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Common Pathogens In Sub-Clinical Mastitis Revealed

09 June 2011

SWEDEN - Staphylococcus aureus and coagulase-negative staphylococci were the most frequently isolated pathogens in a survey of the microbial aetiology of sub-clinical mastitis in cows on dairy farms.

The aim of the survey was to investigate the microbial panorama and the occurrence of acquired antimicrobial resistance. Moreover, differences between newly infected cows and chronically infected cows were investigated.

In total, 583 quarter milk samples were collected from 583 dairy cows at 226 dairy farms from February 2008 to February 2009. The quarter milk samples were bacteriological investigated and scored according tousing the California Mastitis Test.

Staphylococci were tested for beta–lactamase production and presence of acquired resistance was evaluated in all specific udder pathogens. Differences between newly infected cows and chronically infected cows were statistically investigated using logistic regression analysis.

The study found that the most common isolates of the 590 bacteriological diagnoses that were made were Staphylococcus aureus (19 per cent) and coagulase-negative staphylococci (CNS; 16 per cent) followed by Streptococcus dysgalactiae (nine per cent), Str. uberis (eight per cent), Escherichia coli (2.9 per cent) and other Streptococcus spp. (1.9 per cent).

Samples with no growth or contamination constituted 22 per cent and 18 per cent of the diagnoses, respectively. The distribution of the most commonly isolated bacteria considering only bacteriological positive samples were: S. aureus, 31 per cent; CNS, 27 per cent; Str. dysgalactiae, 15 per cent; Str. uberis, 14 per cent; E. coli, 4.8 per cent and other Streptococcus spp., 3.1 per cent.

There was an increased risk of finding S. aureus, Str. uberis or Str. dysgalactiae in milk samples from chronically infected cows than in milk samples from newly infected cows.

Four per cent of the S. aureus isolates and 35 per cent of the CNS isolates were resistant to penicillin G. by beta-lactamase production.

Overall, resistance to other antimicrobials than penicillin G was uncommon.

The report's authors concluded that Staphylococcus aureus and CNS were the most frequently isolated pathogens in cases of sub-clinical mastitis and resistance to antimicrobials was rare.

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