StressBack to Stable Stress






1.  Definition of stress

As with other words, stress is difficult to describe and measure, being very subjective. Broom (1987) described stress as referring to some individual’s state of being overloaded.  There is a need to define stress and frustration more accurately and refine non-invasive techniques for measuring stress hormones like cortisol.  Nyman et al (1996) measured plasma vasopressin levels as mediating stress response and found that AVP (arginine vasopressin) had a role in mediating stress responses in the horse.  Stress is an environmental effect on an individual that overtaxes its control systems and reduces its fitness or appears likely to do so (Broom and Johnson 1993).

 There are three aspects of stress, the stressors, the individual differences and the stress reactions.  The stressors can be physical e.g. thermal and chemical, but they can also be defined by the situation e.g. immobilisation and social isolation.  In the stress reaction the hypothalamus and the central nervous system play an essential role.  The individual difference is the way each animal perceives its environment. 


2.  Welfare implications

Welfare problems arise when individuals fail to cope with their environment or when coping is difficult.   The ultimate criterion of what is detrimental is whether individual fitness is reduced. Ödberg (1987) posed the question; does “coping” warrant welfare?  He also stated that the argument that abnormal behaviour represents coping mechanisms should not induce us to take welfare for granted too quickly as:

  1. It is likely that it only represents the final stage of a distressing process

  2. Little is known about the costs of coping

  3. Not all individuals in the same environment are able to develop such mechanisms


Attempts to cope include:

  1. Functioning of body repair systems

  2. Immunological defences

  3. Emergency physiological responses

  4. And a variety of behavioural responses

Some researchers have taken the “stress” or “disruption of biological functioning” approach to defining poor welfare (Duncan 1996).  There is the paradox that neither the absence of abnormal behaviour like stereotypies, implies no stress, nor that their presence indicated actual stress.  Stress implies poor welfare (Broom and Johnson 1993) but welfare can be poor without stress.

Physiological and behavioural indicators of stress can be used as an index of welfare and provide means of measuring welfare objectively and quantitatively.  The use of physiological indicators of stress can provide a sound ethological base upon which the animal welfare legislation and recommendations of the future can be based. 


3.  Suffering

Dawkins (1990) described suffering as occurring when unpleasant subjective feelings are acute or chronic because the animal is unable to carry out the actions that would normally reduce risks to life and reproduction in those circumstances.  There are some clear links between suffering and welfare.


4.  Stress, Welfare and immunology

Ödberg (1987) studied the relationship between stress and immunology and found that severe and acute stress may suppress the immune response of the animal involved.  He found stress was associated with some form of neurobiological state of uncertainty.  The immune system in one way or another listens to the brain, and brain activity on the hypothalamic level reflects what goes on in the immune system (Ödberg 1987).

Stress comes from not only physical stressors, inflicting some somatic damage, but also from so called psychological stressors, inhibiting the performance of essential behaviour programmes.  A large array of environmental conditions may interfere with the organism’s health (Ödberg 1987) and when changing management regimes and reducing stress we can expect improvements in disease resistance and possibly performance.

The same diagram can use measurements of immune system function, disease condition and hence possible suffering to be indicators of the level of welfare (taken from Broom and Johnson 1993):

Levels of immunosuppression also correlate with levels of welfare.  Welfare includes health and therefore indicators of good and poor health are also indicators of good and poor welfare.  Most research in this area centred on humans.  Susceptibility to disease can be increased by a variety of biological disturbances.  When welfare is good the immune system works effectively to counteract challenges from pathogens, but if there is immunosuppression the animal will have to do more to cope with environmental challenges.

5.  Stress and stereotypies

Although there is a chapter on stereotypies it was thought necessary to describe here the relationship stress has with stereotypies. McGreevy (1998) stated that there was no correlation between the removal of the opportunity to perform these behaviours and a rise in stress measurements. Stereotypic horses had higher stress levels to start with even in the paddock.  When animals with stereotypies are put into better conditions they usually show a reduction in occurrence of the stereotypy but long established stereotypies may continue (Ödberg 1987), possibly implying the animal is scarred.  Stereotypies have also been shown to reduce stress in pigs (Dantzer and Mormede 1993) and in calves (Wiepkema 1987).