Thermoregulatory Mechanism in Birds

29.5.1 Behavioural Responses

Birds are homeotherms which can regulate its core body temperature relatively constant within a narrow range of 41-42 °C. The ideal temperature varies between 19 and 22 °C for optimum production performance in layers.

The increase in the core body temperature of birds increases the respiratory rate. The enhanced respiratory fre­quency leads to severe panting and loss of more water from the respiratory tract as evaporation and results in dehydration and respiratory alkalosis. In addition, air sacs in birds facili­tate air circulation on surface to enhance the heat dissipation through evaporation along with more gas exchange. The birds spend more time on resting with their wings elevated, covering the body in the litter materials, increasing drinking and panting frequency with restricted or reduced feeding, moving and walking. The heat dissipation processes are difficult in birds due to their feather coverage trapping heat to prevent hypothermia. The capillary blood circulation is redistributed throughout the body to increase the sensible heat loss during hot condition.

Birds elicit different behavioural responses during heat stress to prevent further heat increment by panting and wing droop which facilitate heat elimination from the core body to the periphery and the surrounding environment. The impairment in the release of hormones during heat stress activates various behavioural changes such as panting, sand

Table 29.1 The physiological response of birds at different ambient temperature

bathing and standing with wings drooped and lifted slightly from the body to enhance heat dissipation. Heat stress evokes various behavioural responses to heat stress such as preening, feather ruffle and pecking among laying hens and broilers. Though, birds respond to heat stress similarly, however depending upon the intensity and duration, the responses exhibited by each bird in a flock may vary. Such variations among individuals and/or breeds indicate supremacy of the individual and/or breed to combat heat stress. For instance, enhanced alert behaviour, feather ruffle and crowing are frequently found in White Leghorn, whereas a relaxed behaviour and preening can be observed in Red Jungle fowl during heat stress. Further, the conditions that create stress to birds such as transportation, stocking density and social interactions are also influence their normal behaviour.

29.5.2 Physiological Adaptability in Birds

High environmental temperature impacts the physiological functions of poultry species which eventually affect the pro­duction performance of birds. Poultry species, like other mammalian animals, can undergo a number of complex and sequential physiological events which enable them to mini­mize the adverse impacts of heat stress and preserve heat homeostasis (Fig.

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Fig. 29.3 Physiological responses to heat stress in poultry. When birds exposed to heat exposure, HPA axis get activated and birds elicit physiological responses to loss heat load through conduction, convec­tion, evaporation and radiation

condition. Under hot and humid environmental conditions, birds maintain their body temperature through heat dissipa­tion via conduction, radiation, convection and evaporation to the surrounding environment. In a thermoneutral environ­ment, birds dissipate heat through non-evaporative cooling, i.e. radiation, conduction and convection. It is also known as sensible heat loss and is energetically efficient mechanism. In brief, birds increase the blood flow to the body surface area such as wattles, comb, shanks and unfeathered areas under wings thereby heat is lost from the body to the surrounding environment through temperature gradient. The amount of heat dissipation through conduction, convection and radia­tion depends on the temperature gradient between the birds and its surrounding environment. At the same time, nutrient digestibility is reduced since the blood flow and energy are diverted away from vital organs to body surface area, thereby lowering the metabolism and heat production. Increased respiratory frequency during heat stress leads to loss of more carbon dioxide and disturbances in acid base balance.

Rectal temperature reflects the core body temperature in birds and it is one of the biomarkers of heat stress since it indicates the thermal balance in birds. Therefore, changes in rectal temperature in response to heat stress are used to evaluate the degree of adaptability of birds to heat stress conditions. Heat stress conditions significantly increase the body temperature in birds which are exposed to high ambient temperature.

When ambient temperature exceeds the thermoneutral zone, birds switch their cooling mechanism to the evapora­tive heat loss. Generally, all animals when exposed to heat stress starts increasing their respiration rate and effectively reduce the heat load by evaporation of moisture from the respiratory tract. This is particularly important for birds since they don’t have sweat glands and must solely depend on heat loss through respiratory evaporative cooling. Birds tend to increase the heat loss across the respiratory tract surfaces by increasing their respiration rate and decreasing the tidal vol­ume (panting) with rapid vibration of gular membranes (known as gular flutter) to the maximum possible level to maintain their body temperature. However, panting requires increased energy for the increased muscular activity. There­fore, reduced energy efficiency also associated with heat stress. Birds have a special anatomical structure called as air sacs which are very useful during panting, as they enhance air circulation on surfaces of respiratory tract contributing to increased gaseous exchange with the surrounding air and consequently, enhancing evaporative loss of heat. Birds have feathers that help them regulate their body temperature. Feathers provide great insulation during cold weather but inhibit heat loss during summer condition. During heat stress condition, birds’ water intake increases in order to equilibrate the body temperature through conductive heat loss when the water warms the body.

29.5.3 Neuroendocrine Response

Neuroendocrine system performs a major role in sustaining homeostasis and normal physiological functions of birds during heat stress. Heat stress activates the sympathoadrenal medullary (SAM) axis to restore and control homeostasis. The sympathetic nerves sense the increase in environmental temperature and communicate the information to the adrenal medulla which enhances the secretion of catecholamines.

Thyroid hormones play a vital role in the thermoregulation of birds. The stimulation of thermoreceptors activates the hypothalamus-pituitary-thyroid axis leading to increase in thyrotropin releasing hormone (TRH). TRH activates the thyrotropes in the anterior pituitary to secrete thyroid stimulating hormone which acts on the thyroid gland and increases the synthesis of thyroxine (T4). T4 is the source of production of biologically active triiodothyronine (T3) by deiodinase enzymes. However, heat stress inhibits the con­version of T4 to T3 and results in reduced T3 concentration that decreases the metabolic heat increment in birds. Further, decrease in TRH results in reduction of T3 and T4 from the thyroid gland which regulates the basal metabolic rate. Therefore, T3 concentration decreases in heat stressed birds due to a decrease in peripheral deiodination of T4 to T3, whereas T4 level is inconsistent. Heat stress also affects secretory pattern of gonadotrophin-releasing hormone in birds which impairs secretion of steroid hormones, proges­terone, testosterone and oestradiol. Additionally, the biological activity of gonadotropin is depressed in layers during heat stress. Therefore, the neuroendocrine changes are responsible for decrease in growth rate and reproductive performances in heat stressed birds.

29.5.4 Blood Biochemical Response

Heat stress severely affects the metabolic status and physio­logical equilibrium in birds that leads to health problems and high mortality.

The increase in respiration rate during heat stress results in respiratory alkalosis which disrupts the acid base balance and leads to increase blood pH in association with reduced pCO₂. Respiratory alkalosis is associated with decreased growth rate in broilers. Whereas, the metabolic alkalosis occurs when there is a disturbance in the fixed acids and bases in the extracellular fluid. The blood HCO₃ level decreases in panting birds under acute heat stress and more expulsion of CO₂ due to elevated respiration rate helps to bring down the body temperature. The higher level of dehydration that occurs due to more water excretion in faces and urine causes further disturbances in acids base balance.

Heat stress modifies the electrolyte balance which is vital for the maintenance of acid base balance, cellular homeosta­sis, synthesis of tissue protein, electrical potential of cell membranes, enzymatic reactions and osmotic pressure. How­ever, the enhanced excretion of fluid in the urine with more concentration of electrolytes and the disproportion of dietary Na, K and Ca may lead to metabolic alkalosis with increased blood pH and HCO₃. Heat stress impairs function of immune system of birds and enhances the disease vulnerability of birds and leads to high mortality and morbidity.

29.5.5 Metabolic Response

As an adaptive response, birds significantly reduce their voluntary feed intake, digestibility and nutrient utilization to maintain constant body temperature during both acute and chronic heat stresses. This response is considered as the main adaptive mechanism to reduce metabolic heat production (Fig. 29.4). Liver, the important metabolic organ plays a significant role during heat stress condition to maintain the homeostasis by regulating the metabolism of carbohydrate, protein and lipid, thereby controlling the plasma levels of metabolites. However, heat stress impairs the activity of liver, thereby altering the plasma metabolite balance. Proteins, especially amino acids are very essential for growth and concentration of amino acids in the body also regulate feed intake. Heat stress was reported to decrease the amino acids in plasma and tissues of chicken during prolonged heat expo­sure. The altered free amino acids in various tissues like brain and skeletal muscle vary from that in plasma. Similarly, all the altered free amino acids in the various parts of the brain, except for proline and cystathionine are different which indi­cate that alterations in free amino acid contents due to heat stress may be tissue-specific, which is in agreement with the fact that enzymatic activity related to protein metabolism and protein synthesis is tissue-specific.

Glucose is very much essential nutrient since it is consid­ered as a major source of energy for all the tissue, particularly for the brain. Glucose can be converted into a number of intermediate metabolites and is used in synthesis reactions, or utilized for the production of ATP. During stressful condi­tion, liver can generate glucose either by glycogenolysis (from stored glycogen) or gluconeogenesis. Constant supply

Fig. 29.4 Metabolic responses to heat stress in poultry. Heat stress causes decline of feed intake in birds to reduce metabolic heat production. Heat stress also reduces protein and fatty acid synthesis; enhances glycogenolysis and gluconeogenesis in order to provide energy to birds

of energy is crucial to tackle heat stress. Elevated environ­mental temperature increases the rate of glycogenolysis and gluconeogenesis in liver of heat stressed birds. Further, it is essential to sustain the absorption of glucose from intestinal lumen. Glucose transportation in epithelial and non-epithelial cells is carried out by sodium dependant glucose transporter (SGLT) and glucose transporter (GLUT) systems, respec­tively. Heat stress decreases the uptake of glucose from digestive tract. The reduced levels of glucose in serum of heat stressed birds indicate that the birds were in negative energy balance. Research data suggested that many of the metabolites involved in glucose metabolism have been increased in response to heat stress such as glucose 6 phos­phate (G6P) and glucose. The activities of metabolic enzymes involved in glucose metabolic pathways are also increased during heat stress exposure. Phosphoglucomutase, an enzyme that converts glucose 1 phosphate to G6P was significantly increased during exposure of birds to elevated temperature. It was reported that heat stress enhances the gluconeogenesis in liver which was indicated by the higher levels of fructose-6-phosphate (F6P) and fructose­bisphosphatase 2, the enzyme that transform F6P to G6P.

Amino acids are considered as a main source of energy for the liver. In birds, body protein deposition is reduced under elevated temperature condition. Heat stress intensity and duration affect the protein metabolism differently. Acute heat stress depresses the protein synthesis, lowers the protein deposition and increases the protein catabolism in heat stressed animals. Acute heat stress alters the concentration metabolites in plasma, many of which are involved in protein metabolism specifically glycine, serine, threonine, arginine, proline, phenylalanine, cysteine and methionine metabolism in broiler chicken. However, chronic heat stress decreases protein synthesis, catabolism and levels of certain amino acids in plasma. Most of the amino acids are reduced during heat stress condition in chicken except cysteine. The increased serum uric acid levels in heat stressed broiler chicken suggest that the protein might be degraded for energy supply which is supported by the increased levels of some amino acids such as proline, L-cysteine, methionine and threonine in heat stressed birds even though feed intake had been reduced significantly. However, the concentration of glycine decreased in heat stressed birds possibly due to increased uric acid metabolism since glycine is required for uric acid synthesis. Non-protein amino acids such as orni­thine and citrulline levels also increased in heat stressed birds that indicate its role in stress relief and thermoregulation.

Lipids are stored in adipose tissue as triglycerides. Triglycerides when broken down metabolically and release free fatty acids can be utilized for various purposes. Fatty acids like as stearic acid, arachidonic acid, palmitic acid, linoleic acid and oleic acid concentration are declined in birds exposed to heat stress. However, certain fatty acids (like myristate, myristoleate, and palmitoleate) and enzymes involved in fatty acid synthesis were elevated during heat stress exposure in chicken. Acetyl-CoA carboxylase alpha enzyme that converts acetyl-CoA to malonyl-CoA; Acyl- CoA synthetase, which converts myristate to myristoyl- CoA and palmitate to palmitoyl-CoA were increased in heat stress birds (Jastrebski et al. 2017). Concentration of non-esterified fatty acids (NEFA) in plasma also decreased in response to high environmental temperature due to reduc­tion of lipolytic enzyme activity and lipolysis in heat stressed animals. Heat stress significantly influences the TCA cycle since the concentration of α-ketoglutaric acid (intermediate product of TCA cycle) and pyruvate (end product of glycol­ysis) increased in heat stressed birds.

29.5.6 Cellular and Molecular Response

Heat Shock Proteins (HSPs) are highly conserved molecular chaperones well known for their roles as stress response proteins that helps in protein folding and unfolding, assembly of multi-protein complexes, transport and sorting of proteins into subcellular compartments, minimization of protein aggregation and protection of cells against apoptosis. The HSPs are controlled at the transcription level in all organisms however stress factors activate a specific heat shock tran­scription factor (HSF) mainly HSF-1. Further, accumulation of denaturated proteins in the cytosol stimulates HSF-1 as a response to stress. The phosphorylation favours the trimer formation of phosphorylated HSF-1 which enters into the nucleus and binds to promoter region (heat shock element) and enhances HSP gene expression. The increased produc­tion of HSPs selectively binds to the degenerated proteins or aggregated proteins and newly synthesized polypeptides. However, formation of HSP-HSF complex decreases HSF-1 production in negative feedback regulatory mechanism to maintain the internal homeostasis. There are many heat shock protein families which are organized based on their molecular weights as HSP110, HSP90, HSP70, HSP60, small molecule HSPs and ubiquitin. Heat stress activates many genes of HSPs families where the higher expression of Hsp70 promotes lipogenesis and heat tolerance HSPs are essential in protecting and repairing cells and tissues during stress. The production of HSPs is highly essential to restore the survival of stressed cells and maintenance of the internal environment. The exposure of birds to heat stress enhances HSP70 expression which has a wide range of chaperonic activities. HSP70 plays a vital role in cellular protection during hot condition stress and develops the thermotolerance in birds. These chaperones help in maintaining and sustaining the native structure of proteins by protection and stabilization of stress labile proteins, renaturation of denaturated proteins and preventing protein aggregation. They are responsible for refolding of misfolded and aggregated protein.

Genome-wide transcriptomic studies have revealed many pathways that are modified during heat stress in broilers. The pathway analyses determined a wide range of affected cellu­lar responses such as apoptosis, cell cycle, DNA repair, membrane trafficking and immune function. Heat stress activates the genes that are involved in lipid metabolism and increased numerous genes encode enzymes involved in different phases of fatty acid synthesis in birds. Heat stress stimulates the expression of NADPH oxidases and enhances generation of superoxide and other reactive oxygen species (ROS) by transferring electrons across cell membranes. ROS are continually produced in vivo due to metabolic activities and increased significantly during heat stress. The free radicals affect the cellular metabolism by altering the lipid peroxidation reactions compromising biological membrane integrity and functions. The higher level of free radicals damages DNA and proteins and makes them more suscepti­ble to oxidative damage. Further, gene ontology analyses unveiled many pathways are affected during heat stress which were associated with ion channel activities, redox balance and lipid metabolism. The network analyses also determined the interactions of many of these genes with heat shock proteins (HSPs) that regulate heat stress responses.

29.5.7 Biomarkers for Heat Stress in Poultry Birds

The biomarkers are the indices used to assess animals/birds or may be also considered as biological measures of a biological state. Heat stress affects the health, welfare and productive performances of birds (Fig. 29.5). The reduction in glucose and reduction in proteins, calcium, phosphorus and alkaline phosphatase concentrations are some of the biochemical biomarkers in birds during heat stress. Choles­terol content is elevated in liver, breast and thigh muscles of heat stressed birds. Further, creatine kinase and lactate dehy­drogenase are more consistent isozyme in a tissue or organ activity which was increased in heat stress and indicating muscle damage or myopathy. The heterophil to lymphocyte ratio is one of a reliable stress estimator in birds which increases due to environmental or heat stress.

Fig. 29.5 Biomarkers used to determine the heat stress and health status in birds. Biomarkers are biological measures of a biological state. Heat stress affects the health, welfare and productive performances of birds. The changes in the physiological, metabolic process and cellular activities are expressed as intermediate and end metabolites

Further, heat stress stimulates the HPA axis and increases the release of glucocorticoids particularly corticosterone which is an indicator of stress in birds. The higher level of glucocorticoids during heat stress causes a quick influx of heterophils into the blood from bone marrow that increased the concentration of circulating heterophils. Thyroid activities are reduced during heat stress to prevent increment and results in reduced concentration of T3 in birds. The HSPs are important for protecting cells and protein folding and formation during heat stress. The abundance of relative expression mRNA of various HSPs particularly HSP70 has been closely associated with heat stress in birds. Further, heat stress enhances the production of mitochondrial ROS and decreases avian uncoupling protein. The imbalance between ROS and antioxidants in heat stressed broilers results in oxidative stress. Malondialdehyde is a biomarker used to estimate oxidative stress in poultry by indirectly quantifying the level of peroxidation due to ROS. Additionally, the pri­mary redox pathways; superoxide dismutase, catalase and glutathione peroxidase, significantly decrease in heat stressed birds due to higher production of ROS. The lipid peroxida­tion is a major consequence of heat stress where protein carbonyl, MDA, 8-hydroxy-2^,-deoxyguanosine and advanced glycation end products are considered as biomarkers of protein, lipid, DNA and carbohydrate oxida­tion in birds.

29.6