Nearly every animal study article we translate will include some details about how the animals are housed. Currently some major changes are emerging in relation to laboratory animal housing that we are likely to see soon in materials for translation. I have summarized a few below.
Quotes are from publications by the Americal Association for Laboratory Animal Science (AALAS, http://www.aalas.org), and are reproduced here by permission.
Traditionally mice, rats, guinea pigs, and rabbits have been housed in cages with wire mesh sides and flooring. (The style of the cage top depends on whether air is filtered independently through each cage or through the cage rack.) The traditional exception has been for gestating females, moved to bedded cages a few days before delivery, and nursing females with pups.
Wire mesh has several advantages. Urine and feces can be removed regularly from the tray under the cage without handling the animal. The animal is fully visible, and its condition can be readily assessed from outside the cage. Wire mesh is easy to clean, easy to handle, and difficult for the animal to chew through.
However, when compared with a solid floor and bedding, wire mesh appears to be both boring and uncomfortable for the animal. Considerable debate has developed over whether this is actually the case and how animal comfort can be objectively measured. There is no way to ask the animals directly, but researchers who covered one corner of mesh flooring with a piece of hard plastic have reported that their rats preferentially sleep and rest on the plastic rather than on the wire mesh. Some long-term studies have also associated wire mesh flooring with an increased incidence of foot ulcers, although these findings are not universally accepted.
There has been a significant shift over the last few years toward bedded cages, often of solid clear plastic rather than wire or metal. Animals in bedded cages look less bored and uncomfortable (which improves the morale of animal technicians even if benefits to the animals cannot be documented scientifically). But the bedding retains urine and fecal material, increasing the animal's exposure to ammonia fumes. The solid floor also means that the cage can quickly become flooded if the automatic water breaks.
Expect to see continued argument and new developments in this area.
Increased concern about animal welfare, particularly boredom, has led to a number of innovations in the care and feeding of laboratory animals, and a new market for “enrichment treats” and “enrichment devices.” Caged animals don't forage for their food, so enrichment treats are designed to make eating a more involving and attention-consuming operation. For primates this might mean embedding food treats in a plastic matrix or in a piece of carpet so that the animal has to dig each piece out individually. Rabbits reportedly enjoy lettuce and peanut butter smeared on a piece of PVC pipe, and after licking off all the peanut butter “then they proceed to play with the pipe,” according to Connie Arnold of the University of Nebraska Medical Center(TechTalk Vol. 7 No. 4, 2002).
Enrichment devices (toys) add variety to the animal's cage and also provide more exercise. This may be of particular value in studies where it is important that the animal live into old age, such as in long-term carcinogenicity research. There are several challenges, though. The toy must be chew-resistant, and nontoxic if chewed. It has to be easy to take care of, able to withstand appropriate cleaning methods, and cost-effective.
An additional problem is that valuable toxicology data has been accumulated over the years for individual strains of laboratory animals. Particularly in the area of animal behavior, changes in the cage environment could conceivably change the animal's response to test stimuli. Could this invalidate behavioral data from tests performed on animals housed in an enriched environment? We aren't sure.
The amount of space needed by laboratory animals is also being questioned.
Primates, including baboons, play an essential role in current drug testing. Historically both captive-bred baboons and imported wild-caught animals have been available for research use. Now, with imported animals becoming difficult to obtain, it is necessary to supply the demand almost entirely from domestic breeding facilities. But baboons don't always breed like clockwork. Would a less crowded environment improve fertility?
Apparently it does, according to Cary and colleagues of the Oklahoma University Health Sciences Center Primates Facility, in the January 2003 issue of Contemporary Topics. They reported that females in a roomy new indoor-outdoor facility reached their first postpartum estrus after 69 ± 31 days, in comparison to 165 ± 60 days at the old indoor facility, and predict that the number of live births per female per year will be approximately 1.20 at the indoor-outdoor facility in comparison to approximately 0.89 at the indoor facility.
The old facility was small but adequate (per Guide for the Care and Use of Laboratory Animals). The new indoor-outdoor facility provides significantly more space (11.5 times more total surface area and 83.0 times more total volume), and also allows the animals access to natural sunlight in an environment resembling a grade school playground.
The authors suggest that the higher fertility reflects lower stress levels, particularly in subdominant females. They have observed less fighting and wounding at the new indoor-outdoor site, and have needed to perform fewer medical treatments there. They also suggest that the more spacious facility has improved animal nutrition, since the submissive animals are able to eat more and the dominant animals are less likely to spend “a disproportionate amount of time and energy protecting their food versus eating.”
In the same issue of Contemporary Topics, Sharp and colleagues of Wayne State University School of Medicine report that female rats appear to experience less stress from some routine procedures when housed in groups of two or four animals than when housed individually. The researchers used radiotelemetry to measure heart rate, mean arterial pressure, and movement in the cage. Group-housed female rats showed less stress after undergoing restraint and subcutaneous injection, handling and weighing, and handling and vaginal lavage. However, stress levels were the same for group-housed and individually housed animals when transported to a different room for s.c. injection. Exposure to the odor of rat blood caused the same level of excitement in all animals, but values for the individually-housed animals recovered to baseline sooner than the other housing groups.
The authors found no evidence that this level of crowding increased stress level. Resting heart rate was lowest among animals housed four to a cage, even though that animal density would be considered overcrowding by current standards.
There is obviously considerable room yet for debate in these areas. Stay tuned for more developments!