الفهرس 
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Abstract
Six hundred and ninety five lactating cows (2780 quarters) from a total 2300 Holstein Friesian dairy cows at Fayoum district, Egypt were examined for the prevalence of subclinical mastitis and its associated risk factors during the period of December 2016 to November 2017. Four hundred forty four lactating cows with 1145 quarters were positively for SCM with prevalence of 63.88 and 41.18% at cow and quarter level respectively based on California Mastitis Test, Electrical Conductivity with a mean value (6.27 ± 0.066 mS/cm) and Somatic Cell Count with mean value 8.8×105 ± 9.2×103 cells/ml.The risk factors investigated in this study revealed that the highest percentage of SCM was observed in the winter; late stage of lactation and in cows of 4-5 years old. Also the occurrence of SCM was higher in cows with 2nd, 3rd and 1st parity number than those with 4, 5, 6th parity number respectively. The statistical analytical results also revealed that there is highly significance (p = 0.028) between parity and other studied risk factors. The results of chemical analysis of the milk samples along the four seasons of the year which done by the Lactoscan milk analyzer revealed that; the statistical analysis results indicated that there is a significant difference (P < 0.05) in fat % and SCC between winter and autumn, moreover, there is a significant difference (P < 0.05) between (winter and autumn) and (winter and spring) in the parameters of protein, SNF and lactose content. So, milk composition especially contents of fat, protein and lactose were found to be lower with a significant increase in Salt % and SCC of SCM milk samples. This study showed that measurements of SCC can be an effective screening test for abnormalities in milk constituents. It is also concluded that there is a strong negative correlation between SCC and fat %; while there is a strong positive correlation between SCC and salt %. A total of 284 different environmental samples were obtained from bedding, silage, swabs from the milker’s hands and milking equipment were subjected to microbiological analysis and the isolated pathogens could be found in different percentage. In this study also, fifty one BTM samples were examined through the four seasons of the year for the TCC, the data revealed that the highest mean value of the TCC of the BTM was observed in the spring (4.8×104 ± 3.6×103 CFU/ml), followed by summer, winter and autumn seasons with a mean values of 4.4×104 ± 3.4×103, 4.0×104 ± 3.1×103 and 2.7×104 ± 2.3×103 CFU/ml respectively. The mean value of Coliforms count of the examined twenty four water samples from trough showed that the highest mean value 8.3×10 ± 3.45 MPN/ 100 ml was observed at the winter, followed by summer, autumn and spring with a mean values of (4.3×10 ± 1.79, 4.1×10 ± 1.71 and < 3 MPN/ 100 ml) respectively. The bacteriological analysis of 444 SCM milk samples by traditional methods and Vitek 2 compact system revealed that, the most often isolated microorganisms were Staphylococcus aureus 296 (66.66%), Enterococcus species 230 (51.80%), Escherichia coli 210 (47.29%) and Streptococcus agalactiae 106 (23.87%). There is a significant (p < 0.05) relationship between hygienic conditions of environmental samples and prevalence of the SCM pathogens (Enterococcus species, Staphylococcus aureus, Pseudomonas aeruginosa and Escherichia coli) with a chi-square value 41.63, 8.58, 32.68 and 101.19 respectively. The antimicrobial resistance was determined by disc diffusion test and the corresponding resistance genes were detected by PCR. Results of the in vitro susceptibility tests performed and the phenotypes indicated that the highest resistance antibiotic for isolated microorganisms was Penicillin followed by (amoxicillin + clavulanic acid), oxacillin and tetracyclin. In addition, genotypes showed that the tetK and tetA(A) genes were expressed in most examined microorganisms, while blaZ, bla and blaTEM genes were expressed in all examined isolates. Macrolides resistance genes (ermB and ermC) showed variable positive reactions in the examined microorganisms, while methicillin-resistant gene mecA was positive in the examined Staphylococcus aureus and Staphylococcus chromogenes isolate. Moreover, vanA and norA genes were detected in the examined E. gallinarum isolate. However Sul 1 gene was positive 100% in Enterobacter spp. & Citrobacter amalanaticus and in 50% of the examined Klebsiella oxytoca isolates. While Enterobacter speceis was 100% positive for mph(A).