Heat stress has a detrimental impact on the health and productivity of pigs, resulting in significant financial losses. According to calculations by the University of Minnesota (USA), heat stress costs American pig farmers $900 million a year: $450 million is lost at the fattening stage, and the same amount is lost at reproduction.
Heat stress in pigs largely depends on environmental conditions, the animals' physiological state (pregnancy, lactation), housing density, flooring type, the types and efficiency of ventilation and cooling systems, feeding type, and diet composition, among other factors. Heat stress cases are observed early in summer, even when it is not very hot, and their effects are especially noticeable during the summer months and early autumn. Sows, boars, and fattening livestock are most vulnerable to heat stress (Fig. 1).
Fig. 1. The influence of ambient temperature on the average daily weight gain of fattening pigs. Source: https://www.thepigsite.com/
The impact of heat stress on the body
Pigs are more sensitive to high environmental temperatures than other productive animals because they need to keep their body temperature within a very narrow range (about 39°C) for normal life, regardless of how much heat their bodies produce, and the environmental conditions. The comfortable temperature varies depending on age, sex, and stage of rearing. For example, the productivity of sows decreases at temperatures above 22°C, and that of finishing pigs decreases at temperatures above 25°C (with a relative air humidity of 40-70%) (Fig. 2). For piglets, the optimal temperature for growth and development, depending on their age, ranges from 26 to 35°C.
Pigs have limited physiological capacity for self-cooling. They do not have functional sweat glands in their bodies, but instead have an insulating layer of fat that prevents excess heat from escaping. Thus, pigs, to a greater extent than other animal species, rely on reduced metabolic heat production to maintain body temperature in hot weather. This thermoregulation mechanism is associated with a decrease in feed consumption, which negatively affects growth and reproductive capacity. At the same time, increased humidity reduces the efficiency of evaporation from the skin, further aggravating the negative effects of heat stress.
In addition to affecting appetite, high temperatures negatively impact the functioning of the body's cardiovascular, respiratory, and digestive systems. Oxidative stress develops, which manifests as decreased immunity, liver damage, and muscle tissue degeneration. For fattening pigs, oxidative stress, which develops due to heat stress, is dangerous because it causes a sharp drop in live weight gain, reduces meat quality, and can lead to PSE (pale soft watery meat) or DFD (dark dry tough meat) syndromes.
Fig. 2. Critical temperatures for animals of different weights for average daily feed intake (ADFI) and growth (ADG). Source: https://www.thepigsite.com/
Heart Under Attack
Disorders of the cardiovascular system happen because of changes in the heart and the effect of hyperthermia on the central nervous system. In conditions of body overheating, the tone of blood vessels and their blood flow change, tachycardia and arrhythmia develop, and blood pressure drops. At extreme overheating, blood flow velocity decreases, which is attributed to weakened cardiac function.
Heat stroke occurs when animals are exposed to high temperatures for an extended period. Upon autopsy, the myocardium of such animals reveals arteriovenous hyperemia with transition to stasis, oedema, and expansion of the lymphatic spaces, as well as oedema, swelling, and vacuolar dystrophy of muscle fibres. At room temperatures of 23-25°C, animals' breathing and pulse rates increase, and respiratory alkalosis occurs. Neuromuscular excitability rises, and muscle hypertonicity may occur, leading to convulsions and tetany. Animals may act restlessly or, conversely, appear depressed.
Medication care
There are several effective strategies to help reduce the negative effects of high external temperatures, categorised into technical, technological, nutritional, and medicinal approaches. The latter involves the use of veterinary drugs and feed additives (mono- or multi-component), including acetylsalicylic acid (aspirin).
Acetylsalicylic acid is a non-narcotic analgesic in the salicylate group. It helps lower the animal's body temperature by inhibiting the activity of brain and central nervous system areas that control these processes, as well as the production of heat shock proteins (especially HspB1 and Hsp90), which assist cells in managing stress. In addition, acetylsalicylic acid thins the blood, dilates blood vessels, slows the formation of blood clots in the vascular bed, and minimises the ability of platelets to stick together.
The main mechanism of action of acetylsalicylic acid is the inactivation of the cyclooxygenase enzyme, which results in a decrease in the production of inflammatory mediators: prostaglandins, prostacyclin, and thromboxane A2. Slowing down the synthesis of prostaglandins leads to a decrease in elevated body temperature and decreases the sensitivity of nerve endings to pain mediators. The latter, in particular, has a positive effect on the behavioural signs of heat stress: animals become less excitable, behave more calmly, and consume food better.
Irreversible inhibition of thromboxane A2 synthesis in platelets by acetylsalicylic acid prevents spontaneous and induced platelet aggregation, thereby preventing thrombosis against the background of dehydration and hemoconcentration during heat stress, and also facilitates the work of the heart, which is overloaded due to tachycardia, reduced venous return, and oxygen starvation of the organs. This is especially important for fattening pigs with excess body weight.
Daria Chernai
Clinical Research Specialist at BIOTESTLAB
Acetylsalicylic acid is a simple yet effective agent used in strategies to minimise the negative impact of heat stress in poultry and pig farming. In the product line of the company, BIOTESTLAB is the drug Termonorm (acetylsalicylic acid, 700 mg/g).
Heat stress is one of the most common and serious problems in pig farming, especially during periods of high air temperatures. In this situation, the animal's body cannot cope with the excessive heat load, which can lead to heatstroke.
In addition to preventing heat stress, acetylsalicylic acid is also used in pig farming for:
- pathological conditions accompanied by hyperthermia, inflammatory and pain syndrome of weak and moderate intensity of various origins, including fever;
- respiratory diseases;
- MMA syndrome (metritis-mastitis-agalactia);
- rheumatoid arthritis, osteoarthritis, laminitis, and muscle pain.
Even though acetylsalicylic acid has been known for over 120 years, scientific research continues to open new directions and forms of its use in veterinary medicine, particularly in pig farming. A new study conducted in the United States examined the effect of a liquid form of aspirin (Enhanced Liquid Aspirin, ELA) on piglets after weaning.
Results showed that piglets given ELA had significantly greater body weight gain and better intestinal function parameters than the control group. Notably, there was an increase in villus height and in the expression of proteins that maintain intestinal barrier function1.
Termonorm is administered to pigs starting from the first month of life, except for pregnant and lactating sows, as well as animals with digestive tract diseases and blood clotting disorders. Duration of therapy is 3-5 days.
The drug is available as a powder for preparing a solution, which can be administered with drinking water or directly to animals. The solution is prepared following instructions based on the required amount of water (according to drinking standards), dosage, and the animals' weight. For pigs, the recommended dose is 1.5 kg of Termonorm per 1000 litres of drinking water. During this period, pigs should only receive water containing the drug.
The composition of Termonorm includes sodium bicarbonate and aerosil as auxiliary substances. Sodium bicarbonate speeds up the dissolution of the drug. There is no need to constantly monitor or stir the solution. Due to aerosil, the drug does not cake during the entire shelf life (2 years) and remains stable after opening the package (28 days).
It is important to remember that Termonorm is only part of a strategy to control heat stress. Like any other drug, it should be used alongside other measures to minimise the negative impact of high temperatures on pigs' health and productivity.
1Pearce, S. C., & Kerr, B. J. (2025). Acute enhanced liquid aspirin administration improves performance and intestinal function in nursery pigs. Journal of Animal Science, 103, skaf037. https://doi.org/10.1093/jas/skaf037
Water management in summer
- Each pig should be able to drink as much as it needs. Therefore, make sure the watering system is functioning properly (see Table 1).
- Provide a sufficient number of drinkers (Table 2). Make sure they are in good working order and clean.
- Check the quality of drinking water regularly.
- If necessary, add electrolytes and vitamins E, C to drinking water (remember that vitamin C is stable in aqueous solution for only 6 hours).
- Keep the temperature of drinking water as low as possible (ideally 10°C).
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