Effect of automatic cluster flushing on the concentration of Staphylococcus aureus in teat cup liners

摘要

Automatic flushing of milking clusters betweenmilking events is a control measure aimed at reducingtransmission of mastitis pathogens from infectious milkto a subsequently milked cow. We evaluated the effectof flushing with cold water and flushing with watercontaining peracetic acid (PAA) on the concentrationof Staphylococcus aureus in teat cup liners. Thirty-twoclusters in a swing-over milking parlor (Dairymaster,Causeway, Ireland) were subjected to a simulated milkingwith S. aureus–contaminated milk. Sixteen clusterswere not flushed (controls), whereas 8 clusters wereflushed with cold water (966 ± 32 mL) and 8 clusterswere flushed with water containing PAA (200 mL/mL). A random teat cup in each cluster was sampledby rinsing with a phosphate buffer. Teat cup sampleswere cultured on the day following collection on Baird-Parker plates to determine the concentration of S.Aureus. In teat cup samples from control clusters, themean concentration of S. aureus was 2.8 × 10~5 cfu/mL.The concentration of S. aureus was zero in teat cupsamples from clusters flushed with cold water. In teatcup samples from clusters flushed with water containingPAA, the concentration of S. aureus was in generalreduced compared with control clusters, but S. aureuswas not removed completely. However, the automaticcluster flushing did not function properly when clusterswere flushed with water containing PAA; thus, resultsreflected the effect of inadequate function rather thanthe effect of adding disinfectant to the flushing water.Before the main study, we conducted a pilot study toevaluate whether teat cup sampling with swabs andsample analysis with quantitative PCR were appropriatemethods for the main study. Specifically, we evaluatedthe effect of swab sample mass on detection of S.Aureus by quantitative PCR in the laboratory, Further,we compared PCR and bacterial culture on detectionof S. aureus in a suspension following disinfection of thesuspension with PAA. We sampled 20 identical S. aureussuspensions for culture and PCR by swabs beforeand after disinfection with PAA. Swab sample mass wasdetermined by differential weighing and contributed to46% of the variation observed in detection of S. aureusby PCR. Following disinfection with PAA, S. aureusremained detectable by PCR, although culturabilityceased. Based on these results, we sampled teat cupsin the main study with a buffer rinse and quantifiedS. aureus in the samples by bacterial culture. We concludedthat automatic cluster flushing with cold waterwas effective in removing S. aureus from teat cup linersand that addition of PAA was therefore not necessary.