Using the mouse grimace scale to assess pain associated with routine ear notching and the effect of analgesia in laboratory mice.

Social housing is recommended where possible for laboratory mice. In order to achieve this, mice must be individually identifiable. Although, various methods are available, permanent identification is often required, such as ear notching. This method is likely to be painful and to date there is limited literature on pain assessment and alleviation for this routine husbandry practice. Here we aimed to determine if the mouse grimace scale (MGS) could be used to assess pain in C57BL/6 mice following routine ear notching. Langford et al. found that very acute noxious stimuli (i.e. < 10 min in duration) did not produce a change in MGS score in comparison to baseline. Here, no significant difference was found between MGS scores at baseline and immediately post ear notching, potentially indicating that the pain associated with ear notching is either too acute to assess using the MGS tool or the practice is not painful. Studies in other species indicate that ear notching is painful, therefore, unless we can confidently conclude that the process of ear notching is not painful, we should err on the side of caution and assume it is painful due to the large number of mice ear-notched and potential welfare consequences. Alternative methods of assessing pain following this routine practice should be used in order to assess both the potential pain in mice, and the effectiveness of analgesics or local anaesthetics to relieve any associated pain.