Small patches of alopecia are located on the lateral surfaces of the muzzle. They can result from mechanical trauma associated with repeatedly inserting the muzzle through holes in the cage cover or between metal rods forming the food hopper (but is probably more often due to whisker-trimming by cagemates). Histologically, the affected skin may show hyperkeratosis, acanthosis, and mild inflammation (Litterst, 1974).
Mice are known to grow hair in distinct patterns or cycles (Borum, 1954; Chase, 1954; Chase and Eaton, 1959; Argyris, 1963). Occasionally, one may see an entire litter of runted mice about weaning age with complete loss of hair on the torso. A small tuft of hair remains at the base of the tail and normal-appearing hair is present on the head and legs. Such mice generally have severe systemic disease (e.g., mouse hepatitis virus infection) and, presumably, are experiencing a temporary arrest of normal hair growth cycling (J. R. Lindsey, Department of Comparative Medicine, University of Alabama at Birmingham, unpublished).
This condition has been reported in mice (Nelson, 1960), laboratory rats (Njaa et al., 1957; Flynn, 1959; Totton, 1958), and the South African white-tailed hamster (Stuhlman and Wagner, 1971). It is characterized by the appearance of concentric rings around the tail, frequently followed by sloughing of all or part of the tail. The feet also may be swollen and reddened. The cause is thought to be environmental conditions of low (less than 40%) relative humidity and high (more than 80°F) temperature.