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Epidemiology, Genetic and Molecular Characterization of Staphylococcus Aureus in Ohio Dairy Farms

出版	Ohio State University, 2014
URL	http://books.google.com.hk/books?id=oXH9rQEACAAJ&hl=&source=gbs_api

註釋Staphylococcus aureus (Staph.aureus) is the most common contagious pathogen causing intramammary infections (IMI) in cows worldwide. Although practices to control this organism have been advocated for decades, identification of risk factors is crucial in prevention and control of Staph.aureus. The objectives for Chapter 2 and Chapter 3 were to estimate prevalence of Staph.aureus in Ohio dairies and to determine the association of management practices with isolation of Staph.aureus in bulk tank milk (BTM). Questionnaire about herd characteristics, milking procedures, udder health/mastitis control, biosecurity and calf/heifer raising practices were mailed to 780 dairy producers, with a response rate of 49.2%. Staph.aureus prevalence was 48%, 64% and 69% when considering one, two or three BTM samples. Herds with somatic cell count lower than 150,000 cells/mL and milking routine practices such as pre-stripping, pre- and post-dipping and use of single towels per cow were associated with reduced detection of Staph.aureus. Dry-off practices such as abrupt cessation of milking, use of internal teat sealant or blanket dry treatment were associated with herd size. In addition to the presence of Staph.aureus in the infected udder and milk, the cow can also be colonized with the organism on body sites. The objective of the Chapter 4 was to assess the role of teat skin colonization by Staph. aureus in Staph.aureus IMI by evaluating genetic relatedness of Staph.aureus isolates from milk and teat skin of dairy cows using pulsed-field gel electrophoresis (PFGE) and characterizing the isolates based on the carriage of virulence genes. Cows in four known Staph.aureus positive herds were sampled and was found that quarters with teat skin colonized with Staph.aureus were almost five times more likely to be diagnosed with Staph.aureus IMI than quarters not colonized on teat skin. Three main clusters (A, B, C) were identified with PFGE using a cutoff at 80% similarity. All clusters contained both milk and teat skin isolates with 72% of isolates belonging to cluster B. Forty-two virulence factors were screened using PCR and presence of clfA, clfB genes may have contributed to the ability of certain isolates to become the predominant strain. The aim of the study in Chapter 5 was to estimate antimicrobial resistance in Staph.aureus and to determine presence of methicillin-resistant Staph.aureus (MRSA) in Ohio dairy BTM. Staph.aureus isolates for this study were those previously described in Chapters 2. Isolates were tested for antimicrobial resistance by Kirby-Bauer disc diffusion and MRSA confirmation was done by a duplex PCR using femB and mecA genes. MecA was detected in two Staph.aureus isolates from a single farm (herd prevalence = 0.95%), collected at different time. Both isolates appeared highly clonal and belonged to SCC type IV, spa type t021, USA200. These results confirm the presence in low prevalence of BTM MRSA in US dairies. Findings from this study show that Staph.aureus IMI is a multifactorial disease. Characteristics related to cow (colonization on teat skin), or practices performed at farm (pre-strip, pre-, post-dip and use of single towel per cow) should be considered to control Staph.aureus IMI.