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Stress involves outcomes secondary to increased secretion of catecholamines and cortisol. The harmful effects of chronic activation of these hormones have been well described and include adverse metabolic responses that promote dehydration, mental depression, insulin resistance, peptic ulcer formation and increased susceptibility to infection (Greco 1991; Moberg 1985). Chronic stress can also alter metabolism sufficiently to cause weight loss, prevent normal growth, and result in persistent abnormal behavior deleterious to the animal. Stress responses and immunity are also intimately related; stress compromises the immune response, lowering resistance to infection (Griffin 1989). In fact, stress can trigger the shedding of certain viral pathogens, including reactivation of latent viral rhinotracheitis (feline herpesvirus) infections in cats (Gaskell and Povey 1977). In an animal shelter, minimizing stress has the potential to greatly improve animal welfare, decrease infection rates and disease transmission, and enhance adoptability.

A stressor represents any stress‐producing factor or stimulus. Housing cats and dogs in animal shelters presents enormous opportunities for introducing stressors. Stressors may include illness; captivity; transport; crowding; isolation; changes in diet, environmental temperature, light patterns, and/or ventilation; strange smells; noises; other animals; handling and restraint; irregular caregiving schedules; unpredictable daily manipulations; the absence of familiar human contact; and the presence of unfamiliar human contact. In fact, anything unfamiliar to a cat or dog can trigger apprehension and activate the stress response. The severity, chronicity, novelty, predictability, and duration of the stressor, as well as the individual's perception, influence the response to a stressor (McMillan 2002; Moberg 1985). An individual animal's perception of a stressor is influenced by its genetic makeup, personality, and prior socialization and experience.

If allowed, animals employ coping strategies in order to lessen the negative impacts of a stressor (Carlstead et al. 1993; McMillan 2002). There is marked variability among individuals regarding their ability to cope. Some examples of behavioral coping strategies include hiding, seeking social companionship, and acquiring mental stimulation. Those that are successfully able to cope will suffer less from the physical and mental impacts of stress and will adjust better to life in an animal shelter. That being said, it is rare to find an animal that thrives when housed long term in a shelter.

When animals are housed in shelters, stress frequently originates from the loss of control over conditions and lack of opportunities for engaging in active behavioral responses that would serve as a means of coping. When stress is perceived as inescapable or uncontrollable, the resulting stress response is most severe (Carlstead et al. 1993; McMillan 2002). This is an extremely important consideration when designing housing and husbandry protocols for cats and dogs in shelters.