Seasonal changes of some biochemical indicators in four sheep herds from Bulgarian Dairy Synthetic Population

The purpose of the present study was to evaluate seasonal differences in the values of some enzymes and metabolites - ALT, AST, ALP, serum iron, glucose, as well as WBC count and N/L ratio in four herds of sheep from the Bulgarian Dairy Synthetic Population (BDSP), raised at different altitudes. The research was conducted in sheep farms of the Sofia region - Ravnishte (391 m a.s.l.), IAS - Kostinbrod (548 m a.s.l.), Mirkovo (715 m a.s.l.) and Bosnek (940 m a.s.l.). Blood samples were taken from selected 24 animals from each sheep herd. Analysis of biochemical parameters was performed in extracted plasma by Semi-automatic Biochemical Analyzer BTS-350 and WBC (leucocyte subpopulations, resp.) count was performed by 5-Part-Diff Automated Haematology Analyzer. We observed significant differences between seasons in all studied indicators except in WBC count (most in B. and R.). In the summer season there were more significant differences between sheep farms in all studied indicators compared to winter, except about the serum iron levels. All studied indices were within the reference values for sheep, except about ALP activity and serum iron levels, which were lower than limits. The lack of significant difference in total leukocyte count, but the presence of changes in subpopulations, indicated a dynamic adaptation of the immune system to seasonal factors, such as infections, nutrition, stress and parasite loads, i.e., N/L ratio, could be an indicator to seasonal (winter and summer) stress and criterion for selection for stress resistance in sheep, depending on altitude.