B.V. Sunil Kumar, Astha Sharma and Simrinder Singh Sodhi
The condition where there could be an increase in body temperature of an organism which further causes a lot of physiological responses and changes like sweating, panting due to external and environmental factors and forces is defined as heat stress. Such an effect of heat stress has been studied in different organisms. Even laboratory animals like rabbits when exposed to heat stress, showed changes in myocardia in order to adapt to heat stress by inducing myocardial stress protein in order to resist myocardial infarction. Human beings also experience the negative effects of heat stress which can cause health problems like heat stroke that affects their day to day life, work life or can even cause death. Heat stress affects the economy causing a slow pace work mode and affecting medical, animal and agriculture sector as well. It also affects infrastructure.
Poultry industry in India
In India, as compared to beef, poultry meat production and sales have shown tremendous success in the past two decades. The pattern of consumption of food in India has shown a shift from vegetarian to non-vegetarian which has also increased the demand and supply of poultry products. India has high levels of productivity in both layer and broiler type of poultry. Punjab, Karnataka, Gujarat, Tamil Nadu, Andhra Pradesh, Haryana and Maharashtra are the eight Indian states with highest production of eggs. Even in Southern region of India, the cost ofegg and meat production is very low. Nowadays, there is easy availability of veterinary services and medicines which has enhanced the health conditions of livestock and poultry in India. India even exports poultry products to countries like Japan, Korea, Germany and Netherland.
As far the structural setup of poultry industry in India is concerned, the birds are being hatched and raised in breeding farms and sold to other farms, the broiler farms obtain day old chicks and rear them until six weeks and finally sell its meat, the laying farm on other hand, rear hens to collect and sell the eggs. Policies and programmes such as foreign direct investment and all India poultry development programme are also instrumental in promoting the poultry industry.
Heat stress in poultry birds
As in livestock, heat stress also has a huge impact on poultry industry. Heat stress exerts a negative impact on chicken management and reproduction which include physiological changes like respiratory alkalosis due to heavy panting, a reduction in feed intake and efficiency, decreased secretion of estrogen, testosterone, progesterone and thyroid hormones, reduced absorption of calcium and increased secretion of luteinizing hormone, follicle stimulating hormone and glucocorticoids. Heat stress also confers a reduction in quality, number and hatchability of eggs produced, imparting infertility in both female and male chickens.
Heat stress has effects both on laying hens and broilers and accounts for reduced egg quality and egg production and overall reduction in growth. A rise in basophil and lymphocyte ratios has been observed in broilers under heat stress that also showed decreased hematocrit values and increased rectal temperature. An increase in tonic immobility (TI) has also been observed which suggested that broiler birds under heat stress tend to be more fearful.
It has been observed that broiler birds exposed to elevated environmental temperature at an early age (i.e. 24 hours to 5 days old) showed decreased mortality rate when exposed to increased environmental temperature in later life. A significant improvement in feed efficiency was also observed in the birds exposed to an early age mild heat stress but no significant change in body weight.
Heat stress causes oxidative stress in poultry birds leading to reduction in antioxidant status and reduction in fertility. It has been reported that, heat and oxidative stress is associated with dysfunctionality of mitochondria in poultry. They have reported a decrease in metabolic oxidative activity of mitochondria and alteration in antioxidant enzymes’ activity in birds, but dietary interventions such as flavonoids benefitted the chronic heat stressed poultry. Heat stress has also shown to increase muscle and rectal temperature which leads to increase in superoxide production and decrease in body weight more in broiler chicken than in layer chicken.
It has been shown that, poultry birds are more vulnerable to heat stress due to their higher feed conversion efficiency and production performance and suggested many strategies to minimize its effects which included maintaining water and electrolyte balance, feed intake and levels of micronutrients like minerals and vitamins. Heavy metals such as zinc levels in diet also determine the immune response in broiler birds under heat stress. Negative impact on the immune system and overall functioning of poultry birds is also caused by activation of the hypothalamic-pituitary-adrenal axis induced by heat stress which also leads to changes in intestinal mucosa. An increase in the level of lysine requirement in female chicks and reduction in feed intake and weight gain by 22% has been observed both in male and female chicks under heat stress. In layer chicken, heat stress has shown to decrease egg quality and egg production due to increase in pH and a decrease in plasma levels of ionized calcium. It has been shown that administration of vitamin E and C in layer chicken during transportation in dry hot season can amend the negative effects of heat stress while non-administered birds showed a decrease in count of white blood cells, monocytes and eosinophils. It is reported that layer chickens under heat stress showed an enhancement in their immune function and laying performance after supplementation of vitamin A in their diet.
It has been proved that level of Triiodothyronine(T3) is reduced in heat stressed layer chicken and it can be used to indicate long term heat stress in layer chicken. Heat stress has also been reported to increase levels of glutamic-pyruvic transaminase (GPT) and creatine kinase (CK)in chicken, whereas glutamic-oxaloacetic transaminase (GOT) and lactate dehydrogenase (LDH) showed variable activity in the same.
Scientists have reported increased activities of plasma antioxidant enzymes inbirds under heat stress whereas the genotype response with age was not consistent. Layer chicken under heat stress showed great effect on egg quality when supplemented with antioxidant vitamins. Other poultry birds like Japanese quail showed a decrease in egg sale and production as environmental temperature increased cyclically.
Heat Shock Proteins (HSPs):
When exposed to stressful conditions like exposure to UV light, hot or cold conditions, tissue repairing, etc.,cellsexpress certain family of proteins known as heat shock proteins. Animals have shown an increase in production of heat shock proteins under elevated chemical, physiological and temperature stress. These cellular proteins are highly conserved evolutionarily.
HSP belong to a large family of proteins which are involved in protein folding and maturation whose expression is induced by heat shock or other stressors such as anoxia, hypoxia, heavy metals toxicity, or other factors that may induce protein denaturation. Some examples of heat shock proteins include Hsp90, Hsp70, Hsp60, Hsp25 and Hsp 27.
Heat stress and heat shock proteins in poultry
In the last century, as average global temperature increased, rearing of poultry birds has becomedifficult since they tend to undergo heat stress and many heat shock proteins have been reported to relieve that stress and to protect cellular physiology. All cellular organisms have shown that Hsp can mediate resistance to hyperthermia. HSP possess thermotolerance property and help to aggregate toxic proteins which get secreted during hyperthermia. A lot of mechanisms can help in thermotolerance which include: (1) Hsp mediated refolding of proteins and (2) HSP facilitated degradation of proteins in the proteasome pathway.
Effect of heat stress on age at sexual maturity in poultry birds
A study of the effect of frizzle and dwarf gene on broilers reproductive performance under normal and high temperature showed that interactions between temperature and genotype have an effect on reproductive traits like sexual maturity, fertility, egg production, hatchability and chick production.
Another study showed the effect of heat stress on egg industry in United States where birds could not reach at acceptable level of body weight at time of sexual maturity when birds were reared in hot months of the year which delayed the age at sexual maturity. It has been reported that egg weight decreased and egg shell quality deteriorated with every 1o C rise in environmental temperature. There was also a problem in obtaining a desirable and acceptable body weight at sexual maturity.
Normal body temperature of the birds is also affected by the change in environmental temperature. It has been observed that an increase in broiler chicken body temperature under environment of thermo neutral housing condition but a decrease in body temperature when chicken were exposed to high ambient temperature.