Conventional laboratory housing increases morbidity and mortality in research rodents: results of a meta-analysis

Abstract

Background Over 120 million mice and rats are used annually in research, conventionally housed in shoebox-sized cages that restrict natural behaviours (e.g. nesting and burrowing). This can reduce physical fitness, impair thermoregulation and reduce welfare (e.g. inducing abnormal stereotypic behaviours). In humans, chronic stress has biological costs, increasing disease risks and potentially shortening life. Using a pre-registered protocol (https://atrium.lib.uoguelph.ca/xmlui/handle/10214/17955), this meta-analysis therefore tested the hypothesis that, compared to rodents in ‘enriched’ housing that better meets their needs, conventional housing increases stress-related morbidity and all-cause mortality. Results Comprehensive searches (via Ovid, CABI, Web of Science, Proquest and SCOPUS on May 24 2020) yielded 10,094 publications. Screening for inclusion criteria (published in English, using mice or rats and providing ‘enrichments’ in long-term housing) yielded 214 studies (within 165 articles, using 6495 animals: 59.1% mice; 68.2% male; 31.8% isolation-housed), and data on all-cause mortality plus five experimentally induced stress-sensitive diseases: anxiety, cancer, cardiovascular disease, depression and stroke. The Systematic Review Center for Laboratory animal Experimentation (SYRCLE) tool assessed individual studies’ risks of bias. Random-effects meta-analyses supported the hypothesis: conventional housing significantly exacerbated disease severity with medium to large effect sizes: cancer (SMD = 0.71, 95% CI = 0.54–0.88); cardiovascular disease (SMD = 0.72, 95% CI = 0.35–1.09); stroke (SMD = 0.87, 95% CI = 0.59–1.15); signs of anxiety (SMD = 0.91, 95% CI = 0.56–1.25); signs of depression (SMD = 1.24, 95% CI = 0.98–1.49). It also increased mortality rates (hazard ratio = 1.48, 95% CI = 1.25–1.74; relative median survival = 0.91, 95% CI = 0.89–0.94). Meta-regressions indicated that such housing effects were ubiquitous across species and sexes, but could not identify the most impactful improvements to conventional housing. Data variability (assessed via coefficient of variation) was also not increased by ‘enriched’ housing. Conclusions Conventional housing appears sufficiently distressing to compromise rodent health, raising ethical concerns. Results also add to previous work to show that research rodents are typically CRAMPED (cold, rotund, abnormal, male-biased, poorly surviving, enclosed and distressed), raising questions about the validity and generalisability of the data they generate. This research was funded by NSERC, Canada.

Environmental Enrichment for Rats and Mice Housed in Laboratories: A Metareview

“…We suggest that several of these more frequently endorsed elements (i.e., social housing, nesting material, shelters, foraging opportunities) are now sufficiently established that they should be regarded as basic to good rodent housing conditions rather than “enriched”, or that systems that fail to include these features be considered impoverished. Others have suggested that conventional housing should be more formally recognised as a stressful laboratory procedure [ 21 ]. Such framing may help motivate change in laboratories that until now have been unable or unwilling to include these features.…”

“…A systematic review and meta-analysis of 281 rodent enrichment studies concluded that complex housing does not make results any more variable in comparison to conventional laboratory housing [ 46 ]. Another recent systematic review of rodents induced with stress-sensitive diseases found no effect of housing conditions on variation [ 21 ].…”

“…As described by Contreras and Rollin [ 33 ], (p. 21): “One must understand the normal behaviours, environment, and physiologic adaptations that the subject utilises to survive and thrive in the subject’s environment. If one does not appreciate normal, one cannot recognise abnormal.” Studies generally fail to acknowledge that the effects of enrichment are typically measured in comparison to animals that are predisposed to exhibit enhanced symptoms of disease [ 21 ] as a result of heightened stress in conventional cages [ 125 ] or single housing [ 68 , 126 ]. Enrichment is recognised as reducing symptoms for a wide range of rodent disease models, and is therefore recommended as a therapeutic treatment for human populations (e.g., [ 127 , 128 ]).…”

“…Captive environments that prevent natural behaviours can cause frustration, stress, depression, and behaviours indicative of poor welfare [ 15 , 16 ]. For rodents, conventional laboratory cages (which we define here as polycarbonate cages with bedding and possibly nesting material and/or a shelter, with food pellets provided in a metal food hopper) can lead to the development of stereotypic behaviours [ 17 , 18 , 19 ], weakened immune responses [ 20 , 21 ], increased acute hormonal responses to stressors [ 22 ], and behaviours indicative of anxiety [ 23 , 24 ] and pain [ 25 ]. Individually ventilated cages are also increasingly used, offering increased biosecurity but impairing animal thermoregulation [ 26 ].…”

“…Some studies (all from roughly the last decade) refer to the addition of nesting material (to an otherwise barren cage) as enriched (e.g., [ 23 , 38 , 39 ]), although in other studies a cage with nesting material is sometimes used as a condition representing no enrichment (e.g., [ 40 , 41 ]). Other studies refer to cages containing nesting material in addition to other components as their conventional or non-enriched condition (e.g., [ 42 , 43 ]), and many studies do not describe their conventional housing conditions at all [ 21 ], making direct comparisons between studies difficult. Because the term is variously applied, an otherwise barren cage altered in any way might be labelled as “enrichment.” As one example of such practices, a shoebox cage for mice modified to have one indented corner has recently been marketed as enriched [ 44 ].…”