Bulk tank milk ELISA as screening test for Mycoplasma bovis: herd classification based on serology and PCR testing of different age groups

Herd-level infection status for Mycoplasma bovis is commonly assessed by detecting antibodies in bulk tank milk (BTM) samples. However, the association between BTM serology and active circulation of the pathogen at farm level remains largely unclear, in part because non-lactating animals, and particularly calves, fall outside this sampling strategy. This cross-sectional pilot study aimed to categorize 14 dairy herds based on the ID-screen BTM antibody ELISA for M. bovis, and compare those results with PCR testing and serology in adult cows, and PCR testing in youngstock and calves. Serum sample-to-positive percentage (S/P%) from adult cows was measured using the Bio K432 ELISA. Nasal swabs (NS) from adult cows, calves, and youngstock, and vaginal swabs (VS) from lactating cows, were pooled by age group and tested by PCR. Based on manufacturer’s guidelines and optimized cutoff values for the ID-screen ELISA, herds were categorized as BTM negative (S/P% <30%; n = 5), BTM positive (S/P% ≥50%; n = 6) or BTM indeterminate (S/P% 30–50%; n = 3). Median serum S/P% was 26.3 (Interquartile range (IQR); 15.9–42.4), 81.33 (IQR; 52.7-108.8) and 43.4 (IQR; 27.5–95.0) in BTM negative, positive and indeterminate herds, respectively. PCR detected M. bovis only in pooled NS samples from calves and youngstock in two BTM positive herds. Seroprevalence aligned with BTM results for positive and negative herds, but varied in indeterminate herds. BTM testing may serve as a valuable screening tool to distinguish herds likely free of M. bovis from those where additional testing of calves and youngstock is recommended.