Abstract
Injurious home-cage aggression (fighting) in mice affects both animal welfare and scientific validity. It is arguably the most common potentially preventable morbidity in mouse facilities. Existing literature on mouse aggression almost exclusively examines territorial aggression induced by introducing a stimulus mouse into the home-cage of a singly housed mouse (i.e. the resident/intruder test). However, fighting occurring in mice living together in long-term groups under standard laboratory housing conditions has barely been studied. We performed a point-prevalence epidemiological survey of fighting at a research institution with an approximate 60,000 cage census. A subset of cages was sampled over the course of a year and factors potentially influencing home-cage fighting were recorded. Fighting was almost exclusively seen in group-housed male mice. Approximately 14% of group-housed male cages were observed with fighting animals in brief behavioral observations, but only 14% of those cages with fighting had skin injuries observable from cage-side. Thus simple cage-side checks may be missing the majority of fighting mice. Housing system (the combination of cage ventilation and bedding type), genetic background, time of year, cage location on the rack, and rack orientation in the room were significant risk factors predicting fighting. Of these predictors, only bedding type is easily manipulated to mitigate fighting. Cage ventilation and rack orientation often cannot be changed in modern vivaria, as they are baked in by cookie-cutter architectural approaches to facility design. This study emphasizes the need to invest in assessing the welfare costs of new housing and husbandry systems before implementing them.
Highlights
Injurious home-cage aggression in mice affects both animal welfare and scientific validity
This leaves injurious home-cage aggression as the primary welfare issue in mice in terms of potentially preventable morbidity, comprising roughly 15% of morbidities observed in mice in one prevalence study[12], with male mice bearing the brunt of this b urden[13,14,15,16]
As we found no individually ventilated cages (IVC) cages bedded with woodchips, and only a small number of static cages bedded with corncob, we could not separate the effect of these two variables
Summary
Injurious home-cage aggression (fighting) in mice affects both animal welfare and scientific validity It is arguably the most common potentially preventable morbidity in mouse facilities. Mouse welfare has advanced in leaps and bounds in recent years—a biologically relevant[1] and implementable enrichment (nesting m aterial2–10) is widely adopted; and a 95% effective cure for ulcerative dermatitis (which is the most common morbidity in mice, and was previously intractable11) has been identified This leaves injurious home-cage aggression (fighting) as the primary welfare issue in mice in terms of potentially preventable morbidity, comprising roughly 15% of morbidities observed in mice in one prevalence study[12], with male mice bearing the brunt of this b urden[13,14,15,16]. This study aimed to 1) quantify the point-prevalence of fighting (escalated aggression) and related trauma across our institution; and 2) identify potential risk factors associated with fighting
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