Research Rodents Demand Better Housing
For decades, biomedical disciplines have relied on millions of mice and rats to carry out extensive research. Nonetheless, recent inquiries have questioned whether that is both ethical and practical. Why? Because such research causes stress for the animals involved while the majority of animal studies either don’t get published, can’t be replicated, or can’t be translated to human medical outcomes.
Rodent housing is another overlooked factor in the wealth of studies conducted so far. Today, rodent-based research uses over 120 million animals every year, but their barren housing conditions may alter the animals’ welfare and physiology so that they no longer resemble the human subjects they’re supposed to represent. Therefore, improving rodent housing is relevant not just for animal welfare, but also to increase the generalizability of research findings. Currently, translatability rates are low, with around 86–91% of drugs working in animals then failing in human clinical trials.
To address this issue, a team of scientists investigated the detrimental impacts that conventional housing (CH) used in rodent experiments may have on the animals’ health and well-being. They predicted that CH would increase the morbidity of rodents induced with certain stress-sensitive diseases, and also elevate mortality among research rodents in general, compared to animals kept in ‘enriched’ housing (EH). Although housing conditions for research rodents have slowly improved over the last decade, barren cages are still in use and contain little more than food and water. In contrast, in the wild rats dig burrows and build elaborate, well-structured nests as warm, safe resting places. EH are more complex cages containing resources that supposedly make them more similar to rodents’ natural environments.
To determine whether their hypotheses were correct, the team systematically reviewed relevant literature and synthesized findings via meta-analysis. Studies had to be published in English, involve laboratory mice or rats, and report mortality rates. The authors only looked at studies utilizing both EH and CH. Finally, they explored whether the effects of CH differed depending on factors such as species, sex, and disease type.
For feasibility, the team narrowed their analysis to include lab research on five diseases known to be worsened by stress: anxiety disorders, cancer, cardiovascular disease, major depression, and stroke. Using data from over 214 studies and over 6,000 rodents, they found that behavioral restrictions inherent in CH cause sufficient stress to significantly increase the severity of all five stress-sensitive diseases. For example, rodents kept in CH who were induced to have cancer had worsened cancer-related death rates as well as increased tumor numbers, volumes, and weights. CH also worsened factors such as plaque magnitude for rodents with cardiovascular disease, and learned helplessness for animals induced with depression.
Overall mortality rates for CH animals were elevated by about 50%, and keeping animals in EH increased median survival by 9%. The effects of barren housing were consistent regardless of an animal’s sex, species, housing status (i.e., whether they were housed individually or with other animals), and disease status.
For animal advocates, the goal is to remove animals from the research industry altogether. Until then, it’s important to push for more humane, “natural” living standards for animals housed in laboratories. The findings of this study highlight the inadequacies of conventional cages for rodents used in research. The authors note that housing conditions aren’t subject to the same scrutiny as other factors in animal research, which presents a major ethical issue. They also argue that CH should be more logically considered as a deprivation treatment rather than a normal baseline or ‘control,’ while ‘enriched’ is probably not the best term for housing that is merely less inadequate than conventional housing. The authors propose the term ‘CRAMPED’ (cold, rotund, abnormal, male-biased, poorly surviving, enclosed and distressed) to describe the typical research rodent, and they leave readers with one specific question: Can research results stemming from CRAMPED rodents be applicable to the average human, including those who are fit and happy?