Abstract
Stress factors are so numerous and so diverse in their strength and duration that the consequences on animal welfare can be quite varied. The first important distinction concerns the characterization of acute and chronic stress conditions. Acute stress is a short-lived negative situation that allows a quick and quite complete recovery of the physiological balance (adaptation), while chronic stress is a long lasting condition from which the subject cannot fully recover (maladaptation). In the latter case, the direct effects of the stress factors (heat, low energy, anxiety, suffering etc.), as well as the indirect ones (changes occurring at endocrinological, immune system or function level) can be responsible for pre-pathological or pathological consequences which reduce animal welfare. To evaluate the possible chronic stress conditions in single animals or on a farm (in particular a farm of dairy cows), some parameters of the direct or indirect effects can be utilised. They are physiological (mainly hormone changes: cortisol, β-endorphin), behavioural (depression), biochemical (metabolites, acute phase proteins, glycated proteins etc.), as well as performance parameters (growing rate, milk yield, fertility, etc.). Special attention has been paid to the interpretation of cortisol levels and to its changes after an ACTH challenge. Despite fervent efforts, well established and accepted indices of chronic stress (distress) are currently lacking; but without this objective evaluation, the assessment of animal welfare and, therefore, the optimization of the livestock production, could prove more difficult.