Screening of dairy cows’ herd for presence in milk of hemolytic microorganisms in relation to somatic cell content

Inflammation of mammary glands of cows or mastitis is one of the most common infectious diseases of dairy cattle, which causes significant economic losses to livestock and affects the quality of dairy products. This work was aimed at monitoring the presence of hemolytic streptococci and Staphylococcus aureus as the main causative agents of mastitis in the high producing dairy cows and identifying relationship between the milk somatic cell count (SCC) and the infection with S. aureus. Whole herd of clinically healthy Holstein cows of the Central region of Russia from first to seventh lactations ( n = 345) were used for the milk sampling. Depending on the presence of microorganisms in the milk, the cows were distributed between four groups: I - hemolytic streptococci and S. aureus are not found; II - only hemolytic streptococci are detected; III - only S. aureus are detected; IV - both hemolytic streptococci and S. aureus are detected. Determination of antibiotic susceptibility profile of S. aureus isolates was carried out on a special medium using disco-diffusion method. Determination of methicillin-resistant staphylococcus (MRSA) was carried out using the MRSA Alert w/swabs test («HiMedia Laboratories Pvt. Ltd», India). Staphylococcus aureus MRSA ATCC 43300, Staphylococcus aureus ATCC 25923 were used as reference strains. Data processing was performed using MS Excel software. The significance of differences was evaluated by Student’s t test. It was found that only 22.0 % of clinically healthy cows did not contain studied forms of microorganisms in their milk, while in 78.0 % of them the presence of hemolytic streptococci and/or S. aureus in milk was detected. The rate of S. aureus infected animals increased with increasing number of lactations (from 14.7-19.2 % in the first and second lactations to 32.0-45.2 % in animals of the third and more lactations). The higher levels of SCC in the milk of cows in group III with S. aureus (+1191.1×10 3, +1124.3×10 3 and 1099.6×10 3 cells/cm 3 comparing to groups I, II and IV, respectively; p S. aureus in the milk of cows of groups III and IV varied from 3, and from 1.0 to 7.8 lg CFU/cm 3, respectively, and averaged 3.62±0.36 and 3.47±0.23 lg CFU/cm 3, respectively. The associations between titers of S. aureus in the milk of cows and SCC were not identified. More than 95 % of S. aureus isolates were resistant to lincomycin and more than 50 % were resistant to erythromycin and fuzidin. Above 95 % of isolated S. aureus strains were sensitive to oxacillin, gentamicin and vancomycin. Some strains were resistant to oxacillin (1.3 %), so the further study and elucidation of the mechanisms of action using molecular methods are required.