Birds are usually faced with environmental stresses while bred. Stress is defined as a biological reaction of animals to environmental stimuli, being considered a major challenge in the poultry industry, because of its unfavorable effects on growth performance. Both intrinsic and extrinsic negative stressors change body homeostasis in animals. For instance, the Royal Society for the Prevention of Cruelty to Animals (RSPCA), in its welfare standards for meat chicken, has recommended monitoring animals for having access to a thermal comfortable environment at all times, preventing both heat (HS) and cold stress (CS). Moreover, the World Organization for Animal Health (OIE) standards recommends avoiding severe heat, humidity and cold ( Stevenson et al. , 2014 ). In order to prevent these environmental stresses, the OIE has also advised to employ higher air speed, evaporative cooling and reducing stocking density ( Stevenson et al. , 2014 ).

Zhang et al. (2011) have reported that CS (12 ^° C in this study) negatively influences the intestinal system in broiler chicken. CS have negative effects on digestibility ( Onderci et al. , 2003 ), health and comfort in animals ( Tsutsayeva & Sevryukova, 2001 ). CS also impairs egg production and feed efficiency in laying hens ( Sahin et al. , 2002 ). The CS not only decreases growth performance, but also increases energy requirements in broiler chicken ( Blahova et al. , 2007 ). Similarly, HS (30 ^° C in this study) negatively affects the performance of birds ( Yardibi & Turkay, 2008 ; Habibian et al. , 2014 ; Tawfeek et al. , 2014 ). Sohail et al. (2012) reported that broiler chickens submitted to chronic HS had lower feed intake (FI; -16.4%), body weight gain (BWG; -32.6%), and higher feed conversion ratio (FCR; +25.6%) in 42 days of age. HS has negative effects on decreasing growth performance, including decreased protein retention in muscles and increased heat production in broiler chickens ( Zhang et al. , 2012 ). In addition to growth performance, immune responses may be negatively influenced by environmental stresses ( Bartlett & Smith, 2003 ; Niu et al. , 2009b ).

The heterophil-to-lymphocyte ratio has been used as criteria for evaluating the stress, and Campo et al. (2008) showed higher heterophil-to-lymphocyte ratio and lower welfare in broiler chicken submitted to low ambient temperature. Moreover, exposure to stressors provokes neuroendocrine responses in the body and increases the levels of corticosterone ( Iyasere et al. , 2017 ). Corticosterone, an end product of the hypothalamic-pituitary-adrenal axis, has been reported to have regulating effects on metabolism, feed consumption, and immune system ( Zulkifli & Sti Nor Azah, 2004 ). Corticosterone also causes a regression in lymphoid tissues in broiler chickens ( Yang et al. , 2015 ). Therefore, stress can have negative effects on performance and immunity by increasing the levels of corticosterone hormone.

Thus, this study aimed to compare the effects of CS, HS and thermoneutral (TN) treatments on growth performance and on some immune variables in broiler chickens.

Our findings showed that the CS and HS conditions had similar negative effects on growth performance of broiler chickens. Nevertheless, although non-significantly, CS had lower negative effects in comparison to HS. Many authors have informed about the adverse effects of CS and HS on the productive performance of poultry ( Sahin et al. , 2002 ; Star et al. , 2008 ; Mujahid & Furuse, 2009 ; Sohail et al. , 2012 ; Fu et al. , 2013 ; Habibian et al. , 2014 ; Tawfeek et al. , 2014 ).

Thermal stress influences the productive performance of poultry by affecting nutrient metabolism and digestibility ( Zhang et al. , 2012 ) while increasing corticosterone blood levels ( Sahin et al. , 2002 ). The reason for these effects is that stress alters the neuroendocrine system in poultry, by the activation of the hypothalamic-pituitary-adrenal axis, resulting in increased levels of corticosterone ( Quinteiro-Filho et al. , 2012 ). This is likely due to the adverse effects of corticosterone on the digestive system, nutrient utilization and digestibility, which subsequently have negative effects on performance. Indeed, corticosterone decreases food intake, which in turn decreases villus height in the intestine ( Hu & Guo, 2008 ), decreases nutrient utilization, apparent digestibility of protein, and integrity of the intestinal epithelium ( Hu & Guo, 2008 ). Moreover, corticosterone changes the protein metabolism and increases catabolism of structural proteins ( Post et al. , 2003 ; Lin et al. , 2004 ). There are further proof of the link between heat stress, corticosterone and intestinal integrity. For instance, HS-exposed birds showed a reduced villus-height to crypt-depth ratio ( Bozkurt et al. , 2012 ; Deng et al. , 2012 ). We found that the serum concentration of corticosterone was not different between groups on day 21, but it was increased in stressed groups on day 42. Therefore, the CS and HS effects on performance are likely mediated by an increased in the corticosterone levels.

Thermal stress decreased antibody titer against SRBC in our experiment, agreeing with previous reports ( Bartlett & Smith 2003 ; Niu et al. , 2009a , b ; Mishra et al. , 2011 ; Zhang et al. , 2011 ; Collins et al. , 2012 ; Fu et al. , 2013 ; Habibian et al. , 2014 ). HS suppresses the immune response by stimulating the hypothalamic production of corticotrophin-releasing factors (CRF) and increasing the levels of corticosterone ( Quinteiro-Filho et al. , 2012 , 2017 ). Moreover, the increase of corticosterone induced by thermal stress subsequently decreases the levels of IgA and interferon gamma, mRNA expression of pro- and anti-inflammatory cytokines, as well as mRNA expression of avian β-defensin-4, avian β-defensin-6 and Toll-like receptor-2 ( Quinteiro-Filho et al. , 2017 ). Such physiological changes suppress the immune system and increase the penetration the bacteria through the intestinal lining. In addition, thermal stress exerts negative effects on the redox balance, exacerbating the production of reactive oxygen species (ROS) ( Feng et al. , 2008 ) and contributing to the permeability of the gut lining to bacteria. Quinteiro-Filho et al. (2012) indicated that the HS-mediated elevated corticosterone levels increased inflammation and translocation of Salmonella enterica in broiler chickens. Therefore, corticosterone is the main culprit in the observed effects of thermal stress in our study and others', by suppressing the immune system and decreasing food intake, inflammatory cytokines and altering the function of the intestine.

In agreement with our findings, other researchers have reported an increase in heterophil to lymphocyte ratio in stressed animals ( Campo et al. , 2008 ; Prieto & Campo, 2010 ; Felver-Gant et al. , 2012 ). For instance, stressed laying hens presented lower intraepithelial lymphocytes and IgA-secreting cells in the intestinal tract ( Deng et al. , 2012 ). Glucocorticoid hormones reduce lymphocyte counts, because of the higher adherence to endothelial cells and circulating lymphocytes in response to these hormones ( Dhabhar, 2002 ). Shini et al. (2008) reported that chickens treated with corticosterone yielded a higher heterophil-to-lymphocyte ratio and also presented morphological alterations in heterophil size, shape, and granulation, and lymphocyte cytoplasmic properties. Moreover, similarly to our study, other studies have shown that thermal stress decreases the relative weight of lymphoid organs ( Niu et al. , 2009a ; Quinteiro-Filho et al. , 2012 ). The reduced relative weight of the lymphoid organs can be attributed to the corticosterone content. Yang et al. (2015) obtained a lower weight for the lymphoid organs of chickens treated with corticosterone. Glucocorticoids, i.e . corticosterone, prevent glucose transport in a variety of peripheral tissues ( Yang et al. , 2015 ), leading to a decrease in the organs weight. Moreover, glucocorticoids increase apoptosis in spleen in rats ( Collier et al. , 1998 ), which might contribute to these effects. Our data for corticosterone levels and associated effects support these previous observations.

In conclusion, CS and HS had similar adverse effects on the performance and immune system of broiler chickens, and these effects are likely to be mediated by an increase in the serum concentration of corticosterone. Therefore, the stress associated to the adverse thermal environment in the early development of broiler chickens exerts a very negative effect on the animals through well-known physiological mechanisms. Our findings might be useful for improving both the welfare and productivity of broiler chickens.

Authors would like to acknowledge the Immunology Research Center of Tabriz University of Medical Sciences that provided excellent assistance with the preparation of this research. We would like also to thank all of the members of our laboratories for their scientific contributions during these years.