title: Resistance to penicillin of Staphylococcus aureus isolates from cows with high somatic cell counts in organic and conventional dairy herds in Denmark.
abstract: BACKGROUND
  Quarter milk samples from cows with high risk of intramammary infection were examined to determine the prevalence of Staphylococcus aureus (SA) and penicillin resistant SA (SAr) in conventional and organic dairy herds and herds converting to organic farming in a combined longitudinal and cross-sectional study.
  
  
  METHODS
  20 conventional herds, 18 organic herds that converted before 1995, and 19 herds converting to organic farming in 1999 or 2000 were included in the study. Herds converting to organic farming were sampled three times one year apart; the other herds were sampled once. Risk of infection was estimated based on somatic cell count, milk production, breed, age and lactation stage.
  
  
  RESULTS
  The high-risk cows represented about 49 % of the cows in the herds. The overall prevalence of SA and SAr among these cows was 29% (95% confidence interval: 24%-34%) and 4% (95% confidence interval: 2%-5%) respectively. The prevalence of penicillin resistance among SA infected cows was 12% (95% confidence interval: 6%-19%) when calculated from the first herd visits. No statistically significant differences were observed in the prevalence of SAr or the proportion of isolates resistant to penicillin between herd groups.
  
  
  CONCLUSIONS
  The proportion of isolates resistant to penicillin was low compared to studies in other countries except Norway and Sweden. Based on the low prevalence of penicillin resistance of SA, penicillin should still be the first choice of antimicrobial agent for treatment of bovine intramammary infection in Denmark.
authors.affiliation: Department of Animal Health, Welfare and Nutrition, Danish Institute for Agricultural Sciences, 8830 Tjele, Denmark. torbenw.bennedsgaard@agrsci.dk
authors.forename: Torben W
authors.initials: TW
authors.lastname: Bennedsgaard
authors.forename: Stig M
authors.initials: SM
authors.lastname: Thamsborg
authors.forename: Frank M
authors.initials: FM
authors.lastname: Aarestrup
authors.forename: Carsten
authors.initials: C
authors.lastname: Enevoldsen
authors.forename: Mette
authors.initials: M
authors.lastname: Vaarst
authors.forename: Anna B
authors.initials: AB
authors.lastname: Christoffersen
mesh_terms: D000818:Animals; D002417:Cattle; D003430:Cross-Sectional Studies; D003718:Denmark / Q000453:epidemiology; D005260:Female; D008414:Mastitis, Bovine / Q000188:drug therapy* / Q000453:epidemiology; D008892:Milk / Q000382:microbiology; D010403:Penicillin Resistance; D010406:Penicillins / Q000627:therapeutic use*; D015995:Prevalence; D013203:Staphylococcal Infections / Q000188:drug therapy / Q000453:epidemiology / Q000662:veterinary*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000302:isolation & purification
chemical_list: D010406:Penicillins