Prolonged activation of the stress response over time, with persistently elevated cortisol and ongoing sympathetic nervous system activation.

Unlike acute stress, which is time-limited and resolves once the stressor ends, chronic stress involves stress response systems that remain active over days, weeks, months, or years. The underlying causes are typically environmental or social: inadequate housing, restricted social contact, predictable but unavoidable aversive events, training systems built on prolonged pressure, or a relationship of mistrust between an animal and their handlers. A horse kept in long-term stall confinement without turnout or social contact, a dog living in a chronically noisy boarding facility, or a captive primate in an enclosure that does not support species-typical behaviours are all examples of contexts that commonly produce chronic stress.

Chronic stress carries significant welfare and health costs across species. The well-documented effects include immune suppression (chronically stressed animals are more susceptible to infection and recover more slowly from illness), impaired learning (chronic cortisol elevation affects memory formation and behavioural flexibility), altered behaviour (including the development of stereotypies and other coping behaviours), changes in social behaviour, and in long-term cases reduced longevity.

The welfare science consensus is that chronic stress represents a significant welfare burden and that intervention to reduce chronic stressors is one of the most impactful changes that can be made for animal welfare in captive or domestic contexts.

Recognising chronic stress in animals can be challenging because the obvious behavioural signs (high arousal, reactivity) often fade as the animal moves from acute response toward something closer to learned helplessness or general shutdown. A chronically stressed animal may appear quiet and well-behaved while remaining in a state of significant welfare cost. Physiological measures (cortisol patterns over time, heart rate variability, sleep patterns) are often more reliable indicators than behavioural observation alone.

Chronic stress in animals shares many features with the human concept of chronic stress, and similar interventions tend to help across species: predictable environments, control and agency, social connection with conspecifics, opportunities for species-typical behaviour, and the avoidance of repeated unavoidable aversive events.