Online Issues

Journal Vol 48 (1)

A Renewed Look at Laboratory Rodent Housing and Management

Janet C. Gonder and Kathy Laber

Janet C. Gonder, D.V.M., Ph.D., is Principal Consultant, Garber Consulting LLC, Pinehurst, NC. Kathy Laber, D.V.M., M.S., DACLAM, is Professor, Comparative Medicine, Medical University of South Carolina, Charleston, SC, and Director, Virginia Medical University, Ralph H. Johnson Veterans Affairs Medical Center.

Address correspondence and reprint requests to Dr. Gonder, 17 Troon Drive, Pinehurst, NC 28374.

Abstract

Since its publication in 1996, the Guide for the Care and Use of Laboratory Animals (National Research Council, Washington DC, National Academy Press) has become a primary source of information for institutional animal care and use committees (IACUCs) and research facility managers. In the ensuing years, recommendations relating to laboratory animal care have evolved in response to new scientific information and use of new technology such as ventilated caging. In this article, recent publications are examined to determine the potential impact of new scientific evidence on current practices for the housing and care of laboratory rodents. The discussion points out recent advances in technology and new knowledge of the conditions for the housing of various laboratory rodents, including cage space, single versus group housing, ventilated caging systems, thermoregulation, bedding materials, and enrichment. This new information is provided to aid IACUCs and facility managers in making decisions regarding the housing and care of laboratory rodents.

Key Words: caging; housing systems; management; rodent husbandry

Introduction

With the introduction of new technology and review of newly published data, managers of research laboratories must continuously reassess housing and husbandry options for laboratory animals. The 1996 Guide for the Care and Use of Laboratory Animals (Guide¹) (NRC 1996) is a primary reference for animal facility managers and institutional animal care and use committees (IACUCs¹). While much of the information contained in the Guide remains valid today, some new information is now available and should be incorporated into animal care and use programs where appropriate. In fact, the Committee to Revise the Guide prefaced the 7th edition by stating that “. . . the Guide is subject to modification with changing conditions and new information” (p. iii). Until another revision of the Guide is undertaken, new published information must be evaluated to determine current best practice for all aspects of laboratory animal use. It is beyond the scope of this article and of this issue of the ILAR Journal to provide updated information in all areas covered in the Guide. In this article, we focus on the housing and management of laboratory rodents, an area in which new research findings are available and in which new housing methodology has become more widespread.

Cage Space for Mice

Cage space requirements for common laboratory animals have been difficult to determine. In previous editions of the Guide, guidelines have been based on limited data coupled with existing practice and professional judgment, which have frequently relied on real or perceived adverse findings. At present, recommended cage sizes for rodents are primarily based on the weight of the animals and the number of animals per cage. Recommendations regarding cage population density or group composition are not included. The following variables were not taken into account: age, phenotype, gender, social behavior of mouse strains, quality of space (e.g., vertical access), and structures placed within the cages. Although environmental enrichment and social housing for social species are suggested, little information was available at that time to help determine effective environmental enrichment options as they pertain to cage size and social density. In the absence of scientific data to the contrary, many people have anthropomorphized and believed that providing more space is desirable, has no adverse consequences, and would facilitate adding enrichment devices. With respect to rodents, particularly rats and mice, few scientific studies have directly addressed cage space needs without the introduction of confounding variables such as enrichment methods, group sizes, sex, and strain. It is difficult to compare studies and to build on previous work due to the extensive differences in study design and experimental variables that have been measured. The significance and relationship of some of the measured parameters are hard to closely relate to the effects of solely changing cage space. Altering group size as a means of altering housing density is commonly done in cage space studies. As a result, few studies have been conducted that separate the effects of group size and density in studies involving cage space. A variety of mouse strains and ages have also been studied. Other investigators have altered group size and cage density and have incorporated the additional variable of varying types of enrichment. Clearly it is important to consider complexities such as these when cage space issues are discussed.

Smith et al. (2004a) evaluated the impact of altering both floor space and group size in male and female C57BL/6J mice. Mice were housed in a ventilated rack system, in standard shoe-box cages (51.7 in² or 112.9 in²) at the density recommended in the Guide (12 in²/mouse) or at varying higher densities (to 5.6 in²/mouse). It was determined that a housing density of up to 5.6 in² per mouse (approximately twice as dense as Guide recommendations) did not affect testosterone values, food and water consumption, hair loss, body weight, injury, aggressive behavior, or survival, and did not result in histological changes in the nasal passages or eyes. The study did show that microenvironmental parameters such as ammonia, carbon dioxide, humidity, and temperature within the cages were altered; however, these values remained within accepted ranges. The authors concluded that housing mice at twice Guide-recommended densities has no deleterious impact on any of the parameters assessed. The same authors (Smith et al. 2005) extended this study design with other mouse strains (young adult male and female BALB/cJ, NOD/LtJ, and FVB/NJ mice) to evaluate optimal housing densities. The results from this study supported the authors' previous conclusions with the exception of male FVB mice, which demonstrated an early onset of aggression, providing evidence that there is strain variability with respect to cage space.

The optimal housing density for C57BL/6J male and BALB/c female and male mice was also evaluated by Fullwood et al. (1998) and McGlone et al. (2001), respectively, using a design in which the group size stayed constant at three mice but the floor space was altered. The cages were utilized in nonventilated racks, and cage sizes were at the Guide recommendation of 15 as well as 5, 10, and 20 in². Variables evaluated in both strains included measures of growth, survival, adrenal hormones, and immune status as well as for BALB/c mice, measures of behavior. For C57BL/6 mice, the authors concluded that the space recommendations found in the Guide caused a level of immune suppression, based on lymphocyte proliferative response to a T-cell mitogen and on natural killer cell activity, comparable to rodents experiencing stressors such as a metabolic or burn stress. The immunogenic profile was more favorable in mice housed at 10 in² per mouse versus the Guide recommendation of 15 in². The conclusion regarding immunological function was somewhat conflicted because smaller spaces caused an increase in plasma glucocorticoid concentrations and adrenal weights. In this study, space did not measurably influence weight gains; however, mortality was significantly decreased at the higher cage densities of 5 and 10 in² per mouse. Results for BALB/c mice were differentiated by sexes. Females mirrored the findings in C57BL/6 in that the smaller cage space slightly enhanced T-lymphocyte proliferative response to a stimulus, leading the authors to conclude that smaller spaces favor immune function. Females also grew heavier in the smaller cage. Both sexes had reduced mortality in the smaller cages. Male mice spent more time lying down in the smaller cage whereas the female mice showed an increase in grooming and sitting behaviors.

In another study (Van Loo et al. 2001), male BALB/c mice were housed in groups of three different sizes (n = 3, 5, or 8) at two different population densities (approximately the cage space per animal recommended in the Guide or at 50% more space). The level of aggression and the following physiological parameters were assessed: urine corticosterone levels, food and water intake, body weight, tyrosine hydroxylase activity, testosterone levels, and weight of spleen, thymus, testes, and seminal vesicles. Both the frequency and duration of agonistic encounter were affected by both group size and cage size. The most robust finding in this study was that aggression between mice increased substantially with increasing group size. The larger the cage, the higher the duration of agonistic behaviors, and agonistic behavior was observed in both dominant and subordinate animals. Group size and cage size were interactive inasmuch as the frequency of agonistic behaviors was higher for the group of five mice in the larger cage, with no difference noted in either the large or smaller cage in the groups of three and eight mice. In addition, physiological parameters indicated differences in stress levels between dominant and subordinate animals. The authors concluded that the observed decreased level of aggression and number of wounds seen in the smaller cages could be explained by a “crowding” effect. The concept is that there is a curvilinear relationship between crowding and agonistic behavior. Aggression will initially increase with increasing population density as the dominant animal's space becomes invaded. However, with high densities, the available space may be too small to be defended, leading to a decrease in aggression. No weight gain differences based on cage size were observed. This observation supports the work of other authors (Peters and Festing 1990), who discovered that at even greater housing density (approximately half the space recommended in the Guide as constructed by increasing the number of animals per space vs. decreasing space), there was no impact on body or adrenal gland weight. Van Loo et al. (2001) concluded that aggressive behavior in group-housed male BALB/c mice is best prevented by housing the animals in small groups of three to five animals, and that decreasing floor size per animal may be used as a temporary solution to decrease high levels of aggression in an existing social group.

A study documented by Sherwin (2004) challenged that the current space recommendations are less than ideal for mouse housing, drawing parallels from the studies that demonstrated a negative impact of limiting space in animals such as pigs, macaques, hens, and calves. The study design was well conceived in that a group of four CB57 females (vs. previous studies where animals were singly housed) were evaluated to determine the animals' strength of motivation for acquiring additional space. It was determined that the mice were highly motivated for additional space; however, the mice did not discriminate between the amounts of space offered. In addition, the author acknowledged that an animal's visit to the additional space offered could simply have represented exploration or territorial monitoring.

The findings and interpretations of the studies mentioned above clearly challenge the current space recommendations detailed in the Guide. Several of the studies support that smaller spaces both may optimize immune function, which has the ability to significantly affect research outcomes, and may decrease aggression, which can have an impact on morbidity and mortality. Nevertheless, as mentioned above, the inability to make comparisons between studies in concert with the confounding variables found within the studies, or lack of the evaluation of important behavioral parameters including locomotor and exploration activities, only serves to underscore the need for more investigation. The animal care and use community would benefit greatly from determining the optimal cage housing density that supports a physiologically normalized rodent and that, by extrapolation, should also support that animal's well being. However, given the many variables and responses noted above, a single set of recommendations is unlikely.

Environmental Enrichment for Mice

Several articles investigating the influence of adding cage enrichments to mouse cages have been published since the 1996 Guide was released. As was noted with cage density recommendations, comparing studies and building upon previous work are difficult due to the great differences in study design. The neural consequences of environmental enrichment in mice have been reviewed by several authors (Sherwin and Olsson 2004; Van Praag et al. 2000; Wurbel 2001) who have concluded that mice housed in “nonenriched” cages have impaired brain development, stereotypies, and an anxious behavioral profile. Wurbel (2001) also challenged that the level of maternal care provided in a “standard” cage versus a form of enriched cage, which supports the dam's natural behavior of periodically separating from the pups, affects adaptive ability and leaves the offspring more vulnerable to stress.

Van Loo et al. (2001) determined that the type of enrichment strongly affected both aggressive behavior as well as a variety of physiological parameters in male BALB/c mice. They found that adding a rigid shelter to the cage increased intermale aggression, increased urine corticosterone/creatinine ratios, and decreased weight gain—all indicators of stress. However, the provision of nesting material decreased intermale aggression even though the urine corticosterone/creatinine levels were comparable to the control group. The authors postulated that nesting material was preferred because it allowed mice to perform their normal “nesting” behavior and because a material was provided that the mice could actively change to meet their satisfaction. Of note is that the nesting material altered only the quality of the space, whereas the rigid shelter altered both the quality (including adding vertical space) and quantity of the space available.

Conflicting data have been presented by Zhu et al. (2006), who used immediately postweaned male and female C57BL/6 mice housed in groups of six in large cages with multiple enrichment devices compared with mice in smaller cages without enrichment devices. They determined that the enriched groups of mice showed signs of increased anxiety as determined by the plus-maze test. One of the differences noted in this study was that the maze study was performed in the dark phase of the light cycle, whereas the other studies were conducted during the light phase of the cycle.

Kingston and Hoffman-Goetz (1996) used C57BL/6 female mice to test the hypothesis that an enriched environment would act to enhance immune function or would act indirectly to buffer against immunosuppression in times of distress. They were able to demonstrate that environmental enrichment in the form of cage toys (nesting materials, crawl tubes, running wheels) buffered the reactivity of the immune system in response to distress. They reported that long-term environmental enrichment resulted in a lower murine splenic proliferative response to acute stress compared with nonenriched mice. The references cited above help to underscore that although environmental enrichment for mice in general appears to have a positive impact on the animals, it can create a scientific variable that has not been well defined to date.

Cage Space and Enrichment for Rats

Rats in different stages of development (juvenile, postpubertal, and adult) have been shown to respond differently to different cage sizes and stocking density (Arakawa 2005). When the cage size was decreased, juvenile rats displayed decreased locomotion and a lower propensity for exploration, whereas such changes were not evident in postpubertal rats. When the group size was increased, adult rats exhibited a higher level of locomotion. However, these factors did not affect risk assessment behavior of rats in these developmental stages. The author concluded that the effect of stocking density differs depending on the developmental stage of the animal. Juvenile rats showed increased anxiety when space was limited, whereas adult rats showed increased activity.

The foregoing studies, in addition to that of Rock et al. (1997), support the view that a range of cage space allocations are equally acceptable for mice and rats and that increased space can have certain adverse outcomes such as increased aggression between cage mates and increased mortality. These studies demonstrate differences in space usage and physiological effects as well as effects based on age, species, strain, genetic background, and a variety of other variables that include reproductive status and the provision of enrichment materials. These complexities must be weighed when determining appropriate housing for laboratory rodents.

Single Versus Group Housing

At the time of publication of the 1996 Guide, existing literature supported the concept that single housing of social species likely imposed stress that might affect their well-being and have an impact on experimental results. Recent literature also supports the concept that differences exist between individual and social housing, however these relationships are complex and can be modified by other variables such as enrichment items. With regard to cage space, the literature on single versus group housing is confusing and suffers from the lack of a generally accepted framework for interpretation of results or general agreement on definitions and magnitude of differences that constitute stress. Despite these shortcomings, the recent literature has provided a better understanding of the effects of single versus group housing, which is not reflected in the 1996 Guide.

A variety of parameters can be affected by social housing (or lack thereof), hence consistency in housing groups across studies is important. Food consumption appears to be reduced in group-housed male or female rats, which can affect body weight and potentially longevity (Georgsson et al. 2001). Mice appear to select social contact over environmental enrichment materials (Van Loo et al. 2004). Some studies suggest that group housing allows animals to adapt more easily to stressful circumstances compared with singly housed animals, although significant differences in physiological markers for stress are not apparent (Andrade and Guimaraes 2003; Bartolomucci et al. 2003). The makeup of the group and strain/genetic background appears to influence findings. For example, in one study significant differences were seen in responses of group-housed versus singly housed male NIH Swiss mice to standard tests for antidepressant drug activity (Karolewicz and Paul 2001). In this standard test, antidepressant activity of a known test compound was confirmed in group-housed animals but not when tested in individually housed animals. In a study by Perez et al. (1997), there was a statistically significant increase in the food intake of individually housed versus group-housed rats. Significant differences in plasma glucose and triglyceride levels were also noted. These results emphasize the importance of controlling the housing conditions for some test procedures.

Temperature and Thermoregulation

The 1996 Guide provides room temperature recommendations for a number of common laboratory animal species. For many species such as rodents, the basis for these recommendations has been unclear. Recent studies have shown that the thermoneutral zones as well as ambient temperatures selected by mice and rats in temperature gradients are considerably higher than those recommended in the Guide. Thus, rodents must make anatomical, physiological, or behavioral adjustments in order to maintain body temperatures in their thermoneutral zone or preferred range when they are housed at temperatures comfortable for humans (e.g., 22°C). These adjustments are measurable and have been described in recent studies as consequences of altered ambient temperature. In one study (Swoap et al. 2004), cardiovascular parameters were measured with radiotelemetry in mice and rats that were housed in temperature-controlled environments. Small changes in ambient temperature within the range specified in the Guide (18-26°C) and slightly higher (30°C) had a significant impact on cardiovascular parameters. As ambient temperature decreased, mean blood pressure, heart rate, and pulse pressure increased significantly for both mice and rats, with mice demonstrating a greater sensitivity to these ambient temperature changes. When given the opportunity, singly housed, 11 month old CD-1 mice selected an ambient temperature approximately 1°C warmer than group-housed mice (Gordon et al. 1998). However, the thermal preference for young (2-month-old) CD-1 mice was similar for singly- and group-housed mice.

The use of bedding or nesting materials that allow rodents to burrow into or create nests with the materials has been shown to provide a thermal compensating mechanism to achieve ambient temperatures that approach thermoneutrality (Gordon 2004). These data may provide an alternate explanation to psychological enrichment for rodents' use of these materials and their preference for them. Similarly, as noted above, the thermal preferences and effective ambient temperatures may differ between single- and group-housed animals (Gordon et al. 1998), which may also help to explain their preference for social housing. The Guide does suggest that such adaptation is normal, but these recent studies provide a clear basis for this premise.

Bedding and Nesting Materials

The 1996 Guide includes brief discussions of bedding materials but does not provide guidance on nesting behavior or considerations for the use of various bedding materials. Recent studies are available that explore preferences for various types of bedding and provide cautionary information with regard to some potential health effects and experimental impact of bedding materials. Provision of nesting materials for rodents and many other laboratory species is generally considered a type of enrichment. Nesting materials provide an opportunity for animals to exhibit more of their species-specific behavior and, as noted in the section on thermoregulation, exert some degree of control over their environment. In one study (VandeWeerd et al. 1997), six different types of nesting materials were evaluated in a preference test with male and female C57BL/6J and BALB/c mice. Although there were no preference differences between strains or sexes, there were significant differences in the preference of all mice for the various nesting materials. Paper-derived materials such as tissues, towels, and paper strips were preferred over wood-derived materials, although the structure (nestability) of the materials was more important than the type (paper vs. wood). Similarly, Sherwin (1997) studied spontaneous nest building in nonbreeding female TO mice. The mice readily utilized paper toweling or commercially available nesting material for nest building, suggesting an inexpensive means of providing environmental enrichment for mice.

Unlike mice, adult laboratory rats do not spontaneously exhibit nest-building behavior when nesting material is offered. Van Loo and Baumans (2004) have shown that nest building in rats is an acquired behavior. When rats were provided nesting material from birth, they learned to utilize the materials, as has been observed also in wild rats. Similarly, rats have been shown to prefer the type of bedding on which they were raised (Ras et al. 2002).

The choice of bedding or nesting material may also have a significant impact on physiological parameters and therefore on experimental results. As confirmed by Buddaraju and Van Dyke (2003), it has long been known that compounds in untreated pine products induce liver microsomal enzymes. They also demonstrated, however, that this type of bedding has an effect on endocytosis. Bedding materials have also been shown to alter mucosal immune responses in the intestines (Sanford et al. 2002). In contrast, heat treatment of wood product bedding causes a logarithmic reduction in various essential oils and aromatic compounds linked to liver microsomal enzyme induction (Nevalainen and Vartiainen 1996).

The choice of bedding material may also be influenced by unique characteristics of the species or strain housed. Commercial cotton bedding has been shown to be a predisposing factor in the development of conjunctivitis in nude mice (Bazille et al. 2001). Zahorsky-Reeves et al. (2005) reported that changing to the use of paper-based (vs. hardwood) bedding decreased the incidence of blepharitis in a population of nude rats.

In addition to the impact bedding may have on thermoregulation, other physiological parameters, and behavior, the type of bedding has an impact on the cage microenvironment. Smith et al. (2004b) assessed the conditions of the cage microenvironment (ammonia levels, temperature, and humidity) of mice housed in various types of bedding and combinations in static cages. Bedding type had little effect on temperature or humidity; however, there were considerable differences in ammonia concentrations, with the lowest in cages that housed mice on hardwood bedding or a combination of corn cob and alpha cellulose. In addition, not surprisingly, there was a statistically significant decrease in ammonia levels in similarly bedded ventilated cages.

It is important to note that there are complex relationships between various environmental parameters within the primary enclosure. For example, ammonia levels are influenced not only by bedding type and amount as well as high ventilation rates, but also by relative humidity and temperature. Studies by Memarzadeh et al. (1998) have demonstrated that relatively minor changes in temperature and humidity can significantly alter gaseous, and presumably particulate, concentrations in the microenvironment.

Caging and Housing Systems

New, albeit limited, information is available on some caging systems for rodents. The issue of the development of foot lesions on wire-grid floors has been explored and is much more limited in effect than suggested in the current issue of the Guide. Peace et al. (2001) analyzed data and observations from a 2-yr rat study in which groups of rats were housed in either wire-bottom or polycarbonate cages. Abnormalities of the plantar surface of the hind foot were more prevalent in heavier rats than in lighter rats of the same sex regardless of the cage bottom surface, and were more common in wire-bottom cages than polycarbonate. However, despite these differences, lesions were found only after rats had been housed for more than 1 yr. Based on a decreased rate of weight gain and excretion of immunoglobulin A and corticosterone during short-term housing in wire-bottom metabolic cages, Eriksson et al. (2004) concluded that this type of caging was mildly stressful for young adult male rats. In yet another study, however, no differences were found in a large panel of clinical pathology parameters in male Sprague-Dawley rats housed in wire-bottom cages compared with solid-bottom cages with paper-based bedding (Sauer et al. 2006). The parameters evaluated included complete blood count, serum chemistries, coagulation factors, urinalysis, urine creatinine, and urine corticosterone.

Since publication of the 1996 Guide, ventilated caging systems for rodents have come into widespread usage and have even replaced conventional open housing and static microisolation cage housing in many facilities. The microenvironment in these systems has been a topic of study, with focus on gas concentrations, air exchange rates, noise, moisture, required sanitation frequency, and more. Some articles have been published on these issues, and they must be critically evaluated in order to select an appropriate housing system for study requirements. A number of studies have been conducted that have compared the environmental conditions in static versus individually ventilated cages, and in ventilated cages of differing design.

In one study (Tsai et al. 2003), it was demonstrated that the type of housing (ventilated vs. static) may influence reproductive performance, with fewer mouse pups born in individually ventilated cages. Individually ventilated rodent caging systems vary distinctly depending on the manufacturer. Characteristics such as air distribution, exchange rates velocity, and leakage differ and should be considered when selecting a caging system (Tu et al. 1997). Such factors may even have an impact on physiological values. Air change rates in excess of 80 air changes per hour (ACH¹) were associated with increased heart rate and systolic blood pressure in rats (Krohn et al. 2003). In addition, in preference tests, these rats selected cages with lower exchange rates when given the opportunity.

The effect of varying ventilation rates from 30 to 100 ACH was evaluated by Reeb et al. (1998). At all ventilation rates, ammonia concentrations were less than 3 parts per million (ppm¹); carbon dioxide concentrations ranged from 840 to 3300 ppm; relative humidity from 42 to 65%, and temperature from 23.2 to 25.3°C. In the same study, the impact of cage changing frequency was also evaluated. The authors concluded that ventilation rates of 30 ACH were sufficient to maintain an adequate microenvironment when bedding was changed weekly. Reducing cage change frequency to every 2 wk required increasing ventilation rates to at least 60 ACH to achieve a healthful environment as assessed by gaseous and thermal parameters. However, recent evidence suggests that mice can cope with very high ammonia concentrations in the cage microenvironment (Smith et al. 2004a). It is likely that the older literature showing adverse outcomes may have been confounded by underlying infections with agents such as mycoplasma or Sendai virus (Smith et al. 2004b). Protection of caretakers and technical staff from high ammonia levels can be accomplished by the use of ventilated change stations or biological safety cabinets.

In a similar study, Memarzadeh et al. (2004) compared the environment in static versus mechanically ventilated cages, housing groups of five mice on hardwood chip bedding. Four different air flow designs in the ventilated cages were tested. Environmental conditions (temperature, relative humidity, particulates, ammonia, and carbon dioxide) were similar in all four ventilated cage designs. When compared with ventilated cages, static cages had higher relative humidity, higher ammonia levels, and higher carbon dioxide levels. Mice in static cages had relatively lower body weight gain and lower water consumption, but temperature, particulate levels, and food consumption rates did not differ significantly from those in ventilated cages.

As noted above for rats (Krohn et al. 2003), preference testing has shown that when given the opportunity, mice have avoided cages with higher ventilation rates (Baumans et al. 2002). However, provision of nesting material could counteract this avoidance behavior by providing a means by which mice could exhibit preferred nesting behavior and avoid the greater air flow conditions within the ventilated cages. Baumans (2002) also reported that the mice preferred larger cages and cages with the air supply in the cage cover (vs. directed across the cage).

There are a number of other presumed benefits of using individually ventilated cages, including a high degree of containment, high degree of protection from allergens, and a decrease in cage changing frequency. The impact of reduced frequency of cage changes has been studied quite extensively. One study evaluated the impact of change intervals and ventilation rates on the health of breeding pairs and trios of mice (Reeb-Whitaker et al. 2001). Based on ammonia and carbon dioxide concentrations and physiological parameters (breeding performance, weanling weight and growth, plasma corticosterone levels, immunological function, and select histology), they concluded that cage changes once every 14 days and ventilation rates of 60 ACH provided the most optimal conditions for housing. Systems from different manufacturers have been shown to provide different conditions of uniformity and balance of air (Hoglund and Renstrom 2001). Most systems that were studied minimized ammonia concentration and maintained dry bedding conditions for up to 2 wk.

The use of ventilated caging operated at negative pressure to the animal room can effectively reduce personnel exposure to allergens (Thulin et al. 2002) and may reduce the potential for transmission of infectious diseases (Myers et al. 2003). Hasegawa and colleagues (2003) reported intercage transmission of Pasteurella pneumotropica within 4 wk among mice housed in open-air cages in conventional racks, but not between cages in a forced air ventilated microisolation system for up to 12 wk. Intercage transmission of microorganisms may also be influenced by the techniques used for manipulation during cage change operations. Proper cage changing techniques are also required when using forced-air ventilated microisolation caging systems.

Although individually ventilated caging systems have many advantages over conventional cages, these systems pose unique challenges for health monitoring and sentinel programs (Compton et al. 2004a). Strategies have included recirculation of air from individually ventilated cages to sentinel animals as well as more conventional transfer of soiled bedding. One study (Compton et al. 2004b) compared these two methods as well as direct contact of group-housed mice with sentinels. The methods of detection involved transmission challenges following infection of the mice with several agents (mouse hepatitis virus, mouse parvovirus, murine rotavirus, Sendai virus, or Helicobacter spp.). All agents were detected when infected mice were housed in direct contact with sentinels. Mouse hepatitis virus was detected in sentinels with both recirculated air and transfer of soiled bedding. Sendai virus was detected only in sentinels exposed to recirculated air. Both mouse parvovirus and Helicobacter spp. were transmitted in soiled bedding, but the results varied depending on the frequency and amount of bedding transferred. These results demonstrate that it may be necessary for monitoring programs for animals housed in individually ventilated cages to utilize a variety of approaches in order to optimize the ability to detect adventitious agents.

Summary

The discussion above illustrates the complex nature of housing rodents and the potential impact of housing conditions on research variables. While this article focuses on housing systems and enrichment strategies, other aspects of the laboratory environment (e.g., sanitation, illumination, and noise) are not discussed. Recognizing that many gaps still remain in our knowledge, we have attempted to bring together information that may be of use to research scientists, IACUCs, and animal facility managers who use and care for laboratory rodents.

Abbreviations used in this article: ACH, air changes per hour; Guide, Guide for the Care and Use of Laboratory Animals; IACUC, institutional animal care and use committee; ppm, parts per million.

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