Evaluation of the neutrophil-lymphocyte ratio as a measure of distress in rats

The ability to evaluate distress in laboratory animals is needed in order to ensure that husbandry and experimental procedures do not negatively impact animal welfare. Accurate measurement of acute stress and chronic stress, and distinguishing between stress that is harmful (distress) and stress that does no harm (eustress), can be challenging. Whereas corticosterone concentrations are commonly used to measure stress in laboratory animals, the neutrophil-lymphocyte ratio has been proposed as a potentially better indicator of chronic stress. Furthermore, an association between such measures of stress and concurrent behavioral indicators of negative welfare is required to determine their accuracy in evaluating distress. The authors compared serum corticosterone concentrations and neutrophil-lymphocyte ratios to assess acute or chronic stress in male Sprague Dawley rats. Elevated serum corticosterone concentrations, but not neutrophil-lymphocyte ratios, were associated with acute stress exposure, whereas elevated neutrophil-lymphocyte ratios, but not serum corticosterone concentrations, were associated with chronic stress exposure. Because the neutrophil-lymphocyte ratio differences corresponded with a behavioral indicator of distress in chronically stressed rats, it may serve as a valuable tool for the physiological assessment of distress in rats.