الفهرس 
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Abstract
Bovine mastitis is a large-scale contagious disease with important decrease on the economy of milk production. Besides health illnesses of the mammary gland, mastitis can also lead to a huge decrease in milk yield, variations in its quality (change nutritive and technological properties of milk), fertility problems and even systemic diseases. Moreover, causative agents of mastitis with zoonotic potential may lead to a health danger for human populations through the food chain. Results showed that the overall incidence of mastitis in the examined dairy cow was 64.7% which is high with nearly an equal incidence of clinical and sub clinical mastitis (48.2% and 51.8%, respectively). Bacteriological examination of 170 milk samples showed that 110 isolates was Streptococci (64.7%). The predominant one is Streptococcus agalactiae (45.45%) followed by S. uberis (41.82%) then S. dysgalactiae (12.73%).Titration of β haemolysin of S. agalactiae showed that 60% of S. agalactiae was ranged from 1:16 to 1:64. By PCR, the gene responsible for β haemolysin was cylE gene which found in 60 % of examined S. agalactiae isolates, so the result of genotype of β haemolysin is correlated well with the result of the titre of β haemolysin. Hyl gene (hyaluronidase gene) was another gene in S. agalactiae responsible for the spreading of S. agalactiae during infection. It was found in 30% of examined S. agalactiae isolates. Both CylE gene and hyl gene observed in 30% of isolated S. agalactiae. These two genes were responsible for intra cellular survival of S. agalactiae inside macrophage. In conclusion, the current study reported an overall prevalence of Streptococcal species, especially S. agalactiae, associated with mastitis that was a main health problem of dairy cows and will have a drawback on the production of dairy industry and so warrants serious attention in Egypt. Particularly the prevalence of cylE gene encoding β-hemolysin and hyl gene encoding hyaluronidase in the present investigation help in the understanding of the distribution of S. agalactiae and share in the establishing of preventive approaches to decrease the spread of infection.