On-farm culture and pathogen identification: risks and benefits

On-farm culture (OFC) is based on the theory that clinical mastitis caused by Gram-negative pathogens does not require antimicrobial treatment. In OFC protocols, cows with mastitis have an aseptic milk sample taken, which is incubated on agar or in broth. Antimicrobial treatment is withheld in samples with pure growth of Gramnegative species, or no significant growth. In samples with Grampositive species, mixed or contaminated growth, intramammary antimicrobial treatment is given on label.

Several OFC studies have shown that withholding treatment in cases of mastitis caused by Escherichia coli has no detectable impact on treatment outcome (Lago et al, 2011; Vasquez et al, 2017; Fuenzalida and Ruegg, 2019; Schmenger et al, 2020). One criticism of these studies is that they lack statistical power to detect a meaningful clinical difference in cure rate, i.e. the sample size is too small. Two independent studies in the USA have shown a 10–11% numerical reduction in bacteriological cure risk, which was not ‘statistically significant’ (Lago et al, 2011; Fuenzalida and Ruegg, 2019). A recent study in Germany showed more convincing evidence that bacteriological cure rate is unaffected by OFC: 75.3% with conventional therapy compared with 78.4% in the OFC group (Schmenger et al, 2020). Another USA study found similar days to clinical cure, although bacteriological cure was not assessed (Vasquez et al, 2017).