Abstract
BackgroundA nationwide survey on the microbial etiology of cases of subclinical mastitis in dairy cows was carried out on dairy farms in Sweden. The aim was to investigate the microbial panorama and the occurrence of antimicrobial resistance. Moreover, differences between newly infected cows and chronically infected cows were investigated.MethodsIn 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 using the California Mastitis Test. Staphylococci were tested for betalactamase production and presence of resistance was evaluated in all specific udder pathogens. Differences between newly infected cows and chronically infected cows were statistically investigated using logistic regression analysis.ResultsThe most common isolates of 590 bacteriological diagnoses were Staphylococcus (S) aureus (19%) and coagulase-negative staphylococci (CNS; 16%) followed by Streptococcus (Str) dysgalactiae (9%), Str. uberis (8%), Escherichia (E.) coli (2.9%), and Streptococcus spp. (1.9%). Samples with no growth or contamination constituted 22% and 18% of the diagnoses, respectively. The distribution of the most commonly isolated bacteria considering only bacteriological positive samples were: S. aureus - 31%, CNS - 27%, Str. dysgalactiae - 15%, Str. uberis - 14%, E. coli - 4.8%, and Streptococcus spp. - 3.1%. There was an increased risk of finding S. aureus, Str. uberis or Str. dysgalactiae in milk samples from chronically infected cows compared to findings in milk samples from newly infected cows. Four percent of the S. aureus isolates and 35% of the CNS isolates were resistant to penicillin G. Overall, resistance to other antimicrobials than penicillin G was uncommon.ConclusionsStaphylococcus aureus and CNS were the most frequently isolated pathogens and resistance to antimicrobials was rare.
Highlights
A nationwide survey on the microbial etiology of cases of subclinical mastitis in dairy cows was carried out on dairy farms in Sweden
There are no reliable data on the incidence of subclinical mastitis in Sweden, but the annual geometric average bulk-milk somatic cell counts (BMSCC), which reflects cases of subclinical mastitis in a herd, was 190 000 cells/ml in 2009 (Swedish Dairy Association, 2009)
Cows with subclinical mastitis should be considered as a risk for spread of mastitis pathogens within and between herds and are as such of national concern
Summary
A nationwide survey on the microbial etiology of cases of subclinical mastitis in dairy cows was carried out on dairy farms in Sweden. A nationwide survey on the microbial etiology of clinical mastitis was performed in 2002-2003 and revealed that Staphylococcus (S.) aureus, Escherichia (E.) coli and streptococci were the dominating findings [2]. There are no reliable data on the incidence of subclinical mastitis in Sweden, but the annual geometric average bulk-milk somatic cell counts (BMSCC), which reflects cases of subclinical mastitis in a herd, was 190 000 cells/ml in 2009 (Swedish Dairy Association, 2009). This indicates that the incidence should be rather low since a high incidence would be reflected by higher national BMSCC. Cows with subclinical mastitis should be considered as a risk for spread of mastitis pathogens within and between herds and are as such of national concern
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