What happens when pigs experience stress from the environment
Stress is the sum of non-specific response reactions produced by an organism to various extraordinary stimuli. Environmental factors that can cause stress responses in the body are called stressors or stress factors.
After pigs are stimulated by environmental factors, they all produce reactions, but the forms of reaction differ. General stimuli cause specific reactions in the body, such as pigs huddling together when it is cold in winter, and dispersing when it is hot in summer. However, some stimuli (stressors) not only cause specific reactions in the body but also cause non-specific reactions, namely stress responses, which manifest as: ① thickening of the adrenal cortex and increased secretory activity; ② atrophy of the thymus, spleen and other lymphoid tissues, decrease in blood eosinophils and lymphocytes, and increase in neutrophils; ③ hemorrhagic ulcers of the stomach and duodenum. These symptoms are called "general adaptation syndrome (GAS)". Therefore, stress is not an environmental stimulus, but a response of the body.
What is the significance of an animal body producing reactions to stimuli? As discussed in the previous section, the internal environment of higher animals is relatively stable. The specific reactions and non-specific stress responses produced by the body are all for keeping its internal environment relatively stable and enhancing its ability to adapt to environmental changes. The stress response is a physiological response, and its role lies in mobilizing all the body's defense systems to overcome the strong stimulus effects that specific reactions cannot adapt to, thereby expanding the body's range of adaptation and enhancing its ability to adapt to the environment. If the body lacks a stress response or has stress dysregulation, then under any stimulus beyond the range of general physiological regulation (specific reactions), it will lead to disruption of the body's homeostasis (disease) or death. Therefore, stress is a response necessary for the survival of pigs and can expand the range of adaptation of animals.
The occurrence and development of stress can be divided into three stages under typical conditions.
1. The alarm reaction or mobilization stage is the early response of the body to the action of a stressor, in which the GAS response appears. Catabolism predominates, the pig becomes emaciated, a negative nitrogen and carbon balance appears, and the body's resistance is lower than normal. This stage generally lasts 6-48 h. If the stress factor is very strong, the pig may die within 1 h or 1 day; if the body overcomes the influence of the stress factor and survives, it then enters the adaptation stage.
2. Adaptation or resistance stage. The body overcomes the action of the stressor and obtains adaptation. Contrary to the mobilization stage, it manifests as the body's metabolism returning to normal, anabolism predominating, physical condition recovering, resistance rising above normal levels, and at the same time resistance to other stimuli also increasing. This stage generally lasts several hours, several days or several weeks. If the stressor stops acting or its action weakens, and the body overcomes the adverse effects of the stimulus, then stress ends at this stage; in most cases stress only proceeds through the above two stages, but if the stressor continues to act or the body cannot overcome this action, then it enters the exhaustion stage.
3. Exhaustion stage. This stage appears very similar to the alarm stage, but the degree of reaction increases sharply. Although the adrenal cortex is hypertrophied, it cannot produce the necessary hormones. Tissue proteins, fats and other body reserves are decomposed, body weight drops sharply, the acquired resistance is lost again, adaptive function is destroyed, the functions of various systems become disordered, and finally exhaustion leads to death.
When GAS appears during the stress process, livestock productivity, product quality, resistance and feed utilization all decrease. This is because at this time the body mobilizes its whole defense apparatus and energy to resist the stimulation of adverse environmental factors and maintain its life activities, while production consumes the body's nutrients and energy and is unfavorable to maintaining life. Therefore, production activities decrease or stop. Upon entering the adaptation stage, not only does resistance increase, but physiological functions and productivity levels are also restored and compensated. It can thus be seen that the stress response is a reasonable state of special physiological adaptation, and only when the stress response cannot overcome adverse stimuli does it lead to an irreversible exhaustion state. It can be seen that, for the pig itself and its survival, the stress response expands the pig's range of adaptation.
In livestock production, mild stress can improve livestock resistance, productivity and feed utilization, but attention should be paid to controlling the intensity and duration of the action of stressors so that they stop at the adaptation stage and exhaustion reactions are prevented.