| The production and management, prevalence and antimicrobial susceptibility of E. coli, Salmonella spp., Enterococcus spp. and Campylobacter spp. isolated from bovine feces, Staphylococcus aureus isolated from bulk tank milk were compared between organic and conventional dairy herds. Thirty organic dairy herds, where antimicrobials are rarely used for calves and never used for cows, were compared with 30 neighboring conventional dairy farms, where antimicrobials were routinely used for animals for all ages. A seven-page questionnaire was used to assess management and production during 2000--2001. The organic farms had significantly fewer cattle than did the conventional herds. The average daily milk production per cow in organic dairy herds was lower than that of conventional herds. The incidence of clinical mastitis and bulk tank somatic cell count on organic farms was not statistically different from that of on conventional farms. There was little evidence of other fundamental differences between two farm types in other major management and production parameters.; Fecal specimens from ten cows and ten calves on 120 farm visits yielded 1,120 E. coli isolates, 7 Salmonella spp., which were tested for resistance to 17 antimicrobials. A total of 332 Campylobacter spp. isolates were tested to four antimicrobials (ciprofloxacin, erythromycin, gentamicin, and tetracycline). A total of 2,049 Enterococcus spp. were tested to 3 antimicrobials (Quinupristin/dalfopristin, gentamicin, and vancomycin). Of the 118 bulk tank milk samples in Wisconsin, 71 samples (60%) yielded at least one Staphylococcus aureus isolate, and a total of 331 S. aureus were collected and tested for resistance to 15 antimicrobials. The susceptibility of S. aureus were also compared with Danish study. Our study shows significantly lower prevalence rates of AR in E. coli for seven antimicrobials (ampicillin, streptomycin, kanamycin, gentamicin, chloramphenicol, tetracycline, and sulphamethoxazole) in organic dairy herds, as compared to conventional herds. Two Campylobacter isolates from conventional dairy farms were resistant to ciprofloxacin and none of the isolates were resistant to gentamicin or erythromycin. Tetracycline-resistance in Campylobacter was 41.5% (66/159) for organic and 47.4% (82/173) for conventional herds, which was not statistically significant. A significant lower rate of resistant Staphylococcus aureus was detected to only one antimicrobial on organic farms in our Wisconsin study (ciprofloxacin) and on conventional farms in the parallel study in Denmark (avilamycin). Staphylococcus aureus isolates from Wisconsin had higher probability of reduced susceptibility to 7 out of 14 comparable antimicrobials, whereas Danish isolates had higher probability of reduced susceptibility to only two drugs. Differences in antimicrobial susceptibility between organic and conventional farms were small relative to the differences observed between the two countries. Although the organic farms had converted to organic farming methods at least 3 years before our study, antimicrobial resistance clearly presented long after antimicrobial selective pressure had been withdrawn. |
