Comparison of Four Beddings for Ammonia Control in Individually Ventilated Mouse Cages

A Review of the Effects of Some Extrinsic Factors on Mice Used in Research

Animals have been used in research for over 2,000 y. From very crude experiments conducted by ancient scholars, animal research, as a science, was refined over hundreds of years to what we know it as today. However, the housing conditions of animals used for research did not improve significantly until less than 100 years ago when guidelines for housing research animals were first published. In addition, it was not until relatively recently that some extrinsic factors were recognized as a research variable, even when animals were housed under recommended guidelines. For example, temperature, humidity, light, noise, vibration, diet, water, caging, bedding, etc., can all potentially affect research using mice, contributing the inability of others to reproduce published findings. Consequently, these external factors should be carefully considered in the design, planning, and execution of animal experiments. In addition, as recommended by others, the housing and husbandry conditions of the animals should be described in detail in publications resulting from animal research to improve study reproducibility. Here, we briefly review some common, and less common, external factors that affect research in one of the most popular animal models, the mouse.

Rapid ammonia build-up in small individually ventilated mouse cages cannot be overcome by adjusting the amount of bedding

We sought to investigate if varying levels of bedding had an effect on intra-cage ammonia levels in individually ventilated mouse cages (Euro Standard Types II and III). Employing a routine 2 week cage-changing interval, our goal is to keep ammonia levels under 50 ppm. In smaller cages used for breeding or for housing more than four mice, we measured problematic levels of intra-cage ammonia, and a considerable proportion of these cages had ammonia levels at more than 50 ppm toward the end of the cage-change cycle. These levels were not reduced significantly when the levels of absorbent wood chip bedding was either increased or decreased by 50%. The mice in both cage types II and III were housed at comparable stocking densities, yet ammonia levels in larger cages remained lower. This finding highlights the role of cage volume, as opposed to simply the floor space, in controlling air quality. With the current introduction of newer cage designs that employ an even smaller headspace, our study urges caution. With individually ventilated cages, problems with intra-cage ammonia may go undetected, and we may opt to utilize insufficient cage-changing intervals. Few modern cages have been designed to account for the amounts and types of enrichment that are used (and, in parts of the world, mandated) today, adding to the problems associated with decreasing cage volumes.

Effect of Bedding Substrates on Blood Glucose and Body Weight in Mice

Differences in cage microenvironments may contribute to variation in data and affect the outcome of animal studies involving metabolic diseases. To study this, we compared the effects 3 types of bedding-corncob bedding, hardwood bedding, and hardwood bedding plus a cardboard enrichment item-on baseline fasting and nonfasting blood glucose and body weight in mice. Mice housed on corncob bedding showed significantly higher fasting blood glucose than did mice housed on hardwood bedding, with or without the enrichment item. None of the groups showed an effect of bedding type on nonfasting blood glucose levels or body weight. This information informs the choice of bedding substrates for studies that measure fasting blood glucose and potentially mitigates a variable that could confound research outcomes.

Evaluation and Refinement of a Spot-change-only Cage Management System for Mice

Maximizing operational efficiency while maintaining appropriate animal housing conditions is a continuous focus of research animal care programs. Our institution's longstanding approach to cage-change management included scheduled cage changes every 2 wk, with spot changes if cages met established visual criteria during the intervening period. This 2-wk plus spot changing (2WS) practice for mice housed in IVC was problematic during the COVID-19 pandemic when the need arose to minimize workload to reduce on-site staffing out of concern for employee health and possible absenteeism. With the approval of the IACUC, a spot-change-only (SCO) process was adopted, with the requirement to evaluate microenvironmental parameters under both practices to confirm acceptable equivalence. These parameters (humidity, temperature, and ammonia) were evaluated in a controlled study that found no significant difference between the 2 groups. Ammonia levels did not exceed 10 ppm in any group throughout the study. To assess operational differences between these 2 approaches, we collected cage-change data and employee feedback from facilities operating under these schemes. The SCO method required fewer cage changes than did the 2WS method (10.3% per day with 2WS and 8.4% per day with SCO). Despite this benefit, through a Plan-Do-Check-Act process that has been regularly employed at our institution, employee feedback identified important operational challenges associated with the SCO practice. The SCO approach was thus refined into a scheduled spot change (SSC) practice that builds on the SCO model by incorporating a scheduled focused cage evaluation period. Based on subsequent feedback, the SSC was found to retain the efficiency benefits afforded by the SCO model and simultaneously alleviated staff and operational concerns. This result underscores the importance of integrating staff feedback with a performance standard-based approach when assessing cage-change management.

Identification of Rodent Husbandry Refinement Opportunities through Benchmarking and Collaboration

Expanding the use of methods that refine, reduce, and replace (3Rs) the use of animals in research is fundamental for both ethical and scientific reasons. The mission of the 3Rs Translational and Predictive Sciences Leadership Group (3Rs TPS LG) of the International Consortium for Innovation and Quality in Pharmaceutical Development (IQ Consortium) is to promote sharing and integration of science and technology to advance the 3Rs in the discovery and development of new medicines, vaccines, medical devices, and health care products for humans and animals. The 3Rs TPS LG is dedicated to identifying opportunities for member companies to share practices, enhance learning, promote discussions, and advance the 3Rs across the industry. One such opportunity was a benchmarking survey, conducted by the Contract Research Organization (CRO) Outreach Working Group, designed to share practices in rodent husbandry for drug safety research and to identify potential opportunities for refinement. IQ member companies and CROs in Asia, North America, and Europe were surveyed. Areas identified for potential alignment included provision of corncob bedding and wire-grid flooring, management of the nest at cage change, approaches to social housing for male mice, evidence-based enrichment strategies, and evaluating the effects of the timing of studies in relation to the animals' circadian rhythm and light-cycle, with consideration for how such extrinsic factors influence animal welfare and scientific outcomes. This manuscript presents the results of the benchmarking survey, including general trends in mouse and rat husbandry practices in toxicology studies, considerations for social housing, enrichment selection, and potential effects of bedding substrate, emphasizing opportunities for collaboration that can help to identify refinements to rodent husbandry practices.

Carbon Dioxide, Oxygen, and Ammonia Levels in Mouse and Rat Disposable IVC Removed from Mechanical Ventilation

Maintenance of an appropriate microenvironment for rodents used in research is of paramount importance because changes in environmental parameters such as O₂ and humidity can influence animal health and welfare and potentially alter research results. Here we evaluated the microenvironment of mouse and rat disposable cages after removal from mechanical ventilation in order to guide recommendations for their use. Cages with sealed IVC lids, unsealed lids (partially ajar), and lids without the exhaust filter (for rats) or static lids (for mice) were removed from the ventilated rack and were thereafter monitored CO₂, O₂, and NH₃ levels. For mice, effects were investigated under both standard (set point of 72°F/22°C) and thermoneutral (set point of 82°F/28°C) temperatures. When IVC with sealed lids and group-housed C57BL/6J male mice were removed from ventilation under standard temperatures, CO₂ started at 6,600 ± 265 ppm at 0 h and rose to 42,500 ± 7,263 ppm at 1 h, with mice showing a visibly elevated respiratory rate in 1 of the 3 cages; CO₂ stabilized at 26,150 ± 3,323 ppm at 8 h. In contrast, CO₂ levels in cages with single mice were stable after 1 h (1,350 ± 409 ppm at 0 h, 9,367 ± 802 ppm at 1 h, and 8,333 ± 1,115 ppm at 8 h). Findings were similar at thermoneutral temperatures: sealed group-housed mice cages started at 3,617 ± 475 ppm at 0 h and rose to 39,333 ± at 5,058 ppm at 1 h, whereas sealed cages with 1 mouse started at 1,117 ± 247 ppm at 0 h and were 7,500 ± 1,997 ppm at 8 h. IVC with sealed lids and pair-housed Crl:CD(SD) female rats rose to 48,000 ± 2,828 ppm CO₂ and over 70% humidity within 1 h. By 3 h, IVC with sealed lids and singly housed rats had 40,167 ± 5,132 ppm CO₂, and rats were displaying a visually elevated respiratory rate. O₂ levels had an inverse relationship with CO₂ levels. Removing the rat lid exhaust filter was not helpful. However, leaving the IVC lid ajar ameliorated the rise in CO₂ and fall in O₂ for both species. Therefore, IVC with sealed lids and group-housed mice should not be removed from ventilation more than 1 to 2 h; IVC containing pair- or singly-housed rats IVC should not be removed for more than 1 or 3 h, respectively. Whenever possible, such cages should be fitted with static lids, left partially ajar and monitored, or replaced on ventilation.

Validation of Sanitization Practices in Single-use Individually Ventilated Mouse Cages at Standard and Thermoneutral Temperatures

Vivarium husbandry practices are based on performance data and adhere to applicable regulatory guidelines. Refinements in husbandry and optimization of sanitization protocols improve animal wellbeing and help standardize the microenvironment, contributing to research reproducibility. The objective of this study was to evaluate the microenvironment to establish performance standards for mouse husbandry and sanitization, including housing at standard and thermoneutral temperatures. Male C57BL/6J mice were housed singly and in groups in disposable IVCs on α-cellulose or corncob bedding and microenvironmental indicators (ammonia, carbon dioxide) were evaluated. In addition, microbial bioburden tests (ATP and RODAC) were performed on cages and cage accessories on days 0, 7, 14 and, 28 to 30 after cage change. Water testing and aerobic culture of the waterspout of bottles containing chlorinated water were performed to determine acceptable replacement schedules. Ammonia levels remained below the National Institute of Occupational Safety and Health 8-h recommended exposure limit for humans (25 ppm) at all time points for all housing conditions through day 21 for group-housed mice, and through day 30 for singly housed mice. Microbial bioburden results for cage accessories and water testing were acceptable up to 28 d after cage change (RODAC less than 50 CFU; ATP less than 100,000 RLU) at both standard and thermoneutral housing temperatures. Mice remained clinically healthy throughout the studies. These results support site operating practices and verify extended sanitization recommendations per the Guide of the Care and Use of Laboratory Animals in this disposable IVC environment: group-housed mice receive bottom cage and water bottle change up to every 14 d with full cage change (including lid and accessories) every 28 d, and singly housed mice receive full cage change every 28 to 30 d or sooner.

A multicentre study on spontaneous in-cage activity and micro-environmental conditions of IVC housed C57BL/6J mice during consecutive cycles of bi-weekly cage-change

Mice respond to a cage change (CC) with altered activity, disrupted sleep and increased anxiety. A bi-weekly cage change is, therefore, preferred over a shorter CC interval and is currently the prevailing routine for Individually ventilated cages (IVCs). However, the build-up of ammonia (NH₃) during this period is a potential threat to the animal health and the literature holds conflicting reports leaving this issue unresolved. We have therefor examined longitudinally in-cage activity, animal health and the build-up of ammonia across the cage floor with female and male C57BL/6 mice housed four per IVC changed every other week. We used a multicentre design with a standardised husbandry enabling us to tease-out features that replicated across sites from those that were site-specific. CC induce a marked increase in activity, especially during daytime (~50%) when the animals rest. A reduction in density from four to two mice did not alter this response. This burst was followed by a gradual decrease till the next cage change. Female but not male mice preferred to have the latrine in the front of the cage. Male mice allocate more of the activity to the latrine free part of the cage floor already the day after a CC. A behaviour that progressed through the CC cycle but was not impacted by the type of bedding used. Reducing housing density to two mice abolished this behaviour. Female mice used the entire cage floor the first week while during the second week activity in the latrine area decreased. Measurement of NH₃ ppm across the cage floor revealed x3 higher values for the latrine area compared with the opposite area. NH₃ ppm increases from 0–1 ppm to reach ≤25 ppm in the latrine free area and 50–100 ppm in the latrine area at the end of a cycle. As expected in-cage bacterial load covaried with in-cage NH₃ ppm. Histopathological analysis revealed no changes to the upper airways covarying with recorded NH₃ ppm or bacterial load. We conclude that housing of four (or equivalent biomass) C57BL/6J mice for 10 weeks under the described conditions does not cause any overt discomfort to the animals.

Behavioral and Physiologic Effects of Dirty Bedding Exposure in Female ICR Mice

Exposure of sentinel mice to dirty bedding is commonly used in health monitoring programs to screen colonies for clinical and subclinical disease. Despite the potential stressors present in dirty bedding, including but not limited to microorganisms, pheromones, and ammonia, it is unknown whether sentinel mice exposed to soiled bedding experience stress. In this study, select behavioral and physiologic changes associated with stress were assessed in female ICR mice exposed to dirty bedding. Behavioral parameters included evaluation in the home cage and selected behavioral tests; physiologic measurements included neutrophil:lymphocyte ratio and weight. Mice in the acute group were exposed for 24 h whereas mice in the chronic group were exposed for 4 wk. Mice in the chronic group exposed to dirty bedding weighed less at days 21 and 28 than did control mice. Chronic mice exposed to dirty bedding also exhibited decreased net weight gain over the entire study period as compared with control mice. No significant differences were detected in the other behavioral and physiologic parameters measured. These results indicate that dirty bedding exposure may affect sentinel mice, but further investigation is needed to determine the specific mechanism(s) behind the weight difference.