Evaluation of a 3M Petrifilm on-farm milk culture system for use in selective dry cow therapy

As an alternative to blanket dry cow therapy (BDCT), selective dry cow therapy (SDCT) is considered a more judicious approach to antimicrobial use for the purpose of mastitis control during the non-lactating period. The purpose of this research was to evaluate the utility of a 3M Petrifilm-based on-farm milk culture system (OFCS) for use in a SDCT program. A randomized clinical trial was conducted using 16 low bulk tank somatic cell count (<250,000 cells/mL) dairy herds from Prince Edward Island (n = 10) and Quebec (n = 6), Canada. When used to detect intramammary infection (IMI) in cows at drying off, the OFCS performed well with a sensitivity of 85% and specificity of 73%. By comparing producer-derived Petrifilm results to those obtained using an accurate automated reader, it was determined that producers were able to correctly interpret Petrifilm results with an observed agreement of 91% and kappa value of 0.82. The study groups comprising the clinical trial were a positive control group consisting of cows receiving blanket application of dry cow therapy (DCT) with the addition of an internal teat sealant (ITS), and the OFCS group consisting of cows selectively treated at drying off based on OFCS results with ITS alone (Petrifilm negative) or DCT + ITS (Petrifilm positive). No significant differences in postcalving IMI risk and risk of clinical mastitis in the first 120 days of the subsequent lactation were detected between the study groups. Furthermore, no significant differences were observed between study groups regarding test day milk yield and somatic cell count in the first 180 days of the subsequent lactation. Petrifilm-OFCS-based SDCT enabled a reduction in DCT of 21% as compared to BDCT. The effect of ITS on the interdependence of quarters towards the acquisition of new IMI (NIMI) with coagulase negative staphylococci (CNS) over the dry period was also investigated. It was demonstrated that in cows that were infused with ITS, the presence of CNS in another quarter at drying off was a risk factor for CNS NIMI, but this association was absent in cows without ITS infusion. This unexpected finding may have resulted from inadvertent introduction of CNS during ITS infusion. In quarters of cows without a CNS IMI at drying off (i.e. without a source of contagious CNS), ITS was protective against CNS NIMI suggesting that CNS infection over the non-lactating period can be partly attributed to CNS species located within the environment. According to economic analyses, OFCS-based SDCT resulted in a marginally higher total cost per cow than BDCT. However, OFCS-based SDCT provided the combined benefits of lowering DCT treatment risk without increasing the risk of postcalving IMI, and was superior economically to a SDCT program based on somatic cell count and clinical mastitis history. Overall, the Petrifilm OFCS was an accurate diagnostic tool that was effective in reducing DCT use without compromising the health, welfare, and future milk production of the cow.