Caging Rodents

Small rodents, e.g., rats, mice, hamsters, gerbils, and guinea pigs, are usually housed in solid bottom shoebox-type cages with several centimeters of various types of bedding materials covering the cage bottom. Open stainless steel wire-bottom cages without bedding material are considered less desirable in terms of animal welfare but are occasionally used when required to achieve the scientific objectives.

Figure 8.26 Rats group housed in an old rabbit cage. The complexity of the environment has been increased with hay and a branch on which the rats can climb to a shelf.

Large guinea pigs are sometimes housed in rabbit cages, especially ones with perforated plastic floors. Rabbit cages are particularly adaptable to creating a highly enriched environment for housing rats (Figure 8.26). Rodent shoebox-type cages are made of various types of plastic materials. The material most frequently used at this time is polycarbonate [Lexan® (GE) or Makrolon® (Bayer)]. This material is transparent, rigid, durable, and sanitizable. Its "softening temperature" is 152 to 157°C (305 to 315°F), and it withstands autoclaving at 120°C (250°F) but eventually becomes brittle after multiple exposures to autoclave temperatures. A newer copolycarbonate [Apec® (Bayer)] better withstands higher temperatures, thus it is often referred to as "high temperature polycarbonate," is reported to hold up better under repeated autoclaving. Recently, polysulfone [Radal® (Amoco)] was introduced as a cage material. It is a durable plastic that is reported to be able to handle thousands of repeated autoclaving while maintaining impact strength and transparency. Polysulfone, due to its brownish color, has about 35% less light penetration than polycarbonate. This might not be a problem in many laboratory facilities; it may, on the contrary, be beneficial, because light intensity may be high to make caretaking easier without illuminating the animals too much. The sole drawback with polysulfone is the initial cost, which is high in comparison to the other materials but may be cost effective in the long run because of the high durability. Other types of plastic are also used for rodent caging. Polypropylene is a light, flexible, material with high chemical inertia and thermal resistance up to 120°C. It may be translucent or opaque, depending on whether it is a copolymer or a reinforced copolymer. Polystyrene is rigid, with low impact and heat resistance. It is usually used to form disposable cages that are suitable for toxic or radioactive applications when decontamination of the cage is impractical or too dangerous for personnel.

Much emphasis has recently been given to environmental enrichment for all species, including rodents. The value of enrichment for the well-being of rodents is controversial and will probably be a source of debate for years to come. Experiments with pen housing and different ways to increase the complexity of the primary environment for the animals are currently being performed (Figure 8.26). The challenge is to balance seemingly conflicting requirements to provide for animal well-being through cage enrichment with features required to provide for routine animal care and cage sanitation, to assure animal health, and to successfully achieve the research goals.

Solid bottom shoebox-type cages may be covered with wire bar lids that leave the cage interior open to the room environment, or may be covered with filter tops that provide a barrier between the cage microenvironment and the room macroenvironment that prevents the spread of airborne infections between cages. The most common type of filtered top is a rigid inverted shoebox with an air filter insert that covers the top of the cage and overlaps the sides to form the microbiologic equivalent of a petri dish. These are known as microisolation cages. Microisolation cages have proven to be effective at protecting rodents from microbial contamination when used in combination with a HEPA-filtered mass air displacement cabinet used any time the microisolation cage is opened for cage changes or performing

Figure 8.27 A rack with static microisolator cages adjacent to a Type II A biosafety cabinet with a microisolator cage inside the cabinet. The combination of the microisolation cage and cabinet makes up what is known as a "microisolation cage system." The microisolation cages are only opened inside the cabinet, including for cage changes or research procedures.

Figure 8.27 A rack with static microisolator cages adjacent to a Type II A biosafety cabinet with a microisolator cage inside the cabinet. The combination of the microisolation cage and cabinet makes up what is known as a "microisolation cage system." The microisolation cages are only opened inside the cabinet, including for cage changes or research procedures.

procedures on the animals (Figure 8.27);46-50 however, in the static air microenvironment of the cage, animals are subjected to elevated levels of ammonia, carbon dioxide, moisture, and heat.51,52,53 The longer the time between cage changes, the higher the ammonia levels. There are many variables that affect the ammonia levels, e.g., days between cage changes, relative humidity levels, and the type of bedding, but in general, two changes per week is considered the minimum number of changes for a microisolator cage housing the maximum capacity of mice. Ventilated microisolation caging was designed to better control the microenvironment.54,55 Directly ventilating each microisolation cage with HEPA-filtered air significantly slows the buildup of ammonia, etc., in the cage, thus decreasing the cage change frequency to once a week or even once every two or three weeks.56-60 Ventilated cage racks are fitted with HEPA filter blower units that deliver HEPA-filtered air to each cage through a system of manifolds. Some ventilated racks also capture the air coming from each cage and HEPA filters it before dumping it into the room (Figures 8.17 and 8.18) or, even better, directs the air from the cages directly into the room exhaust (Figures 8.16 and 8.19). Some ventilated microisolation cage racks that control the cage supply and exhaust air can selectively maintain the air pressure in the cage, either positive or negative relative to the room air pressure. When exhausted directly into the room exhaust ducts, HEPA filtration is not necessary, but furnace or higher grade filters are desirable to reduce the amount of dust dumped into the exhaust ducts and especially onto the heat recovery system. A significant "fringe benefit" of ventilated cage racks that HEPA filter air coming from the cages or directly exhaust the cage air from the room is the reduction of animal allergens in the workers' environment because allergies to animal allergens are the most significant occupational hazard for personnel working with animals.40,61 Another advantage is that it allows for high-density housing. Mobile double-sided ventilated mouse cage racks commonly hold up to 140 cages (7 cages wide x 10 rows high x 2 sides) each with the capacity for up to five mice. Static racks may even be stacked higher, further increasing housing density but also increasing the ergonomic problems and possibly injuries associated with reaching the upper cages.

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