Evaluation of a 3M Petrifilm on-farm culture system for the detection of intramammary infection at the end of lactation

The purpose of this study was to evaluate a 3M Petrifilm-based on-farm culture system for the detection of intramammary infection (IMI) in low somatic cell count (SCC) cows (<200,000cells/mL) at drying off. The main objectives were to determine the test characteristics and the predictive values of the Petrifilm on-farm culture system. The ability of dairy producers to correctly classify cows as infected or uninfected based on Petrifilm culture and a set colony count threshold was also assessed. A total of 360 cows originating from 16 low bulk tank SCC (<250,000cells/mL) dairy herds were enrolled at drying off. Enrolled cows had an expected dry period of 30–90 days, a SCC<200,000cells/mL on the last 3 tests prior to drying off, no clinical mastitis in the same time period, and no antibiotic treatment in the last 14 days. Quarter milk samples were collected on the day prior to drying off, and a composite milk sample was created by combining 5mL of milk from each quarter sample. Composite milk samples were cultured on-farm using the Petrifilm culture system, which provided results within 24h. Quarter milk samples were cultured in a reference laboratory, and the results were aggregated to the cow level. On the day of drying off, the Petrifilm was read by the producer and cows were classified as positive if ≥5 colonies (equivalent to 50 colony forming units/mL) were present. When read by the producer, 47.8% of the cows cultured negative on Petrifilm and were infused with only an internal teat sealant at drying off. The test characteristics of the Petrifilm on-farm culture system were calculated by comparing the producer-derived Petrifilm results to those obtained by standard laboratory culture. The sensitivity and specificity of the Petrifilm on-farm culture system were 85.2% (78.5–90.5) and 73.2% (66.4–79.3), respectively. The negative predictive value of the Petrifilm test system was high (86.6%) when estimated using the prevalence of IMI in this data set, and the positive predictive value was moderate (70.9%). An automated 3M Petrifilm reader was used to obtain accurate colony counts. The agreement between Petrifilm results obtained by the producer and those obtained by the automated Petrifilm reader was high, with a kappa value of 0.82 (0.75–0.89).