Indian Journal of Animal Research

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Exploring Thermoregulatory Responses and Hormonal Changes in Heat Stressed Assam Hill Goats 

S. Deori1,*, S.N. Abedin1, H. Chakravarty1, R. Katiyar1, B.U. Choudhury1, E. Syiem1, S. Doley1
1Division of Animal and Fisheries Sciences, ICAR Research Complex for NEH Region, Umiam-793 103, Meghalaya, India.

Background: Livestock productivity and health are adversely affected by heat stress (HS). The aim of this study was to determine the effects of HS on the thermoregulation and hormonal profile in Assam Hill Goats reared in the subtropical hilly regions of Meghalaya so as to arrive at a conclusion on its heat adaptation abilities. 

Methods: An experiment was conducted during the year 2022 for 120 days with twelve (12) Assam Hill Goats, aged between 3-5 months, randomly divided into 2 groups (n=6 per group). The goats in group I were exposed to HS through natural exposure to sunlight during the extreme summer months (May to August) for 07 hours (8 AM to 3 PM) during grazing. The control animals (CON, group II) were maintained under shade with no exposure to sunlight. Thermoregulatory responses viz., rectal temperature, respiratory rate and pulse rate and body weight data were recorded every 15 days interval of the sampling period. The hormonal profile viz., serum T3 (nmol/L), T4 (nmol/L) and Cortisol (ng/mL) were also determined. 

Result: Results revealed a significant (p<0.05) increase in rectal temperature, pulse rate (day 45 onwards to day 120) and respiratory rate in HS group in comparison to CON. A significantly (p<0.05) lower average daily gain was recorded in HS group. Serum T3 and T4 concentrations were significantly (p<0.05) reduced and cortisol concentration was significantly (p<0.05) elevated in HS group in comparison to CON. In conclusion, Assam Hill Goats exposed to environmental HS shows elevated thermoregulatory responses, decreased thyroid hormone activity and increased cortisol activity to reduce production to support life sustaining activities in the hilly regions. 

In the tropics, goats hold great importance as valuable small ruminant resources, especially for impoverished families residing in rural areas. They possess a compact body structure that enables them to dissipate heat effectively by exposing a large surface area in proportion to their weight. The tropics’ diverse climatic conditions have led to the development of distinct adaptive mechanisms in goats, enabling them to cope with a wide range of stressful environmental conditions. Environmental stress triggers noteworthy effects on thermoregulation, live body weight and physiological parameters in goats, as observed during both dry and wet seasons.
       
The adverse impact of heat stress (HS) on livestock productivity and health is well-established and it can severely disrupt the physiological equilibrium of goats. Consequently, it is essential to investigate goats’ heat adaptation abilities in harsh environments from a scientific perspective multiple authors have suggested that the most effective thermo-physiological indicators for objectively assessing animal welfare in challenging conditions are rectal temperature, respiratory rate and blood indices (Helal et al., 2010; Sanusi et al., 2010). HS is more prevalent during the dry season, especially when there is high environmental temperature, high relative humidity and prolonged exposure to direct sunlight. Specific responses and reciprocal regulatory influences involving their nervous, endocrine and immune systems have been implicated in this process (Castanheira et al., 2010).
       
Animals employ various adaptations to deal with HS, such as an increased sweating rate, elevated respiration rate, vasodilation resulting in enhanced blood flow to the skin surface, reduced metabolic rate, decreased dry matter intake (DMI) and changes in water metabolism, among other mechanisms (Baumgard and Rhoads, 2013; Sejian et al., 2018). The levels of circulating T3 and T4 experience a reduction of approximately 25% as well, mirroring the decrease in metabolic rate, dry matter intake (DMI) and overall production during HS conditions (Silanikove, 1992).
       
The objective of the current study was to measure the physiological, growth and hormonal responses to extreme HS conditions in Assam Hill Goats exposed to prolonged sunlight since no priori information is available on the effects of HS on this native breed, which is widely distributed in Assam and adjoining areas of Meghalaya.
Animal care
 
The present study was reviewed and approved by the Institutional Animal Ethics Committee (IAEC) of ICAR RC for NEH, Umiam, India.
 
Study area and design
 
The study was conducted in the Livestock Farm, Indian Council of Agricultural Research (ICAR) Complex for North-Eastern Hill Region (NEHR), Umiam, Meghalaya, India. In the present study, twelve (12) Assam hill goats, aged between 3-5 months were randomly divided into two groups, consisting six (6) animals each. The goats in group I (HS) were exposed to heat stress through natural exposure to sunlight during the extreme summer months (1st May to 31st August, 2022) for 07 hours (8 AM to 3 PM) during grazing. The control animals (CON, group II) were maintained under shade with no exposure to sunlight during the entire experimental period. Roughage and concentrate feed were supplied every day at adequate amount to all the animals in each group with free access to drinking water. 
 
Collection of data
 
Climatic data
 
Ambient temperature (°C) and relative humidity (RH%) were measured two times daily (morning and evening) using a thermometer and barometer respectively, throughout the study period. Temperature-humidity index (THI) was calculated using the formula as reported by Helal et al., (2010). Further, the climatic variables were crosschecked with the data provided by the State Meteorological Department, Meghalaya.
 
Thermoregulatory and live bodyweight data
 
The rectal temperature (°F), respiratory rate (Per minute) and pulse rate (Beats per minute; bpm) of each goat were measured on three occasions (Morning, afternoon and evening) at 9:00, 13:00 and 16:00 hours every 15 days throughout the sampling period and the mean value of the three readings was calculated. To measure rectal temperature, a digital thermometer with a disinfected sensory tip was inserted into the rectum when the thermometer displayed the temperature reading in degrees celsius (°C) and then readings were converted to Fahrenheit (°F). Respiratory rate was determined by counting the number of abdominal movements per minute. Pulse rate was recorded by placing the fingertips on the femoral arteries of the hind limb for one minute as reported by Sanusi et al., (2010). The body weight (Kg) of goats in each treatment group was measured and the average daily gain (g) was calculated.
 
Hormonal profile
 
Monthly blood samples were collected from all animals to measure the serum levels of T3 (nmol/L), T4 (nmol/L) and Cortisol (ng/mL) in both groups. Aseptic techniques were employed to collect the blood samples from the jugular vein using a 5 mL syringe. The serum samples were then separated and analyzed for hormone estimation using a species-specific ELISA kit provided by ELK Biotechnology, Co. Ltd.
 
Statistical analysis
 
Statistical package for social sciences programme version 23 (IBM SPSS) was used for statistical analysis. Data obtained from thermoregulatory, bodyweight and hormonal indices were expressed as Mean±SE and analyzed using the Student’s ‘t’ test. P<0.01 and P<0.05 were considered highly significant and significant at 99% and 95% confidence interval respectively.
Climatic data
 
Table 1 shows the THI during the experimental period. The experiment was carried from 1st May  to 31st August, 2022. During the period the THI ranged from 71.27 to 74.39.
 

Table 1: Average monthly temperature, relative humidity and THI during the experimental period (1st May, 2022 to 31st August, 2022).


 
Thermoregulatory responses
 
Rectal temperature
 
Fig 1 depicts the trend of rectal temperature (°F) in both the experimental groups. The rectal temperature significantly (p<0.05) increased in HS goats in comparison to CON. Even under challenging weather conditions, goats exhibit a certain level of thermoregulatory control to uphold their body temperature within a narrow range. In a thermo-neutral setting, rectal temperatures usually range from 38.3 to 39.9°C. Rectal temperature measurement is widely employed as a prominent indicator of body temperature, offering insight into the core temperature (Al-Tamimi, 2007). It is recognized as a crucial physiological parameter for assessing animal welfare in hot environments (Silanikove, 2000). The outcomes of the present investigation indicated a significant (p<0.05) elevation in rectal temperature on all days, except at 90 days, during periods of HS. Current results were in accordance with the reports of Kandasami et al., (2023) in Kanni Adu and Kodi Adu goats of Tamil Nadu, where HS caused marked increase in rectal temperature during summer. These heightened rectal temperatures observed in the context of hot summer conditions unequivocally demonstrate that the animals were subjected to HS (Alamer and Al-Hozab, 2004; Al-Haidary et al., 2012; Minka and Ayo, 2012). In the face of elevated temperatures, animals employ strategies to maintain thermal equilibrium and dissipate excess heat from their bodies. One notable response is an evident increase in the respiratory rate or panting. However, if the body is unable to effectively maintain this thermal equilibrium, it leads to an elevation in body temperature (Marai et al., 2007). The outcomes of the present study are in line with published research on Saudi Arabian goats (Al- Samawi et al., 2014), indicating concurrence with the previous findings. However, these results contradict the reports associated with Black Bengal goats where heat treatments did not increase skin and rectal temperatures (Alam et al., 2011).
 

Fig 1: Variation in rectal temperature in control (CON) and heat stressed (HS) group.


 
Pulse rate
 
Fig 2 depicts the trend in pulse rate (bpm) in the different experimental groups. There was no significant (p>0.05) change in pulse rate on day 0, 15 and 30 days, but a significant (p<0.05) increase was recorded from day 45 onwards till 120 days in the HS group in comparison to CON group. The results of our study are consistent with the reports of Alam et al., (2011) in Black Bengal goats and Kandasami et al., (2023) in Kanni Adu and Kodi Adu goats. The pulse rate primarily indicates the stability of circulation and the overall metabolic condition. When exposed to high environmental temperatures, the pulse rate tends to increase (Aboul-Naga, 1987). In goats, it was found that the pulse rate during summer was considerably higher compared to winter (Ismail et al., 1995). This similar pattern was also observed in grazing goats (Khan and Ghosh, 1989). An increase in the pulse rate promotes enhanced blood flow from the core to the body’s periphery, leading to increased heat loss through both sensitive means (conduction, convection and radiation) and insensitive means (Water loss through diffusion from the skin) (Marai et al., 2007).
 

Fig 2: Variation in pulse rate in control (CON) and heat stressed (HS) group.


 
Respiration
 
Fig 3 depicts the trend in respiration rate (per minute) in the experimental groups. Results revealed a significant (p<0.05) increase in respiratory rate in the HS group in comparison to the CON. The present results were in agreement with previous findings where it was reported that respiration rate can be elevated through HS in goats (Habeeb et al., 1992; Minka and Ayo, 2012; Kandasami et al., 2023). The respiratory rate was employed as a measure of HS and served to assess the detrimental impact of environmental temperature (Alamer and Al-Hozab, 2004). For goats, the respiration rate is higher during summer compared to winter (Fahmy, 1994). In essence, the increased respiration rate during summer signifies HS and can be considered one of the mechanisms employed by animals to dissipate excessive heat through evaporation.
 

Fig 3: Variation in respiration rate in control (CON) and heat stressed (HS) group.


 
Live body weight
 
Table 2 depicts the average body weight gain in the experimental goats in both the treatment groups. Results revealed a significantly (p<0.05) lower average daily gain in the HS group in comparison to CON. Current results are in agreement with the reports of Pragna et al., (2018) who reported reduced average daily gain in Osmanabadi and Malabari heat stressed goats. Upon exposure to high temperatures, goats activate their physiological adaptability to maintain homeothermy (Aleena et al., 2018). These adaptive processes consume considerable energy, leading the animals to redirect their energy from productive pathways towards the adaptive pathway (Sejian et al., 2018; Aleena et al., 2018). This behavior, characterized by a reduction in production to prioritize life-sustaining activities, is typically observed in adapted goat breeds.
 

Table 2: Effect of heat stress on the body weight gain (Mean±SE) of Assam hill goats.


 
Hormonal profile
 
Table 3 depicts the serum concentration of hormones viz., T3 (nmol/L), T4 (nmol/L) and Cortisol (ng/mL) in the experimental groups. T3 and T4 were important hormones in regulating energy balance (Hefnawy et al., 2011). Environmental temperature was the main external regulator of the thyroid gland activity (Dickson, 1993). The present study revealed that T3 and T4 serum concentrations were significantly (p<0.05) reduced in HS animals in comparison to CON. These outcomes are in agreement with the findings reported previously, indicating a decrease in blood concentrations of T3 and T4 during thermal stress (Silanikove, 2000). Furthermore, an inverse relationship between temperature and the activity of thyroid gland hormone secretion (T3 and T4) has been observed in sheep (Starling et al., 2005) and goats (Todini et al., 1992). The thyroid and adrenal glands play vital roles in animal adaptation by regulating heat production within the organism. When animals are exposed to high temperatures, the reduction in thyroxine secretion occurs as a response to reduced thermogenesis requirements, representing a significant step in countering heat stress (Coelho et al., 2008). This decrease in thyroid hormone release is influenced by the impact of heat on the hypothalamic-pituitary-adrenal (HPA) axis, leading to a decrease in thyroid hormone release and subsequently reducing basal metabolism. Similarly, during cold stress, the increased levels of T3 and T4 stimulate oxygen consumption and heat production in cells, resulting in an elevation of basal metabolism (Bernabucci et al., 2010).
 

Table 3: Effect of heat stress on hormonal profile (Mean±SE) of Assam hill goats.


       
In response to stressful conditions, the activation of the hypothalamic-pituitary-adrenal (HPA) axis leads to an increase in plasma cortisol concentration (Silanikove, 2000; Mormède et al., 2011). Our study revealed that the exposure of Assam Hill Goats to heat stress resulted in an elevation of serum cortisol concentration. These results are consistent with the findings of Zhengkang et al., (1994), who observed a significant (p<0.01) increase in blood cortisol concentrations in relation to higher environmental temperatures. Additionally, Mormède et al., (2011) reported an increase in plasma cortisol concentration when animals were exposed to direct sunlight during hot summer conditions. Higher plasma cortisol concentrations in animals subjected to heat stress were also reported (Wise et al., 1988). The elevation in cortisol levels can be attributed to the direct impact of heat stress, particularly from solar radiation, as stress conditions activate the hypothalamic-pituitary-adrenal (HPA) axis (Mormède et al., 2011). The hypothalamus responds by releasing corticotrophin-releasing factor (CRF), which acts on the anterior pituitary, leading to the release of adrenocorticotropic  hormone or corticotrophin (ACTH). This, in turn, stimulates the adrenal cortex to produce corticosteroid (Cortisol) hormones (Mormède et al., 2011). Cortisol, the primary glucocorticoid, plays a crucial role in various physiological processes, including the regulation of metabolism, body water distribution, electrolyte balance and blood pressure. Its secretion triggers physiological changes that enable animals to cope with the stress caused by a hot environment. In the initial stages of acute heat stress, the animal’s response is primarily emotional rather than thermoregulatory. However, during severe acute heat stress, cortisol likely contributes to the induction of hyperglycemia, facilitating the anticipated increase in glucose utilization (Bernabucci et al., 2010).
In conclusion, Assam Hill Goats exposed to environmental HS shows elevated thermoregulatory responses, decreased thyroid hormone activity and increased cortisol activity to reduce production to support life sustaining activities in the hilly regions. Such behavioural changes are aimed to increase the animal’s basal metabolism and is a typical characteristic of goat breeds towards climatic adaptation.
The authors thankfully acknowledge The Director, ICAR Research Complex for NEH Region, Meghalaya, India and ICAR-NICRA for providing necessary facilities to conduct the research work.
We certify that there is no conflict of interest with any financial organization regarding the material discussed in the manuscript.

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