Effect of presampling procedures on real-time PCR used for diagnosis of intramammary infections with Staphylococcus aureus in dairy cows at routine milk recordings

Aseptic procedures for milk sample collection are considered crucial for bacterial culture to avoid misdiagnosis and subsequently unnecessary treatment or culling. The objective of this field study was to investigate the effect of presampling procedures on the PCR-positivity at cycle threshold value <= 37 of real-time PCR assay to detect Staphylococcus aureus from composite milk samples at routine milk recordings while accounting for known cow-level risk factors. A total of 1,199 dairy cows from 6 herds with conventional milking parlors were sampled and tested by PCR in 2011. Following the farmers' routine premilking preparations, 624 cows of the 1,199 cows were randomly selected for bacterial culture preceded by presampling procedures. These procedures were: cleaning of udder teats, removing the first streams of milk, and 70% alcohol teat disinfection. Data on parity, somatic cell counts, days in milk, daily milk yield, fat %, and protein % were extracted from the Danish Cattle Database, whereas energy-corrected milk was calculated based on the latter 3. The within-herd prevalence of intramammary infections with Staph. aureus was 31%, ranging from 16 to 48% in the 6 herds. Univariable analysis showed that the presampling procedures, somatic cell counts, energy-corrected milk, and days in milk were significantly associated with PCR-positivity, whereas parity was not significant. A multi-variable model with herd as random effect showed that presampling procedures decreased the chance of being PCR-positive to 0.75 (95% CI; 0.58-0.97) compared with cows where the presampling procedures were not carried out. In conclusion, presampling procedures decreased the cow's chance of being PCR-positive to Staph. aureus. Presampling procedures appeared to improve the specificity of PCR for detection of Staph. aureus by reducing false positives through destruction of Staph. aureus bacteria colonizing or contaminating the teat skin, orifice, and canal. Random herd effects accounted for only 8.9%, indicating that the cluster effect due to herd management on the PCR positivity to Staph. aureus was virtually negligible.