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Selective and deferred treatment of clinical mastitis in seven New Zealand dairy herds
Bates, A.; Laven, R. Bork, O.; Hay...
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Objectives: This study focused on evaluating the ability of a novel on-farm diagnostic system for bovine mastitis (Mas- tatestÒ ) to control antibiotic usage whilst achieving equivalent bacteriological and clinical cure rates alongside long term individual somatic cell count (ISCC) outcomes as conventional treatment choices.
Mastitis is the most frequent reason for antibiotic use in New Zealand dairy cattle and technologies reducing this use contribute to responsible product stewardship. Rapid identification of pathogen and antibiotic susceptibility facilitate targeted treatment but currently involve a minimum 24 hours delay. Studies from confinement systems where Gram-negative organisms are responsible for a significant proportion of mastitis, indicate selective treatment can reduce antibiotic use without reducing clinical or bacteriological cure. However, in New Zealand’s seasonal, pastoral dairy system, mastitis is dominated by Gram-positive organisms and if treatment is deferred, it is vital both short- and long-term clinical health outcomes are not compromised.
Materials and methods: MastatestÒ is an on-farm or clinic diagnostic system for bovine mastitis indicating the pathogen and its antibiotic sensitivity within 24 hours of sampling.
Mild to moderate mastitis cases in the 100 days after calving in 6,467 cows from 7 farms were milk sampled and randomlyallocatedtoapositivecontrolgroup(non-selective treatment) or a culture-based treatment using MastatestÒ. All milk samples were processed on-farm.
For the positive control, the quarter was treated immediately with 3 treatments of procaine penicillin every 12 hours. For the selective treatment group, treatment was delayed for 24 hours and then informed by pathogen and antibiotic sensi- tivity from the MastatestÒ result. Gram-negative and no-growth quarters were untreated. Gram-positive quarters were treated with the antibiotic for which the lowest in vitro antimicrobial sensitivity was reported.
Re-sampling was carried out from affected quarter(s) approximately 21 days after initial diagnosis and cultured for bacterial identification. Clinical recurrence within 60 days and ISCC data was recorded at herd tests over the duration of the lactation. Antimicrobial usage and days of milk withhold pending clearance of residues were also noted.
Results: There was no difference in bacteriological or clinical cure rate between the two treatment groups. Out of 535 quarter cases, 451 (84%) were bacteriologically cured at re-sampling and 43 (8%) were re-diagnosed with clinical mastitis within 60 days of the original diagnosis. Bayesian models predicted no difference in the cure proportion by treatment group but the probability of a bacteriological cure for cows infected with Staph.aureus was significantly less than that for Strep. uberis in both groups. There were numerical differences in the median bacteriological cure proportion by farm, but the coefficients spanned zero and overlapped. There was no evidence for a significant interaction between treatment group and farm, nor between treatment group and pathogen.
Final herd test ISCC - 225,000 cells/mL (95% predictive interval (PI) =25,000-4,543,145) - and days of milk withhold from supply - 5.7 days per quarter case (95% PI=1.0-6.5) - did not differ between groups. There were numeric differences in the median predicted ISCC by pathogen and farm, but the coefficients spanned zero and overlapped. There was no evidence for a significant interaction between treatment group and farm, nor between treatment group and pathogen.
Antibiotic usage was 24% less (95% PI = 12-47%) in the selective group with the model predicting that the there was a 98% chance that antibiotic usage in the selective group (1.3 daily doses per case, 95%PI=1.1-1.6) was less than in the non-selective group (1.7, 95%PI=1.4-1.9).
Conclusions: This study suggests that on farm decisions about deferred treatment of mastitis using MastatestÒ to identify the intramammary pathogen can reduce the antimicrobial usage with no loss in bacterial or clinical cure and with no effect on ISCC over the lactation.
Keywords: Mastitis, deferred treatment, antibiotic sensitivity, Mastatest.
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Affiliation of the authors at the time of publication
Vetlife Scientific Ltd, Temuka, New Zealand;
School of Veterinary Sciences, Massey University, New Zealand;
Mastaplex Ltd, Dunedin, New Zealand;
Vetlife Oamaru, Oamaru, New Zealand;
Vetlife Temuka, Temuka, New Zealand;
Centre for Dairy Excellence, Geraldine, New Zealand
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