The relationship between management factors, antimicrobial sensitivity and prevalence of staphylococcal mastitis pathogens in a random sample of Prince Edward Island dairy herds
This study, an investigation of coagulase positive (CPS) and coagulase negative (CNS) staphylococcal mastitis in 44 randomly selected dairy herds in Prince Edward Island (PEl) had three major objectives. These are: 1. to survey dairy management factors and determine the effect of management factors on the herd prevalence of staphylococcal mastitis pathogens. 2. to survey antimicrobial sensitivity of Staphvlococcus isolates from the study herds, and assess which management factors affect sensitivity of staphylococcal mastitis pathogens to these antimicrobials. 3. to determine if significant correlations exist between the prevalence of staphylococcal mastitis pathogens and their in vitro antimicrobial sensitivity. Information on dairy management practices and antibiotic use was sought through a mail out questionnaire. A response rate of 77% was realized. Adoption rates of 79% and 48% for postmilking teat dipping and dry cow therapy for all cows in a herd were observed. Cloxacillin benthazine and Oxytetracyline hydrochloride were the most widely used dry cow products, on 48% and 32% of farms respectively. Seventy eight percent of the farms used products containing beta lactams, 48% of these contained Penicillin G. Weighted least squares multiple regression was used to relate the within herd prevalence of CPS or CNS to management factors and, separately, to dry cow products. For CPS, farms that dipped teats after milking were significantly associated with low prevalence. The number of times the milking machine was checked had a weak positive correlation with prevalence. Dry cow products used had no association with the prevalence of CPS. Freestall and ties tall versus loose housing, high-line milking system versus bucket milking, and pre-partum teat dipping were associated with low prevalence of CNS. Farms that used a dry cow product containing procaine penicillin G and novobiocin sodium were associated with low prevalence of CNS, while farms that used any dry cow product containing novobiocin were associated with high prevalence. All CPS isolates and a sub-sample representing 25% of CNS isolates were tested for sensitivity to 13 antimicrobials using the disc diffusion method. Among CPS, sensitivity was above 91% to all the antimicrobials tested except penicillin G, ampicillin, neomycin, and polymyxin B. Among the CNS, sensitivity was above 91% except penicillin G, ampicillin, polymyxin B, tetracycline, and sulphamethoxazole / trimethoprim. There was between farm variation in sensitivity for both CPS and CNS. When farm factors were taken into account, the proportions of CPS sensitive to tetracycline, sulphamethoxazole / trimethoprim, and nitrofurantoin were significantly higher than CNS. The proportions of CNS sensitive to neomycin and polymyxin B were significantly higher than CPS. Sensitivity to penicillin G, ampicillin, and erythromycin was strongly affected by farm level factors. Weighted least squares multiple regression was used to relate the proportions of CPS and CNS sensitive to penicillin G or tetracycline to management factors and, separately, to dry cow products. Among CPS, post-milking teat dipping was associated with high sensitivity to penicillin G and tetracycline. The milking herd proportion culled because of mastitis had a negative correlation with sensitivity for tetracycline. Neither the herd proportion receiving dry cow therapy nor the dry cow products used had any association with CPS sensitivity to Penicillin G or tetracycline. Among the CNS, post-milking teat dipping, dipping milking cluster between cows and the herd proportion receiving dry cow therapy had positive correlations with sensitivity to penicillin G. The milking herd proportion treated for clinical mastitis and the herd proportion receiving dry cow therapy had negative correlations with CNS sensitivity for tetracycline, while the herd proportion culled because of mastitis had a positive correlation. Dry cow products used predicted CNS sensitivity to penicillin G and tetracycline. The association between the proportions of CPS or CNS sensitive to penicillin G and tetracycline in vitro and the herd prevalence were sought after taking into account farm factors using the partial correlation coefficient. No significant associations were found.