Seasonal variation in serum enzyme activities of Nagavali sheep under tropical conditions

Introduction

Sheep contribute substantially to food production, fibre, rural employment, and household income. As homeothermic animals, they maintain thermal balance through physiological and behavioural adjustments. Blood biochemical traits are useful indicators of metabolic status, but they may vary with breed, sex, age, nutrition, health, reproductive status, season, diet, and management [1,2,3,4] . Among climatic variables, ambient temperature and humidity strongly influence livestock productivity and welfare. Increasing thermal load can alter feed intake, water balance, respiration, metabolism, production, and reproduction [5,6] . The Nagavali sheep, also known as Vizianagaram or North Coastal sheep, is a recently recognized breed from the north coastal region of Andhra Pradesh. Breed-specific reference information on seasonal serum enzyme activity remains limited. The present study therefore aimed to assess seasonal changes in serum AST, ALT, and ALP activities in rams, ewes, and the total herd, and to relate these changes to the prevailing THI.

Materials and methods

Study location and animals

The investigation was conducted at the Livestock Research Station, College of Veterinary Science, Garividi, Sri Venkateswara Veterinary University, Vizianagaram district, Andhra Pradesh, India (18.28946° N, 83.55393° E). One hundred apparently healthy Nagavali sheep aged one year or older and maintained under comparable management were selected. The study population comprised 50 ewes and 50 rams. Sampling was conducted in May, August, and December, representing summer, monsoon, and winter, respectively.

Temperature-humidity index

Dry-and wet-bulb temperatures, minimum and maximum temperatures, and relative humidity were recorded daily at 13:00 h using a digital thermo-hygrometer. THI was calculated as THI = 0.72(Tdb + Twb) + 40.6, where Tdb is dry-bulb temperature and Twb is wet-bulb temperature (°C) [7] .

Blood collection and serum preparation

Approximately 5 mL of blood was collected aseptically from the jugular vein of each sheep between 07:30 and 08:30 h in each season. For serum separation, 2.5 mL was transferred to a clot-activator vacutainer. Samples were labelled, transported to the laboratory on ice, allowed to clot for 2 h, and centrifuged at 5000 rpm for 5 min. Serum was transferred to labelled microcentrifuge tubes and stored at -20 °C until analysis.

Serum enzyme assays

Serum AST, ALT, and ALP activities were measured using commercial Erba diagnostic kits and a Thermo Scientific Multiskan spectrophotometer according to the manufacturer's instructions. AST and ALT were determined using kinetic methods, and enzyme activities were expressed as IU/L [21] .

Statistical analysis

Seasonal variation in serum AST, ALT and ALP activities was analysed separately for rams, ewes and the total herd using one-way ANOVA. Significant differences among seasonal means were identified using Duncan's multiple range post-hoc test. Statistical significance was accepted at P≤0.05.

Results

Seasonal temperature-humidity index

The mean THI was highest in summer (80.02 ± 0.43), followed by monsoon (70.51 ± 0.41) and winter (67.89 ± 0.27) (Figure 1). Based on the thresholds cited in the source manuscript, summer conditions represented moderate heat stress, whereas monsoon and winter conditions were comparatively comfortable.

Serum enzyme activities

Season significantly affected serum AST activity in rams, ewes, and the total herd (Table 1). AST was highest in summer, intermediate in monsoon, and lowest in winter.

ALT activity was lower in winter than in summer in rams and the total herd; in ewes, winter values were lower than both summer and monsoon values. ALP activity was also higher in summer and monsoon than in winter.

Discussion

The higher summer THI observed in this study indicates greater thermal load during that season. Similar seasonal increases in THI have been reported in small ruminants [9] . Sheep and goats are generally considered relatively tolerant of harsh climates, but prolonged exposure to high THI can still affect production, physiology, and welfare [10,6] .

Using the classification cited in the source manuscript, THI values of 72 or below indicate no heat stress, 73-77 mild stress, 78-89 moderate stress, and values above 90 severe stress [7] . Accordingly, the summer THI of approximately 80 represented moderate heat stress, whereas monsoon and winter conditions were below or near the comfort threshold [8] .

Aspartate aminotransferase

Serum AST activity was markedly higher during summer in rams, ewes, and the total herd. This pattern agrees with reports of elevated AST during hot seasons in indigenous sheep and other small ruminants [5,3] . Increased AST may reflect altered hepatic and muscular metabolism, greater membrane permeability, or tissue strain under thermal stress. Temperature-related acceleration of enzymatic reactions has also been proposed [12] .

Alanine aminotransferase

ALT activity was generally higher in summer and monsoon and lower in winter. Comparable seasonal increases have been documented in sheep and goats [5,15] . Because ALT participates in conversion of alanine to pyruvate, increased activity during summer may be associated with enhanced gluconeogenic demand or altered liver function under heat stress [14,13] .

Alkaline phosphatase ALP activity was higher during summer and monsoon than during winter. ALP originates mainly from liver, bone, and other tissues, and seasonal variation may reflect changes in hepatic function, bone turnover, growth, and energy metabolism. Similar thermal effects on enzymatic activity have been described in goats and sheep [17,18,19,20] . The present results should be interpreted with caution because the study involved a single flock and three discrete seasonal sampling periods. Potential effects of age, physiological state, nutrition, and repeated sampling were not modelled separately. Nevertheless, the findings provide preliminary breed-specific information that may assist clinical interpretation and seasonal management of Nagavali sheep.

Conclusion

Summer conditions were associated with the highest THI and increased serum AST, ALT, and ALP activities in Nagavali sheep. Winter values were consistently lower, particularly for AST and ALP. Seasonal thermal load should therefore be considered when interpreting serum enzyme profiles in this breed. The values reported here may serve as preliminary baseline data, although confirmation in larger, multi-flock studies using repeated-measures or mixedeffects models is warranted.

Acknowledgement

AcknowledgementThe authors acknowledge the staff of the Livestock Research Station, the in-charge of the Livestock Farm Complex, and the Associate Dean, College of Veterinary Science, Sri Venkateswara Veterinary University, Garividi, for permission, facilities, and support. FundingNo external funding was received for this study. The research was carried out as part of the Master's degree programme of the first author using the facilities available at Sri Venkateswara Veterinary University.

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