Pigs and poultry are particularly sensitive to heat stress as they lack the ability to sweat. Heat stress occurs when the environmental temperature and humidity rises above the point where the animal can release more heat to its surroundings than it is producing. With the hot summer months approaching fast, let’s look at the physiological response triggered by heat stress.

1. Physiological responses to heat stress are controlled by the nervous system, brain and kidneys (Martinez-Mirό et al., 2016; Ross et al., 2017).
2. Stress stimulus triggers the sympathetic nervous system resulting in the release of Catecholamines, such as epinephrine and norepinephrine, from the adrenal gland (Martinez-Mirό et al., 2016).
3. Epinephrine results in increased heart rate, panting, and blood sugar levels, while norepinephrine is responsible for vasoconstriction and rising blood pressure (Calefi et al., 2017; Martinez-Mirό et al., 2016).
4. Continued stress triggers the release of Adrenocorticotrophic hormone (ATCH) from the pituitary gland, corticotropin-releasing hormone from the brain and ultimately the production and release of cortisol from the adrenal gland (Nawaz et al., 2021; Martinez-Mirό et al., 2016).
5. Cortisol is a steroid hormone that increases blood sugar through glucose production, specifically from fat and muscle breakdown. Cortisol also plays a role in the immune system (Campos et al., 2017).
6. Cortisol negatively impacts reproduction as it suppresses luteinizing hormone, ultimately affecting estrogen, progesterone, and testosterone production. The reproductive processes most sensitive to stress are ovulation, expression of sexual behaviour and implantation of the embryo, since they are directly controlled by the neuroendocrine system (Ross et al., 2017; Einarsson et al., 2008).
7. During times of heat stress, the animal reduces feed intake to reduce metabolic heat production. A consequence of this is reduced fat breakdown and increased whole body insulin sensitivity (Fernandez et al., 2015a&b; Pearce, 2011).
8. Reduced blood flow to the gut and organs result in the occurrence of Leaky Gut Syndrome – impaired intestinal integrity allowing pathogens, food and toxins to enter the blood (Ortega & Szabo 2021).
9. Reduced blood and nutrient flow to cells, tissues and organs may result oxidative stress, ultimately impairing the ability of the cells to function (Stewart and Cabezón, 2016).
10. Panting disrupts the bicarbonate buffer system in the blood. This may induce respiratory alkalosis and metabolic acidosis (Stewart and Cabezón, 2016).
11. Heat stress impacts the mRNA expression of genes. Goel et al. (2021) found that the expression of glucose and fatty acid receptors in the gut is decreased, leading to a reduction in the absorption of glucose and fats from the intestine.
12. Heat stress negatively impacts the immune system by suppressing the development of antibodies utilized by the acquired immune system while increasing pro-inflammatory signaling cells (Hirakawa et al., 2020; Goel et al., 2021; Ganesan et al., 2016).

The physiological impact of heat stress on our animals is not purely linked to increased body temperature. Rather, the overall consequences of heat stress are impaired gut integrity, oxidative stress, Acid-Base imbalances, disruption in gut microbiota, neuroendocrine changes, reduced blood flow, energy production, suppressed immunity and impaired reproduction. As a result, the approach to managing heat stress must be an integrated approach involving both nutrition and management.