Availability of feces-free areas in rodent shoebox cages

The Guide for the Care and Use of Laboratory Animals (the Guide) recommends that terrestrial mammals be provided space free of urine and feces in which to rest. To evaluate the feasibility of meeting this recommendation, the author examined the availability of feces-free resting areas in standard rodent cages over time. Adult rodents (C57BL/6J mice and Wistar rats) were housed singly, in pairs or in trios in shoebox cages at densities that met the space recommendations of the Guide. As housing density increased, the availability of unsoiled resting space declined. For C57BL/6J mice housed singly, in pairs or in trios, most cages lacked unsoiled resting area within 3-6 days (depending on cage size), 2 days or 1 day, respectively. Similarly, for Wistar rats housed singly, in pairs or in trios, most cages lacked adequate unsoiled resting space within 3 days, 2 days or 1 day, respectively. Because most cages lacked adequate unsoiled resting space within 3 days of housing animals, the author concludes that standard cage change frequencies of once a week for adult C57BL/6J mice and twice a week for adult Wistar rats may be inadequate to provide unsoiled resting areas for rodents.