Non-invasively Assessing Disturbance and Stress in Laboratory Rats by Scoring Chromodacryorrhoea

In rats, like many rodents, Harderian glands next to the orbits secrete porphyrins, lipids and other compounds. High levels of secretion lead to chromodacryorrhoea (red or “bloody” tears), often taken as a sign of stress or disease. Here, we developed a scoring system for recording chromodacryorrhoea in a quantitative way, and investigated whether the low-level, transient Harderian secretions of normal, healthy rats correlate with low to moderate levels of stress or disturbance. Rather than exposing our subjects (24 Lister Hoodeds, housed in 11 single-sex cages) experimentally to stressors, we made opportunistic use of three likely sources of low-level stress within the unit: 1) building maintenance work, taking several hours and involving several potential stressors; 2) visits by unfamiliar humans, and the other mild sources of disturbance normal in an animal unit; and 3) social status within the cage. The mean daily chromodacryorrhoea score increased most with the severe disturbance of building maintenance work (F1,9 = 602.67, p << 0.0001), and also increased — though to a lesser extent — with the mild disturbance of visitors and similar (F1,9 = 8.77, p = 0.008), while being the subordinate member of a cage-group had a smaller effect still (F1,6 = 7.86, p = 0.03). Individual rats scored consistently across treatment conditions, and there was also significant inter-observer reliability between independent scorers. We therefore suggest that scoring chromodacryorrhoea could be a simple, practical and non-invasive way of sensitively assessing the impact on rats of housing, husbandry, or procedures.