Selective and deferred treatment of clinical mastitis in seven New Zealand dairy herds.

Mastitis is the most frequent reason for antibiotic use in New Zealand dairy cattle and technologies reducing and targeting this use contribute to responsible product stewardship. Rapid identification of pathogen and antibiotic susceptibility facilitate targeted treatment but currently involve a minimum 24 h 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. Mastatest® is a diagnostic system for bovine mastitis indicating the pathogen and its antibiotic sensitivity within 24 h of sampling. This study focused on evaluating this system's ability 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. Mild to moderate mastitis cases in the 100 days after calving in 6467 cows from 7 farms were milk sampled and randomly allocated to a positive control group non-selective treatment or a culture-based treatment. All milk samples were processed using Mastatest®. For the positive control, the quarter was treated immediately with 3 treatments of procaine penicillin every 12 h. For the selective treatment group, treatment was delayed for 24 h and then informed by pathogen and antibiotic sensitivity from the Mastatest® 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 antibiotic residues were also noted. There was no difference in bacteriological or clinical cure rate between the two treatment groups. Final herd test ISCC and days of milk withhold from supply did not differ between groups. Antibiotic usage was 24 % less (95 % predictive interval = 12-47 %) in the selective group. This study suggests that on farm decisions about deferred treatment of mastitis using Mastatest® 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.

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