Explainer: What Is Heat Stress?

Rising global temperatures could result in more frequent and intense heat waves, which, in turn, could adversely affect humans and animals due to acute heat stress, resulting in casualties, illness, and decreased productivity and fitness, especially in animals.

Increasing temperatures also have profound effects on endocrine responses in animals, disrupting critical hormonal balance, leading to physiological and behavioural changes.

What is heat stress?

Heat stress occurs when an animal’s body temperature exceeds its ability to dissipate or release heat. This leads to physiological strain and related health consequences. Severe and acute heat stress can result in heat exhaustion, heat stroke, and even death, if left untreated. It can be caused by a combination of factors including high temperatures, excessive humidity, and prolonged exposure to heat without adequate cooling mechanisms.

In animals, including humans, heat stress disrupts thermoregulation, which is the body’s ability to maintain its internal temperature despite a change in the external environment. This leads to dehydration, increased heart rate, and excessive sweating or panting. When the body’s heat load exceeds its capacity to cool through inherent mechanisms — such as sweating, radiation, or convection — the core body temperature rises, which can impair cellular function and organ systems.

Chronic exposure to high temperatures — a looming reality in the current climate scenario — can alter endocrine and metabolic functions, impacting overall health, reproductive performance, and productivity.

How does heat stress affect animal hormones? 

Studies have found that climate change-induced heat stress and seasonal variations significantly affect hormone production and regulation in cattle, horses, goats, macaques, birds, and rodents. Extreme temperatures cause fluctuations in hormones essential for body functions, such as cortisol, aldosterone, thyroxine, triiodothyronine, prolactin, estradiol, testosterone, and gonadotropins.

In dairy cattle, heat stress has been found to cause increased cortisol levels and insulin, and reduced thyroid hormone concentrations. Cows also show disrupted endocrine balance and metabolism, which affect the reproductive, renal, and circulatory systems. Behavioural changes include reduced feeding, lower productivity and milk yield, and increased respiration rate. In sows, high temperatures were found to impact the HPA axis, which is the link between the brain and adrenal glands that controls stress responses and hormone release, causing ‘summer infertility’ or a drop in fertility during hot months.

A study looking at the effects of heat stress on livestock in India found prolonged presence of elevated catecholamines and the hormones epinephrine and norepinephrine responsible for the ‘fight or flight’ response. Prolonged elevated levels of these hormones can result in irregular heartbeat, high blood pressure, and other complications in the body. Yet another study conducted on farm animals recorded similar observations, including decreased growth and milk yield due to a stressed pituitary gland.

Diminished thyroid function was also observed in rabbits. Chronically low thyroid function in mammals can result in body weight fluctuations, intolerance to cold, and fatigue. Goats were also found to show impaired thyroid function and metabolism, and degeneration of liver and endocrine tissue due to over-activation of the HPA axis due to excessive heat.

Rats have also been found to exhibit temperature-related symptoms of stress and anxiety at higher temperatures.

Polar bears show significantly high traces of cortisol in their faeces that correlated with thermoregulatory issues and physiological stress. Shrinking food and habitat due to climate change add to the stress on polar bears, affecting their body condition and survival of cubs.

Some goats also show elevated cortisol levels in response to heat stress, while others show climate-change related weight fluctuations. A study on Apennine chamois, a species of goat-antelope, found increased aggression in the group-living herbivores. Competition and aggressive behaviour intensified with rising temperatures and decreasing rainfall, highlighting how warming and droughts can cause resource-related aggression. Another study conducted on Alpine chamois yearlings in the Swiss Alps found that higher than normal temperatures during birth and suckling periods resulted in the young ungulates being underweight by about 3 kgs. Other goats show impaired thyroid function and metabolism, and degeneration of liver and endocrine tissue due to over-activation of the HPA axis due to excessive heat.

Impacts of heat stress on the endocrine systems of marine animals are not as straightforward to assess as studying terrestrial mammals. But some studies have found that fish too experience elevated cortisol levels in warmer waters, which can impair immunity and overall health. Climate change and warming-related ocean acidification and hypoxia trigger stress in marine life like rockfish, resulting in peaking cortisol levels and physiological stress. Increasing sea surface temperatures and related phenomena were also correlated as stressors in baleen whales, showing up as cortisol concentrations in their earwax.

How else does heat affect the animal body?

The first response to heat stress is the activation of the HPA axis – a communication system between the hypothalamus, pituitary gland, and adrenal glands which helps with stress management. The response also includes a rise in cortisol to initiate counter-heat measures. However, heat also affects other physiological systems due to the excessive activation of the HPA axis.

Studies indicate that excessive heat impacts blood characteristics in dogs (lower white blood cell counts and haemoglobin) and protein expression in cattle.

It also affects the reproductive system of animals in multiple ways – disrupting hormones, influencing offspring sex determination, causing deformities, metabolic issues in young animals, and reducing health and immunity.

Male Wister rats exposed to chronic heat exhibit oxidative damage in testicular tissues, reduced testosterone production, and impaired fertility. Bama miniature pigs also show testicular damage and disrupted sperm production due to oxidative stress.

Several studies have found an association between higher temperatures and violent behaviour among people. Dogs too exhibit a similar trend in aggression, with warmer temperatures increasing their irritability and the likelihood of biting people.

Mitigating and managing heat stress for animals has several overlaps with the strategies advised for humans. Recommendations for livestock include housing fewer animals together, providing high quality feed and ample water, using sprinklers and misters to cool the area, providing cool shade, and improving ventilation. Some studies have explored the potential of red grape juice and dietary supplements in combating the oxidative stress caused due to heat in animals.

Propionate supplements, vitamin D3 and calcium, and niacin were also found to be helpful in managing the endocrine and physiological impacts of heat stress in some cows. The poultry industry supplements fats, yeast, antioxidants, and electrolytes to manage heat stress in affected poultry. However, prevention – an increasingly difficult challenge – remains the most effective strategy.

Heat waves have increased in intensity and frequency, and are projected to get worse, hinting at possibly hotter and drier conditions that would add to the stress directly or indirectly. The effects of heat stress on reproduction, endocrine functioning, and health are extensive and require further study to develop methods to mitigate and manage the impacts on the animal, especially in the wild. Forming informed context- and landscape-relevant management strategies could be key to effectively adapting to a rapidly warming world.

(Published under Creative Commons from Mongabay India. Read the original article here)