Decreased interest in social interaction
Decreased play behavior
Decreased grooming behavior
There will be some variation in how rapidly these behavioral changes set in and in the degree to which they appear, depending upon the pathogen involved. Some diseases will cause a rapid and severe onset of lethargy and anorexia, while others may develop more slowly, and the behavioral signs may be less obvious. The status of each individual’s immune system may also affect the degree of illness experienced and thus the degree of behavioral change.
An animal’s coat can provide important clues regarding its health status. Grooming behavior has evolved in mammals to serve a variety of purposes, depending upon the species. These behaviors may spread natural body oils throughout the coat, contributing to coat health and thermoregulation, as well as effectively decreasing ectoparasite loads (Hart 2011). Saliva contains a variety of antibacterial and wound‐healing substances, so that the predisposition for animals to lick body parts and wounds is likely an evolved behavioral tool for decreasing the incidence of infection (Hart 2011). When animals fail to practice normal self‐grooming behavior, it should serve as a warning sign that something is wrong.
4.3.1 Cats
Some dogs can be adept at hiding their illnesses, but cats are even better at it. This may be due to the cat’s unusual position of being both predator and prey, depending upon the environment. Anorexia is often the first sign noted by caretakers of sick cats. The fastidious nature of the cat contributes to the ability to mask signs of disease. For example, if cats have diarrhea, they are likely to clean themselves, removing all signs of the mess, until they become too ill to do so. The more sedentary and nocturnal nature of the cat may also cause caretakers to overlook inactivity due to illness until it becomes severe. Unkempt hair coat in a cat should be immediately noted and a possible cause investigated because the cat must be either ill, injured, or otherwise impaired in its movement in order for it to stop grooming itself.
A variety of different studies have suggested that monitoring sickness behaviors in the cat may be an excellent means of evaluating feline welfare and that cats’ behavior is a more reliable indicator of their level of stress than their physiological responses (Stella et al. 2013). One study demonstrated that the presence of unusual external events is enough to increase the risk of sickness behaviors in cats (Stella et al. 2011). When cats are exposed to multiple unpredictable stressors, including exposure to unfamiliar caretakers, an inconsistent husbandry schedule, and discontinuation of play time, socialization, food treats, and auditory enrichment, they demonstrate a higher incidence of sickness behaviors (Stella et al. 2013). These behaviors include increased vomiting (Stella et al. 2013), decreased food intake, avoidance of elimination for 24 hours, and elimination outside the litterbox (Stella et al. 2011).
4.4 The Role of Stress
Nowhere else is the interplay between behavioral and physical health more apparent than when looking at the impact that stress plays on every aspect of health. Increasingly, science is uncovering the myriad of different ways in which stress affects living organisms at every stage of development. Much controversy exists about how to define stress, so for the purpose of this chapter, stress (or stressors) is defined as anything that disturbs or threatens homeostasis. These stressful forces may be physical, chemical, or emotional and typically result in physiological or behavioral responses as the organism attempts to restore homeostasis. The physiological events that occur during an acutely stressful event are intended to be adaptive, and, in most cases, they do succeed in helping an organism maintain homeostasis by adapting to the stressor. When stress is chronic and unremitting, or the individual cannot successfully act in such a way as to decrease the stressors, a variety of physiological events can conspire to damage the overall health and well‐being of the organism. Thus, in the long term, the stress response can be maladaptive.
There are two primary components of the stress response, involving two different endocrine systems. The first is the sympathetic nervous system response. Within seconds of perceiving a stressor, the sympathetic nervous system begins secreting norepinephrine, and the adrenal medullae begin secreting epinephrine. This begins to prepare the body for “fight or flight.” The second system is the hypothalamic‐pituitary‐adrenal (HPA) axis, generally believed to be the body’s primary stress‐responsive physiological system (Hennessy 2013). When the HPA axis is triggered, the hypothalamus releases corticotrophin‐releasing factor that triggers the release of adrenocorticotropic hormone from the pituitary gland. This hormone then stimulates the release of glucocorticoids from the adrenal cortex. Several other hormones, including prolactin, glucagon, thyroid hormones, and vasopressin, are secreted from various other endocrine organs. The overall effect of these circulating hormones is to increase the immediate availability of energy, increase oxygen intake, decrease blood flow to areas not critical for movement, and inhibit digestion, growth, immune function, reproduction, and pain perception. In addition, memory and sensory functions are enhanced. Essentially, the goals of all of this physiological activity are to make more energy available for immediate use and to put on hold any and all processes that are not involved in immediate survival.
Acute stress has been shown to enhance the memory of an event that is threatening (McEwen 2000). This is clearly adaptive if it allows the organism to form strong associations, enabling it to avoid dangerous things in the future. Knowing this should increase animal handlers’ awareness of the important and lasting impact that their behavior and actions can have on an animal. An unpleasant handling experience may have long‐term, negative effects on the animal’s behavior, ultimately making that animal less adoptable.
If the stress response continues, for whatever reason, cardiovascular, metabolic, reproductive, digestive, immune, and anabolic processes can all be pathologically affected. The results can include myopathy, fatigue, hypertension, decreased growth rates, gastrointestinal distress, and suppressed immune functioning with subsequent impaired disease resistance. Chronic stress can even lead to structural and functional changes in the brain, and when extreme conditions persist, permanent damage can result (McEwen 2000). It is believed that when dealing with chronic stress, the HPA axis becomes dysregulated, and the various components of the system may no longer respond in the predicted fashion. For example, in some cases, chronic stress results in adrenal hypertrophy and elevated levels of glucocorticoids, while adrenocortical‐stimulating hormone (ACTH) levels remain unchanged. At this point, the dysregulation results in an HPA axis that is no longer able to respond appropriately to future stressful events, and measurements of glucocorticoid levels may become less meaningful (Hennessy 2013).
Stress can arise from a variety of different sources, both physiological and psychological. Physical stress can be caused by hunger, thirst, pain, exposure to extreme temperatures, disease, illness, and sleep deprivation. Psychological stress can result from exposure to novelty, unpredictable environments, social conflict, and constant exposure to fear‐ or anxiety‐provoking stimuli as well as any other situation that leads to chronic frustration or conflict. A lack or loss of control is another important psychological stressor. In fact, novelty, withholding of reward, and the anticipation of punishment (not the punishment itself) have been found to be the most potent of all psychological stressors (McEwen 2000).
A variety of different means have been used in an attempt to measure physiological stress, including but not limited to measuring glucocorticoids and their metabolites in hair, urine, feces, blood, and saliva. Glucocorticoids in blood and saliva do appear to measure the condition of the animal at that moment, whereas glucocorticoids in urine, feces, and hair reflect the condition of the animal over a longer time frame (Hennessy 2013). ACTH and luteinizing hormone‐releasing hormone stimulation tests have also been used to measure adrenal and pituitary sensitivities, respectively, and one study demonstrated increased HPA responsiveness and reduced pituitary sensitivity occurring in the face of chronic stress (Carlstead et al. 1993).
