Identification of Sick or Dead Mice (Mus musculus) Housed with 6 Grams of Crinkle Paper Nesting Material

Although nesting material is beneficial to the welfare of laboratory mice, provision of appropriate amounts may impair visualization of the mice. In anticipation of our academic research institution transitioning to providing 6 grams of nesting material to all mice, we conducted a 2-step prospective epidemiologic study to 1) evaluate whether 0, 2, or 6 grams of nesting material alters the ability to identify sick or dead mice, and 2) evaluate the number and severity of health concerns identified in the presence of 6 grams of crinkle paper nesting material at cage-side health check as compared with cage change. Animal Treatment Reports (ATRs) and death incidences were collected across a variety of research and breeding uses. This information was used to determine if nesting material prevented prompt identification of mice in need of veterinary attention. The clinical health condition category (CHCC) was determined based on the severity of the animal's health condition on initial veterinary exam. Additional assessment determined if the identification of the animal's condition was a success (early-stage or mild illness when first identified) or a failure (late-stage or endstage illness when first identified). Mice that died spontaneously were also assessed with regard to which observation activity was being performed at the time of the animal's identification (daily health check or cage change) and location of the mouse in relation to the nest. The results showed that nesting material did not cause a significant increase in the severity of CHCCs at the time reported for veterinary evaluation. Successful identification of health concerns occurred significantly more often than failures. Death rates were similar between all nesting groups, and dead mice were more likely to be located outside of the nest. In summary, nesting material did not hinder the ability to identify mice in need of veterinary care during routine cage-side health checks and did not critically affect the ability to identify mice that died spontaneously. These results indicate that mice can receive appropriate amounts of crinkle paper nesting material without lowering the ability of staff to recognize mice in need of veterinary attention.