General parity between trio and pairwise breeding of laboratory mice in static caging

Group size planning for breedings of gene-modified mice and other organisms following Mendelian inheritance

Colony management of gene-modified animals is time-consuming, costly and affected by random events related to Mendelian genetics, fertility and litter size. Careful planning is mandatory to ensure successful outcomes using the least number of animals, hence adhering to the 3R principles of animal welfare. Here we have developed an R package, accessible also through an interactive public website, that optimizes breeding design by providing information about the optimal number of breedings needed to obtain defined breeding outcomes, taking into account specific species, strain, or line properties and success probability. Our software also enables breeding planning for balanced male-to-female ratio or single-sex experiments. We show that, for single-sex designs, the necessary number of breedings is at least doubled compared to the use of all born animals. While the presented tool provides preset parameters for the laboratory mouse, it can be readily used for any other species.

Effects of Housing Density on Reproductive Performance, Intracage Ammonia, and Welfare of Mice Continuously Housed as Breeders in Standard Mouse and Rat Caging

Maintaining compliance with cage density recommendations in The Guide for the Care and Use of Laboratory Animals precludes continuous trio breeding in standard-sized mouse cages. This study evaluated and compared several parameters of reproductive performance, intracage ammonia concentration, and fecal corticosterone levels in 2 strains of mice, C57BL/6J (B6) and B6.129S(Cg)-Stat1tm1Dlv/J (STAT1-/-), housed as continuous breeding pairs or trios in standard-sized mouse cages, and continuous breeding trios in standard-sized rat cages. Reproductive performance data indicated that STAT1-/- trios raised in rat cages weaned significantly more pups per litter than did STAT1-/- trios raised in mouse cages, and B6 mice had higher pup survival rates at weaning than did STAT1-/- mice in mouse cages housing continuous breeding trios. In addition, the Production Index was significantly higher for B6 breeding trios in rat cages than for B6 trios in mouse cages. Intracage ammonia concentration increased with cage density, with significantly higher ammonia concentrations in mouse cages housing trios compared with rat cages housing trios. However, fecal corticosterone levels did not differ significantly regardless of genotype, breeding configuration, or cage size, and daily health checks revealed no clinical abnormalities under any of the conditions evaluated. These results suggest that, although continuous trio breeding in standard-sized mouse cages does not seem to compromise mouse welfare, it offers no advantage in reproductive performance compared with pair breeding, and in some cases, it might be disadvantageous in this regard. Further, high intracage ammonia in mouse cages containing breeding trios might necessitate more frequent cage changes.

Effects of Trio and Pair Breeding of Mice on Environmental Parameters and Nasal Pathology and Their Implications for Cage Change Frequency

According to the Guide, cage change frequencies must be considered when cage density requirements are exceeded. We monitored ammonia, carbon dioxide, cage wetness, health status, and breeding parameters of trio and pair breeding cages containing CD1 mice in ventilated and static microisolation caging (4 cages per condition) daily for approximately 6 wk. Minimum cage change frequencies for each condition were determined on the basis of performance data. At 3 d after cage change, static trio and pair cages had average ammonia levels of 74 and 38 ppm. Ventilated cages remained below the 25ppm threshold reported to be potentially deleterious for mice until at least day 7 after cage change. By 7 d after cage change, ammonia levels had risen to an average of 100 ppm and 64 ppm in static trio and pair cages and to 34 ppm and 20 ppm in ventilated trio and pair cages, respectively. Ammonia levels in ventilated cages continued to rise slowly through day 14 after cage change. CO₂ levels exceeded 5000 ppm in all groups at 2 d after cage change. Pair breeders in ventilated cages took the longest-10 to 14 d-to reach cage wetness threshold scores. On day 7, pups in trio static cages were noted to have decreased and squinted eyes, whereas in ventilated cages containing trios and pairs, these clinical signs were rare to absent. Histologically, there was an increasing incidence and severity of nasal lesions in weanlings with increasing housing density and decreasing ventilation, consistent with nasal epithelial toxicity. Given these parameters, we concluded that under the current husbandry conditions, it may be necessary to change breeders in static cages more frequently than every 7 d. Additional studies are necessary to evaluate the effects of more frequent cage changes on reproductive parameters, given that cage changing is stressful for mice and affects breeding results.

Recommended housing densities for research mice: filling the gap in data-driven alternatives

Space recommendations for mice made in the Guide for Care and Use of Laboratory Animals have not changed since 1972, despite important improvements in husbandry and caging practices. The 1996 version of the Guide put forth a challenge to investigators to produce new data evaluating the effects of space allocation on the well-being of mice. In this review, we summarize many studies published in response to this challenge. We distinguish between studies using ventilated or nonventilated caging systems and those evaluating reproductive performance or general well-being of adult mice. We discuss how these studies might affect current housing density considerations in both production and research settings and consider gaps in mouse housing density research. Additionally, we discuss reliable methods used to monitor and quantify general well-being of research mice. Collectively, this large body of new data suggests that husbandry practices dictating optimal breeding schemes and space allocation per mouse can be reconsidered. Specifically, these data demonstrate that prewean culling of litters has no benefit, trio breeding is an effective production strategy without adversely affecting pup survival and well-being, and housing of adult mice at densities of up to twice current Guide recommendations does not compromise well-being for most strains.-Svenson, K. L., Paigen, B. Recommended housing densities for research mice: filling the gap in data-driven alternatives.

Effect of Continuous Trio Breeding Compared with Continuous Pair Breeding in 'Shoebox' Caging on Measures of Reproductive Performance in Estrogen Receptor Knockout Mice

Some performance standards for continuous trio breeding in 'shoebox' cages for inbred stocks and outbred strains of mice challenge the minimum floor space recommendations in the 8th edition of the Guide for the Care and Use of Laboratory Animals. In our study, we evaluated whether continuous trio breeding could be successfully applied to a breeding colony of genetically engineered mice housed in shoebox cages with a floor area of 67.6 in2. Mice heterozygous for genetically engineered mutations to estrogen receptors and their wildtype counterparts were continuously bred as trios or pairs. Confounding environmental factors were controlled through standardized husbandry practices and husbandry, and all mice were bred simultaneously to control for temporal factors. Several measures of reproductive performance-including number of litters per female, production index, interlitter interval, litter size at birth, litter size at weaning, weaning rate, and body weight of pups at weaning- were evaluated over approximately 6 mo. Regardless of genotype, interlitter interval, litter size at birth, and litter size at weaning were significantly lower for trio-bred mice than for pair-bred mice. In addition, significant interactions emerged between genotype and breeding strategy for these reproductive measures. Furthermore, significant differences between genotypes occurred for interlitter interval and weaning rate, regardless of breeding strategy. Underlying mechanisms to account for effects of genotype on interlitter interval and the interaction of genotype with breeding strategy were unclear but may reflect effects of overcrowding and reproductive suppression.

Effect of Continuous Trio Breeding Compared with Continuous Pair Breeding in 'Shoebox' Caging on Measures of Reproductive Performance in Estrogen Receptor Knockout Mice

Some performance standards for continuous trio breeding in 'shoebox' cages for inbred stocks and outbred strains of mice challenge the minimum floor space recommendations in the 8th edition of the Guide for the Care and Use of Laboratory Animals. In our study, we evaluated whether continuous trio breeding could be successfully applied to a breeding colony of genetically engineered mice housed in shoebox cages with a floor area of 67.6 in2. Mice heterozygous for genetically engineered mutations to estrogen receptors and their wildtype counterparts were continuously bred as trios or pairs. Confounding environmental factors were controlled through standardized husbandry practices and husbandry, and all mice were bred simultaneously to control for temporal factors. Several measures of reproductive performance-including number of litters per female, production index, interlitter interval, litter size at birth, litter size at weaning, weaning rate, and body weight of pups at weaning- were evaluated over approximately 6 mo. Regardless of genotype, interlitter interval, litter size at birth, and litter size at weaning were significantly lower for trio-bred mice than for pair-bred mice. In addition, significant interactions emerged between genotype and breeding strategy for these reproductive measures. Furthermore, significant differences between genotypes occurred for interlitter interval and weaning rate, regardless of breeding strategy. Underlying mechanisms to account for effects of genotype on interlitter interval and the interaction of genotype with breeding strategy were unclear but may reflect effects of overcrowding and reproductive suppression.

Retrospective Analysis of Reproductive Performance of Pair-bred Compared with Trio-bred Mice

Federal guidelines provide recommendations regarding the minimum of floor space that should be allotted for breeding laboratory rodents. Mouse mating systems used by a variety of institutions frequently deviate from these recommendations. Regulatory agencies suggest that deviations from established guidelines should be assessed on an institutional basis and recommend periodic review by the local IACUC. Mouse breeding data, collected in a laboratory animal management software program at a single institution, were retrospectively analyzed to determine the effects of 2 breeding schemes on reproductive performance. Data were analyzed over a 20-mo period from all inbred strains of mice breeding in the vivarium. Variables included total number of pups per litter, pups per female, and litters per female. Data indicated that the numbers of pups and litters per dam do not differ between trio-bred (one male, 2 female) and pair-bred (one male, one female) mice. However, more pups were weaned per litter in trio-bred (mean, 5.8 pups) when-compared with pair-bred (mean, 4.7 pups) mice. These results suggest that allocating less than the recommended amount of floor space is not detrimental to the overall breeding success of the strains of mice examined.