Evaluation of a 3M Petrifilm on-farm mastitis culture system and treatment decision algorithm for clinical mastitis in Canada

Mastitis represents the most costly infectious disease of dairy cattle and results in the highest proportion of antibiotic use on dairy farms. Rapid and accurate diagnosis of pathogens causing clinical mastitis (CM) would allow selective treatment strategies which are expected to reduce antibiotic use without compromising short or long term clinical outcomes. A randomized clinical trial was conducted on 48 herds across Canada to test the utility of a 3M Petrifilm on-farm culture system (OFCS) for the diagnosis of CM. First, the stability of 3M Petrifilms following re-packaging in the OFCS and storage at room temperature was tested in the laboratory setting. The ability of dairy producers to use the OFCS accurately to identify and selectively treat mild to moderate CM cases caused by gram positive pathogens was then examined. Clinical and bacteriologic cure, relapse risk and risk of secondary antibiotic use OFCS-based treatment and treated control (TC) groups were determined. The economic impact of using the OFCS and implementing selective culture-based treatment strategies was compared to the blanket therapy approach. Finally, the minimum inhibitory concentration (MIC) of pathogens isolated from pre-treatment and follow-up milk samples were determined and the impact of using the OFCS on post-treatment resistance patterns was examined.;Conditions under which Petrifilms were stored did not significantly alter the sensitivity or specificity of the OFCS in the lab. On farm, dairy producers identified 81.8% of CM cases caused by gram positive pathogens using the OFCS providing that on average, herds experienced at least 1 clinical case per month throughout the clinical trial. Sensitivity was reduced to 63.8% when herds with lower incidence of CM were included in the analysis. Attention to aseptic technique, incubator maintenance, culture protocol and colony identification were identified as potential factors in the accuracy of on-farm diagnosis.;A significant reduction in clinical cure probability and the days to clinical cure were observed in the OFCS treated group compared to the TC group which was attributable to incorrectly diagnosed cases by the dairy producer. No significant differences in bacteriologic cure probability were detected between OFCS and TC groups; however, further analysis detected a significant difference in bacteriologic cure probability in false negatively diagnosed cases. No statistically significant differences in clinical and bacteriologic cure or days to clinical cure were detected in correctly diagnosed cases. Primary antibiotic use was reduced in the OFCS group with no significant difference in risk of receiving secondary antibiotic treatment compared to the TC group. The cost of mastitis was increased in the OFCS group due primarily to longer milk withhold time for gram positive cases which were delayed treatment for 24 hours and due to a reduction in cure rates in false negatively diagnosed gram positive cases. However, substantial savings of over ;Overall, the 3M Petrifilm OFCS was effective in reducing antibiotic use and long term clinical outcomes were not compromised compared to the blanket therapy approach. The greatest potential for economic return in using an OFCS for selective treatment strategies is in herds with accurate diagnosis of treatment and non-treatment cases and in herds with a higher incidence of coliform and no growth (non-treatment) cases.