Microbial aetiology of acute clinical mastitis and agent-specific risk factors.

A nation wide study on the microbial aetiology of cases of acute clinical mastitis in Swedish dairy cows was conducted with the aim to investigate changes in the microbial panorama compared to a previous study performed 1994-1995. Another aim was to investigate some agent-specific environmental and individual risk factors. Milk samples were collected from 987 udder quarter cases from 829 cows during six 2-month periods from May 2002 to April 2003, and data on risk factors and demography were collected at sampling by means of a questionnaire. In total, 1056 bacteriological diagnoses were made. The most frequently isolated bacterial species was Staphylococcus aureus constituting 21.3% of the diagnoses, followed by Escherichia coli (15.9%), Streptococcus dysgalactiae (15.6%), Streptococcus uberis (11.1%), coagulase-negative staphylococci (6.2%), Arcanobacterium pyogenes (6.1%) and Klebsiella spp. (4.2%). Samples with no growth or contamination constituted 10.6% and 4.5% of the diagnoses, respectively. A major shift in the panorama of udder pathogens was not observed compared to the survey in 1994-1995. Isolation of Klebsiella spp. was strongly associated with the use of sawdust as bedding material. On the other hand, using sawdust as bedding reduced the risk of isolating S. uberis relative to using straw or peat. The risk of isolating E. coli increased with increasing milk yield and was higher in loose housing systems than in tie stalls. Isolation of S. aureus was associated with tie stalls, and A. pyogenes with low yielding cows and teat lesions. S. dysgalactiae infections were also associated with teat lesions.