A Comprehensive Assessment Of Animal Welfare In Regards To Cage Size For Trio-Bred C57Bl/6 Mice

Abstract

The current edition of the Guide for the Care and Use of Laboratory Animals makes recommendations on minimum floor space requirements for housing female mice with litters. As a result, these recommendations preclude trio breeding of mice in standard "shoebox" cages, which impacts institutions that historically have permitted trio breeding of mice. The intent of this study is to assess the effects of cage size and paired versus trio breeding on the welfare of breeding mice and their litters, as assessed by various parameters including reproductive efficacy, weight gain, behavioral assessment, biological stress markers, microenvironment, the histopathology of respiratory tissues, and genetic expression. The 2x2 design of the study utilized C57BL/6 mice housed in either a pair- and triobreeding scheme within either a 67 in2 (170 cm2) standard "shoebox" mouse individually ventilated cage (IVC) cage or a 132 in2 (335 cm2) standard rat IVC. No significant differences among these four groups were noted in any of the measured parameters with the exception of (1) a significant effect of cage size on microenvironment (specifically, elevated ammonia levels in smaller cages irrespective of pair or trio housing) and (2) two marginally significant behavioral affects that are not internally corroborated. Despite the increased ammonia concentrations observed in smaller cages, there were no significant gross or histologic changes noted in respiratory tissues of the trio bred mice in standard cages compared to the other groups. We conclude that a larger floor space can reduce intra-cage microenvironment parameter levels, but there are no differences in any other welfare parameter assessed in this study when comparing rat-cage sized housing to standard mouse cages. Moreover, there is no evidence of reduced welfare based on trio housing as opposed to pair housing irrespective of cage size.