ABSTRACT
Our current investigation was aimed to study differential influence of heat stress on defensive functions of blood polymorphonuclear cells among native and crossbred cows by culturing them in vitro after animal exposure to in vivo heat stress. Blood was collected from 18 cows which were divided in to three groups based on their breed as, group I (Sahiwal), group II (Tharparkar) and group III (Crossbred). THI was calculated by temperature and relative humidity (RH). PMN were isolated and were incubated at 38°C. Phagocytosis and lymphocyte proliferation was then investigated by NBT and MTT assays, respectively. The overall mean THI was significantly (p < 0.5) higher during summer (88.41 ± 1.54) when compared to the winter season (64.75 ± 0.97). There is a significant high lymphocytic proliferation in Group II when compared to Group III cattle. Also, there is a significant decrease in PA during the summer when compared to winter in all the breeds and in group III there is significant decrease in LPA during summer. The results of the present study suggest that indigenous cattle Tharparkar exhibited more tolerance to heat stress than the Group-I and Group-III during summer. However, in winter, all the breeds showed similar.
1. Introduction
The major constraint on dairy animals’ health and productivity in tropical and subtropical areas is heat stress due to high ambient temperature and humidity. Heat stress induces immuno-suppression in dairy cows, increasing the risk of mastitis and milk somatic cell counts (Lecchi et al. Citation2016). The differential influence of heat stress on native and crossbred cows their leukocytes activities have not been fully clarified. Indigenous cattle play a crucial role in the livelihood system and the well-being of the traditional rural farmers of India. High producing crossbred cattle gradually substitute them. Although crossbreds are high yielding, they demand substantial initial investment as well as high maintenance costs due to the inability to sustain extreme climatic conditions like heat stress, which is unaffordable to the majority of the farmers (Kalyani et al. Citation2018). The indiscriminate breed substitution in the native cattle population has been an incredible loss to India's rural traditional agriculture. The immediate and appropriate measures are needed to conserve indigenous bovine genetic resources; otherwise, irrecoverable damage is likely to occur in the subsistence agriculture system of India. Not enough field works on the aspect of the adaptive capability of indigenous cattle and crossbred cattle, especially to the extreme climatic conditions that are predominant in a tropical climate like India, have been accomplished so far.
In addition to the above, severe heat stress also compromises the immunity in terms of in vitro activity of leukocytes resulting in the higher incidence of infections during periods of hot weather and the circulating leukocytes mostly peripheral blood mononuclear cells (PBMCs) were considered as an effective model to understand the heat stress response in human (Hunter-Lavin et al. Citation2004), goat (Sharma et al. Citation2013; Dangi et al. Citation2016), cattle and buffalo (Kishore et al. Citation2014). Therefore, the current experiment was conducted to evaluate the differential effect of heat stress on certain Immunological responses in native and crossbred cattle to evaluate their resilience to thermal stress.
2. Material and methods
All the experiments, procedures, and protocols on animals were conducted following the approval of the Ethics Committee, Indian Veterinary Research Institute, No.F.1-53/2012-13/J.D(R).
2.1. Experimental animals and site
The experiment involved 18 healthy lactating native (Sahiwal/Tharparkar) and crossbred cattle (Hariana× HolstienFriesian / Brown swiss / Jersey) of the same age group between 4 and 5 years maintained at Livestock Production and Management Section, Indian Veterinary Research Institute, Izatnagar, UP, India. The institute is located at an altitude of 564 feet above the mean sea level, at the latitude of 28°21′N and longitude of 79°24′E having sub-tropical climate. The experimental cows were in second to fourth parity in their mid-lactation stage. Animals were maintained under isomanagerial conditions of housing, feeding, and milking.
2.2. Experimental design
All selected 18 animals were divided into three equal groups (n = 6) with consideration of breed, as Group I (Sahiwal), Group II (Tharparkar), and Group III (Crossbred). Temperature and Relative humidity of the animal sheds were recorded daily. The immune status during the study period was evaluated through two types of immunity assays by using PBMCs and Neutrophils isolated from the blood within two hours of sample collection.
2.3. Blood sampling
Blood samples were collected at the weekly intervals for six weeks during summer and six weeks during winter, through jugular puncture using heparin as an anticoagulant for isolation of PBMC, Neutrophils, within two hours of sample collection.
2.4. Measurement of meteorological variables
The minimum temperature, maximum temperature, relative humidity (RH) were recorded at the experimental shed daily during the experiment period using dry and wet bulb thermometer, and then the THI, a measure of thermal load on animals, was calculated from a formula by McDowell et al. Citation1976.
2.5. Determination of phagocytic activity and lymphocyte proliferation assay (LPA)
Peripheral blood mononuclear cells were isolated using Histopaque according to the method described by English and Andersen (Citation1974), and The lymphocyte proliferation and Phagocytic assays were done as per the method described by Khatti et al. (Citation2017).
3. Result
3.1. Temperature humidity index
The variations in the temperature-humidity index (THI) during different weeks in summer and winter are depicted in . During the study period, the lowest THI was recorded during the winter, and the highest THI was recorded during the summer. The overall mean THI was significantly (p < 0.5) higher during summer (88.41 ± 1.54) when compared to the winter season (64.75 ± 0.97).
Figure 1. Temperature and humidity index (THI) during the study period in summer and winter.
3.2. Immune responses
The variations in phagocytic activity and lymphocyte proliferation index among the groups during summer were presented in and (A), respectively, and during winter were presented in and (B), respectively. Significantly (p < 0.05) higher phagocytic activity was found in 3rd week during summer in Group I and Group II cattle and significantly (p < 0.05) higher phagocytic activity was observed in 3rd week followed by 6th week during winter. Whereas there was no significant (p > 0.05) variation in lymphocyte proliferation within a group between weeks both in winter and also in the summer season. Also, significantly (p < 0.05) lower proliferation rate was observed in Group III when compared to Group I during summer. The overall PA and LPA between summer and winter seasons among the breeds under study were depicted in and (C) respectively. The PA decreased significantly during summer when compared to winter among all the groups and also significantly (p < 0.05) higher phagocytic activity was observed in Group II when compared to Group III cattle during winter whereas the lymphocyte proliferation decreased significantly (p < 0.05) during summer when compared to winter in Group III but not in Group I and II cattle.
Figure 2. (A) Weekly Phagocytic activity in crossbred, Tharparkar and Sahiwal during Summer (The bars bearing alphabets (a,b,c) denotes significant difference (p < 0.05) between weeks within a breed). (B) Weekly Phagocytic activity in crossbred, Tharparkar and Sahiwal during Winter (The bars bearing alphabets (a,b,c) denotes significant difference (p < 0.05) between weeks within a breed). (C) Over all mean Phagocytic activity in crossbred, Tharparkar and Sahiwal cattle during Summer and winter. (The bars bearing alphabets (x, y) denotes significant difference(p < 0.05) between seasons within a breed).
Figure 3. (A) Weekly LPA S.I Values in crossbred, Tharparkar and Sahiwal cattle during Summer. (B) Weekly LPA S.I Values in crossbred, Tharparkar and Sahiwal cattle during Winter. (C) Overall seasonal LPA S.I Values in crossbred, Tharparkar and Sahiwal cattle during Summer and winter (The bars bearing alphabets (x,y) denotes significant difference(p < 0.05) between seasons within a breed).
4. Discussion
The Livestock Weather Safety Index (LWSI; LCI, 1970) is a benchmark commonly used to assign heat stress levels to normal, alert, danger, and emergency categories. In the LWSI, THI values ≤74 are classified as alert, >74 THI < 79 as a danger, and 79 ≤ THI < 84 as an emergency. As per the above index of THI, during the winter season, the mean THI of 63.5 ± 1.80 indicated that the animals were under the thermal-comfort zone, and during the summer season, the mean THI of 88.09 ± 1.80 indicated that the animals were under moderate to high-intensity thermal stress.
In our investigation, as animals are exposed to natural climatic conditions and the PBMC were isolated and cultured in vitro, we observed that In vivo heat stress is accountable for a decline of in vitro neutrophil phagocytosis and lymphocyte proliferation in crossbred cattle (Group III), whereas, in native cattle (Group I & II) even though there is decrease in neutrophil phagocytosis during summer there is no significant difference (p > 0.05) in LPA values, between summer and winter. The discrepancy between the study groups could be due to variation in breed of experimental animals, and it may be suggested that rising body temperatures during thermal stress might not be linked with the same degree of immune cell efficiency amendment in the three breeds (Bhanuprakash et al. Citation2016). Thus the present study indicated that the impairment of cellular functions might initiate at a different level of thermal stress in the proposed breeds.
Also, significantly lower values of LPA and PA in crossbred cattle (Group III) when compared to native cattle (Group I & II) in both summer and winter seasons could be attributed to their increased serum cortisol levels, which are observed in our earlier study (Tejaswi Citation2018). Further, paralleling the increase of cortisol hormone production from the adrenal cortex, activation of the HPA axis could also result in catecholamine production from the adrenal medulla (Carrasco and Van de Kar Citation2003). Thus, the probable cause of diminished activity of the neutrophils and proliferation of lymphocytes could be due to increase of glucocorticoids (Cook et al. Citation2002) and also could be due to decrease in number of viable cells due to heat stress (Kamwanja et al. Citation1994).
5. Conclusion
The reduced lymphocyte proliferation and phagocytic index (LPA and PA) stipulate the decline in immune cell reactivity and the higher susceptibility of infections during summer in crossbred when compared to native cows. Moreover, Indigenous cattle, particularly Tharparkar, exhibited more tolerance to heat stress than the Sahiwal and crossbred during summer.
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No potential conflict of interest was reported by the author(s).
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References
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