The term "stress" has been widely used in biology to describe a set of physiological and behavioral changes elicited by aversive stimuli. In 1929, Cannon described stress as the sympatho-adrenomedullary (SAM) system's attempt to regulate homeostasis when threatened by a variety of aversive stimuli or stressors. Later on, Selye conducted some of his classic studies on the response of the hypothalamic-pituitary-adrenal (HPA) axis to noxious stimuli and suggested that the organism reacted in a non-specific manner to a wide variety of aversive stimuli, mainly with an increase in the HPA axis activity. Both the HPA axis and the SAM system are generally considered to be the two main elements of the stress response, and plasma levels of glucocorticoids have been widely used as measures of stress.¹ The problem with this approach, however, is that the HPA axis and the SAM system have a crucial function in energy mobilization and redistribution of nutrients to active tissues and both aversive (e.g., fighting) and rewarding situations (e.g., play and mating) may elicit a similar physiological stress response. Therefore, if stress is perceived as potentially negative, it may be misleading to consider stress as synonymous with the HPA axis activation. On the other hand, there is now sufficient evidence showing that it is not the physical nature of an aversive stimulus that has negative consequences on the animal, but rather the degree to which the stimulus can be predicted and controlled. As a result, it has been suggested that the term "stress" should be restricted to conditions where an environmental demand exceeds the regulatory capacity of the organism, in particular when such conditions include unpredictability and uncontrollability.

Current research on stress biology has addressed the role of the brain. Several areas of the brain are involved in the organization of responses to aversive or threatening stimuli, and these areas interact extensively. Neurons in the hypothalamus, for example, are sensitive to internal physicochemical stimuli and to external physical and psychosocial stimuli. To a great extent, the stress response is mediated by the corticotropin-releasing factor (CRF) that is secreted mainly by the paraventricular nucleus of the hypothalamus.

Stress causes a variety of behavioral changes in dogs and cats, and many of such changes may have a negative effect on the human-animal bond or on the health of the animal.²

Stress often results in a decrease in feed intake in a variety of species, and this seems to happen rather frequently in cats, particularly when stress and diet change occur at the same time. Stress-induced anorexia in cats may lead to a potentially fatal condition known as hepatic lipidosis. Hepatic lipidosis seems to be particularly common in overweight cats that stop eating due to a stress response. Occasionally, however, stress has the opposite effect, and it has been suggested that stress may be one of the factors contributing to obesity in both dogs and cats.

Aggressive behavior has many different causes, and there is evidence that stress may increase aggression.³ This effect may be partly mediated by a decrease in serotonin activity in chronically stressed animals. Redirected aggression is a common type of aggression in cats, which is very dangerous due to its unpredictable and violent nature. Redirected aggression in cats is suspected when a cat behaves aggressively toward an irrelevant but accessible target at least once, and the primary inciting stimulus is inaccessible or no longer available. Redirected aggression may serve as a coping mechanism when cats are exposed to stressful events.

Another behavioral problem that is sometimes caused by stress in cats is urine marking. Urine marking is a normal behavior seen most frequently (but not exclusively) in intact males. Cats can mark with urine when they want to create a sense of familiarity when placed in an unknown environment or when they want to signal their presence to other cats. Furthermore, stress increases the likelihood of cats showing urine marking.⁴ Changes in the environment and inter-cat conflict are the most common stressors that may cause urine marking. Stress may also contribute to interstitial cystitis, which is another cause of inappropriate elimination in cats. Interstitial cystitis is a very painful condition and therefore has to be considered a major welfare issue. Several internal and environmental risk factors have been identified for this disease. For instance, both local bladder abnormalities and an abnormal hypothalamus-pituitary-adrenal response play a role in the pathophysiology of feline interstitial cystitis.

Repetitive behaviors are often related to stress and are therefore potential indicators of poor welfare. Repetitive behaviors of dogs and cats include a variety of behaviors such as pacing, over-grooming, self-mutilation, and fly-biting, among many others.

Repetitive behaviors may be caused by a variety of medical conditions.⁵ However, repetitive behaviors are shown by apparently healthy animals and appear to be caused by a suboptimal environment. Chronic stress (e.g., separation-related problems or conflicts between dogs that live together) and inability to perform highly motivated behaviors (e.g., exploratory behavior, play behavior) and repeated conflict situations (e.g., inter-cat conflict) can result in animals developing repetitive behaviors. Some individual animals seem to be more predisposed to develop repetitive behaviors than others when the environment is inadequate and such predisposition results from a combination of genetic factors and early experience.⁶ Once initiated, repetitive behaviors may be perpetuated through several mechanisms, including learning. Owners tend to reinforce these behaviors as they try to calm down the animal when it performs the repetitive behavior. As a consequence, the animal learns that doing these behaviors prompts owner attention. Self-rewarding is another mechanism that can perpetuate the repetitive behavior.⁶ In addition, animals subjected to high levels of stress during early periods of development (neonatal and socialization period), when the nervous system is still maturing, are more susceptible to suffer irreversible changes in the structures involved in the stress response and in the basal ganglia, which are involved in motor control. These animals are more likely to exhibit behavioral problems such as repetitive behaviors.

In general, there are many different strategies to treat or prevent stress in animals. Adequate husbandry during early periods of development and environmental enrichment programs may be very useful to prevent and reduce stress.

References

1.  Jensen P, Toates FM. Who needs 'behavioural needs'? Motivational aspects of the needs of animals. Appl Anim Behav Sci. 1993;37:161–181.

2.  Rochlitz I. Feline welfare issues. In: Turner DC, Bateson P, eds. The Domestic Cat: The Biology of its Behaviour. 3rd ed. UK: Cambridge University Press; 2014.

3.  Heidenberger E. Housing conditions and behavioural problems of indoor cats as assessed by their owners. Appl Anim Behav Sci. 1997;52:345–364.

4.  Pryor PA, Hart BL, Bain MJ, Cliff KD. Causes of urine marking in cats and effects of environmental management on frequency of marking. J Am Vet Med Assoc. 2001;219:1709–1713.

5.  Luescher AU. Repetitive and compulsive behaviour in dogs and cats. In: Horwitz D, Mills DS, eds. BSAVA Manual of Canine and Feline Behavioural Medicine. 2nd ed. Gloucester: BSAVA; 2010.

6.  Mason GJ. Stereotypies: a critical review. Anim Behav. 1991;41:1015–1037.