The pork market is influenced by external factors to production that modify the price of pork. Pig farm efficiency must guarantee maximum yields to reduce production costs. These costs are related to the cost of inputs and feed, the costs associated with treatments and diseases, and even the amount of meat imported into each country.

For this reason, environmental stress is a condition that affects the productive efficiency of swine. The environment where pigs grow is composed of a series of factors that must be maintained in balance to guarantee animal welfare and productive efficiency in swine. Otherwise, alterations in that environment trigger the physiological stress response, which has diverse effects that, if not controlled in time, can negatively affect the pig herd.

Stress in swine: definition and stages of development

Stress is a physiological response of animals to events that represent a risk or danger to their life. This condition can be initiated by external stimuli or environmental changes that generate a response cascade in the nervous, immune, and endocrine systems that seek to generate responses to these stimuli to protect life. Stress responses pass through the hypothalamic-pituitary-adrenal axis. Thus, it has been described that stress in animals has three developmental phases:

Alarm phase

The alarm phase of stress is the first reaction that occurs when animals are in danger or perceive a notable change in the environment. This response seeks to adapt the animal to the change through a series of physiological responses. The alarm phase is characterized by its short duration and instantaneous beginning.

Animals in the alarm phase of stress have increased heart and respiratory rates to improve oxygenation of high activity zones such as the musculoskeletal system. In addition, there is increased efficiency of cardiac output and mobilization of immune system cells. On the other hand, changes such as rapid eye movements, vocalizations, nervous body movement, pallor of the mucous membranes, and muscular rigidity can be seen in swine. These body changes are produced through a cascade of reactions mediated by hormones such as cortisol or adrenaline produced in the adrenal gland of swine.

Resistance phase

During the resistance phase, the stressor stimulus is maintained, and the animal must adapt to the new conditions to find balance. For this reason, stress is sometimes considered necessary in order to adapt to environmental conditions. In the resistance phase the hormonal cascade has slowed down, so cortisol and adrenaline have decreased. Therefore, the signs observed in swine during the alarm phase have disappeared.

Exhaustion phase

In the exhaustion phase of stress, the swine have depleted the capacity to adapt to environmental stimuli, which is why this phase is also called distress. The exhaustion of the animals’ adaptive response occurs because the stimulus is prolonged in time or very intense.

Some of the signs observed in swine during the distress phase are tremors, decreased weight gain, behavioral alterations, stereotyped behaviors, hyperventilation and tachypnea, dry mucous membranes, and weakness. In addition, some gastrointestinal signs observed are correlated with increased diarrhea and generalized inflammation.

In this phase of stress, animals have serious effects on their organs, on their productive efficiency, and even generate predisposition to diseases and increased mortality.

Environmental stress in pigs: causes and consequences

Air quality

Air is a key factor in the environment in which swine live. Elements that remain in the air such as bacteria, pollutants, and dust can affect respiratory function and the local immune system. In this way, poor air quality can cause stress on the respiratory system which predisposes to the entry of microorganisms.

These pollutants cause irritation and inflammation that damages the respiratory system’s defense mechanisms. If these noxious elements remain over time or are intense, respiratory distress occurs, which also affects the animals at the metabolic level.

Two of the most important air pollutants are ammonia and hydrogen sulfide, which are produced by swine excretions. These particles are harmful to both animals and farm workers; in the case of hydrogen sulfide, it is highly harmful if the amounts exceed 1000 ppm. In the case of ammonia, amounts above 50ppm already affect the respiratory system of pigs. This stress generated by air pollutants decreases weight gain in pigs and affects piglet growth.

Feeding

The nutrients provided in the diet of pigs must meet their needs according to their productive stage (piglets, pregnant sows, rearing, growing). Therefore, each group of animals has different nutritional needs that must be supplied through the feed. Otherwise, the animals can get into a state of stress due to insufficient nutrients or, in more severe cases, a catabolic state. Thus, animals begin to expend body reserves for metabolic maintenance, which decreases weight gain, compromises gestation, and increases malnutrition stress.

Population density, noise, and management

The number of pigs that remain in each production space (density) is related to stress, as the higher the density, the more uncomfortable the animals feel due to lack of movement, increased temperature, and environmental pollutants. This density management is a critical point since it can predispose to behavioral alterations, episodes of aggressiveness, or excessive heat, which increases stress in the animals.

On the other hand, stressful stimuli associated with noise also affect swine significantly. Excessive environmental noise increases stress as animals perceive these as potential threats.

Likewise, handling procedures (dehiding, dehorning, treatments, vaccination, herd changes, mobilization) cause stress because they generate unfamiliar stimuli for the animals and in some cases temporary pain.

Temperature

The temperature in swine is a critical factor as they are susceptible to heat stress or thermal stress. Pigs have a deficient system to eliminate heat due to their small number of sweat glands, so they eliminate heat by other mechanisms such as conduction thanks to the permanent contact with the ground or radiation eliminating heat from the environment. In cases of elevated temperatures, pigs suffer from excessive heat, which triggers stressor mechanisms and can lead to death if this stimulus is not stopped, entering a state of thermal shock. (Table 1).

Heat stress depends on environmental temperature and humidity. For this reason, it is closely correlated with herd management, since the higher the stocking density, the more this problem increases. In addition, ventilation, and the production system (open or closed) are key factors in the control of environmental temperature.

Brightness

Sows are susceptible to the effect of light on the farm. Sows accustomed to closed and controlled production systems must have precise hours of light and darkness to avoid behavioral changes.

In cases where brightness has abrupt or unexpected changes, the stress response is triggered. In the case of pregnant sows, this seriously affects the viability of the gestation.

Rearing stage

Piglets entering the rearing stage stop suckling and must switch to feeding, which is a stressful event during the pigs’ lifetime. This represents a challenge at the immune and gastrointestinal system levels since the changes generated can cause disease processes.

Criteria for assessing animal welfare in pigs

Stress has been used as an indicator to evaluate animal welfare in livestock production. Therefore, some criteria allow to know the animal welfare on a swine farm and to evaluate if the stress levels in a group of animals are significant and are affecting production.

Behavior

Behavior is a measure at the ethological level that allows the evaluation of pigs’ welfare. To determine the status of this, behaviors such as sudden paralysis, attempts to escape from pens, changes in feeding or water consumption, alterations in posture or movement patterns are reviewed. In addition, in a clinical examination, the respiratory rate, heart rate, temperature, and condition of the skin and extremities can be completed to rule out changes. On the other hand, it is important to determine if vocalizations or episodes of aggression are increased.

Morbidity and mortality

Stress in swine can predispose to increased incidence of disease or mortality. Prolonged stress states have an immunosuppressive effect that is the gateway for infectious diseases in any system such as the respiratory or digestive. In addition, other non-infectious diseases such as lameness, ulcers, skin lesions, and bruises can also increase if there is stress in the herd. On-farm examinations should determine these changes to decrease morbidity and mortality associated with acute or chronic stress conditions.

Among the causes of morbidity, lameness, and associated musculoskeletal problems can be genetic, sleep and house hygiene conditions, and even diet. These lameness and alterations generate pain in the animals, which increases stress, which is a vicious circle because the higher the pain, the stress continues to increase, weakening the hooves and affecting the healing processes.

Reproductive efficiency

Stress states significantly affect pregnant sows as well as piglets. Thus, changes generated by this stressful condition can decrease conception rates, the number of piglets born alive, and milk production. On the other hand, factors such as stillbirths, mummies, abortions, and embryo loss may be increased under stressful conditions. Therefore, evaluating all reproductive parameters is a good indication of the state of animal welfare on a farm.

Measurement of physiological parameters

In the evaluation of animal welfare, some parameters are evaluated to determine the degree of stress in production animals. These parameters are correlated with hormones that are released during moments of stress such as ACTH, which stimulates the secretion of cortisol that generates physiological responses in stressed animals.

The precise method to evaluate stress in pigs is through serological tests in saliva where cortisol is measured. In unstressed pigs, the level of cortisol is 2 ng/mL in saliva, while when undergoing stressful processes such as weighing it can reach 6 ng/mL. In the evaluation of this hormone in blood (a more invasive and stressful method by itself) values can reach 70 ng/mL in pigs before slaughter (Martin, 2013). For this reason, hormone evaluation is an indication of the welfare status of the animals.

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Prevention strategies against stress in swine

• Control excess humidity in the litter or floor of swine production to avoid foot lesions, reduce air pollution by ammonia and ensure animal welfare.
• Evaluate stocking density and establish an adequate range for swine flocks, which reduces heat stress, overcrowding and avoids aggressions.
• Use of Alquernat Yucca which is a natural supplement that reduces ammonia concentration. This product, developed by Biovet S.A., is composed of natural extracts based on yucca and combines the effect of saponins, cimenol ring, and glycocomponents. These compounds select and control bacterial populations, improve intestinal absorption and decrease the production of ureases. Therefore, Alquernat Yucca reduces ammonia air pollution by up to 75%, according to field tests. In this way, the product has a positive effect on animal welfare, reduces stress, and improves production performance.
• Install ventilation systems that help circulate the air, which reduces temperature and prevents heat stress, as well as avoids the accumulation of microorganisms in the air.
• Administer Alquernat Immuplus, a product developed by Biovet S.A., that strengthens the immune system in pigs thanks to the plant extracts it contains. Stress can affect the immune system of animals predisposing them to diseases due to the immunosuppressive effect of cortisol. This product strengthens innate and specific immunity, being useful in young animals with an immature immune system against challenges by pathogens. In addition, the use of Alquernat Immuplus improves the response to vaccination and pharmacological treatments which are stressful events, thus increasing their efficacy.
• Monitoring morbidity and mortality rates to identify prevalent diseases in the herd are detrimental to animal welfare.

Conclusions

Stress is a physiological response of animals to stimuli that may represent a threat to their life. However, when this condition is prolonged in time or is very intense, it can have harmful effects on swine.

There are three phases of stress described that generate different responses at the physiological level. The first phase is the alarm phase where nervous and hormonal responses are triggered to respond instantly to the stimulus. Then, the resistance phase generates an adaptation of the swine to the environment. Finally, when stress is maintained, the exhaustion or distress phase is generated, where the pathological effects start and affect production.

Environmental stress can be caused by distinct factors such as air quality, feed, stocking density, noise, management, temperature, and luminosity. There are criteria to evaluate the level of stress and animal welfare in the pig farm through the evaluation of behavior, morbidity and mortality, and reproductive efficiency.

Finally, there are strategies to prevent environmental stress in swine by using ventilation systems, improving hygiene conditions, administering pronutrients, and evaluating the health status of the farm.