Failure of exit-race teat spraying to control Corynebacterium bovis colonisation.

When an automated exit-race teat sprayer replaced a conventional teat dip cup for the application of a disinfectant containing 0.5 per cent iodine, there was an increase in the level of intramammary infection by Corynebacterium bovis at drying off from approximately 25 per cent of quarters to approximately 75 per cent of quarters. When the peak level of infection had been reached half of the clinical mastitis in the herd was caused by C bovis, and these were recurrent and chronic infections. There was some evidence that the increase in C bovis infection increased the bulk milk cell count. There were no changes in the rates of infection by major pathogens or by coagulase-negative staphylococci, another important secondary pathogen. The reintroduction of teat dipping rapidly reduced the rate of mastitis infection and the level of infection was reduced to approximately 20 per cent of quarters in about 12 months.