The impact of poor health, pain, distress and other adverse states on animals will often have a direct effect on their welfare and their quality of life. The question is how do we know that it is having this effect and how can we measure its degree of impact? The umbrella term used to describe such adverse states in the USA research regulations is “pain and distress” whereas in Europe the equivalent phrase is “pain, suffering, distress and lasting harm”. Perhaps the US Congress, when it approved those words, was interpreting the word “distress” as ‘any adverse state other than pain’. After all, pain is a common, well-defined experiential phenomenon, well understood by the public, well characterized in terms of its anatomy, physiology, neurology and pathology, and several modalities are available for its treatment and avoidance. Distress, on the other hand, is nowhere near as clear-cut, and Europe is in a similar state to the US, as there are no accepted definitions or measurements of “distress” or “suffering”. (“Lasting harms” can be identified as experimentally induced or genetic conditions such as diabetes, blindness, deafness, paraplegia, and so on.) However, in Europe, despite any universal agreed definitions of distress and suffering, there is a general understanding of what distress etc might encompass in terms of animal wellbeing. It would, for example, include adverse feelings such as hunger, thirst, excessive hotness or cold, disorientation, frustration, boredom, deprivation (e.g. maternal, social), grief, and so on. All these may require definitions or descriptions but perhaps the important thing is to be able to recognize such states when they occur, to be able to assess them (qualitatively or quantitatively), to know how to avoid causing them and how to treat them if they are not the desired experimental outcome or goal for study. It is not important to come up with a ‘definition’ per se, i.e. these adverse states need to be ‘operationalized’. The integrated responsiveness of animals While pain can be separated as a mental and physiological phenomenon, it would be wrong to assume that it occurs in isolation. All vertebrates and many invertebrates are complex organisms that have evolved with mechanisms to protect themselves from physical damage and to maintain an internal physiological homeostasis. In vertebrates it is clear that the nociceptive system, the sympathetic adrenal medullary axis (SAM), the hypothalamic-adrenal cortical axis (HPA) and the inflammatory response, together with the senses of sight, hearing, touch, olfaction and taste, form part of an integrated system for the protection of the whole animal. As such they may all respond together or separately, but more often all these systems are activated at roughly the same time as part of a longer-term harm-preventing strategy, together with healing, to restore an animal to normal health and vigor. Take, as an example, a dog with a broken leg going to the veterinarian. It is in pain from the fracture and its corticosteroid levels will be raised along with the inflammatory ‘repair’ response. The vet manipulates the leg in order to determine where it is broken and in so doing causes that animal some pain (probably fairly severe). The next time the vet approaches, the dog remembers the pain that was caused by this person and it becomes frightened (mental feeling) leading to the SAM and HPA responses. Consequently, the animal has now been motivated to respond not only to the pain but also to the fear; i.e. it responds as an integrated whole and not as a series of separate systems. In research and as clinicians we only measure part of this overall responsiveness depending on our interests.

for lore....follow link

https://www.wellbeingintlstudiesrepository.org/cgi/viewcontent.cgi?article=1013&context=acwp_asie