As in previous posts, allostasis refers to the body’s dynamic process towards maintaining homeostasis. When an animal faces various stressors – physical, emotional, and environmental – its body ramps up hormones, heart rate, and other physiological responses to meet the demand. This is the body’s way of preparing the animal to handle a situation – and in the short-term, it’s a survival mechanism and that’s good thing.

While allostasis is essential for survival, when the body is faced with constantly having to adapt to stressors, the ‘load’ of that allostatic process can take a toll. The term ‘Allostatic Load’, introduced by Bruce McEwen and Eliot Stellar in 1993, refers to the cost of this adaptation. It’s the cumulative burden of ‘wear and tear’ on the body from repeated or chronic stress responses.

Bruce McEwen and John Wingfield went further and proposed two distinct types of allostatic load – Type 1 and Type 2 – offering valuable insights into how an animal’s body can respond to stress. Type 1 occurs when the body faces a situation requiring more energy than it has available (starvation or extreme illness). In response, the body prioritises survival and goes into emergency mode. This reduces the allostatic load in the short term by shutting down non-essential functions and focusing on survival, allowing the animal to conserve energy and hopefully find a way to replenish its reserves. Once the crisis passes and energy levels recover, the body can return to normal function.

Type 2 occurs when the body has enough energy but faces constant stressors (like those stressors experienced by animals in captivity). Unlike Type 1, the body doesn’t go into emergency mode, and it doesn’t trigger the ‘escape’ response. Instead, it triggers the secretion of glucocorticosteroids along with other mediators from the nervous system, immune system, and brain. If this process continues for a prolonged period, the body’s ability to adapt weakens, leading to long-term damage and potential health problems like Pituitary Pars Intermedia Dysfunction (PPID), and other metabolic and systemic issues.

The only way to improve the animal’s ability to adapt and cope with Type 2 allostatic load is to remove the stressor, or to at least reduce the stressor to a level where the animal can adapt to it.

The concept of allostatic load highlights a significant challenge when considering our animal patients. Wild animals in their natural habitats, have more control over their environment – choosing when and where to access food, water, exercise, and social interactions. This allows them to manage stress and potentially trigger the “emergency/survival” response (Type 1) during extreme situations, conserving energy for survival. However, in our domesticated animal patients, this natural stress management is often disrupted. Owners control their access to food, water, exercise, and social interactions, all of which have the potential to become chronic stressors if not managed well.

Allostatic loading will accumulate over time, and whilst some aspects may have been treated or removed, some will have lasting physiological, psychological, or mechanical effects. As an osteopath it is important to consider the history of the animal; their stressors over time; and their current environment and influencing factors in our analysis of the animal and in the approach to their health.

Chronic exposure to stressors can also interfere with the normal ‘set point’ for stress responses – recognised as an alteration of the set point in a feedback loop. When this happens, the feedback loop doesn’t recognise the elevated levels of stress as being ‘abnormal’, because the body has not been given the opportunity to reset itself to the normal parameters. At this point, even small triggers can be enough to overload the animal because the set point of what the animal can deal with is already elevated. An example of this in humans is an increase in blood pressure. When blood pressure stays high for a prolonged period, the body’s regulatory systems may adapt by resetting their reference point for what’s considered “normal.” This means the body no longer recognises the higher blood pressure as abnormal and doesn’t attempt to bring it down to the original, healthier level. This resetting of the baseline leads to the maintenance of high blood pressure, which can have significant negative consequences for health.

Managing and reducing/removing stressors is crucial to prevent high allostatic loading and overload. By educating owners and developing proactive strategies to provide animals with more control over their surroundings; opportunities for exercise; and social interaction (think Five Freedoms), and minimising as many stressors from their daily life as possible, we can help them manage their environment more effectively and prevent the detrimental effects of chronic allostatic load.