Impact of bovine subclinical mastitis and effect of lactational treatment

This thesis aimed to quantify the impact of subclinical mastitis in dairy cattle in the Netherlands and to explore the epidemiologic and economic effects of antimicrobial treatment of recently acquired subclinical mastitis during lactation. First, the occurrence of (sub)clinical mastitis was estimated in a one-year observational study. It was concluded that herds in the Netherlands varied substantially in their mastitis occurrence, indicating room for improvement of udder health. The relation between subclinical mastitis (indicated by a somatic cell count (SCC) ?200,000 cells/ml) and clinical mastitis was investigated next. Primiparae with a high composite SCC had a fourfold higher risk to develop clinical mastitis than primiparae with a low SCC. This was twofold higher for multiparae. Approximately 25% of clinical mastitis cases can be prevented when high SCC cows are prevented or removed from the dairy population. The therapeutic effects of antimicrobial treatment of recently acquired subclinical mastitis during lactation were studied in a randomized field trial in 40 Dutch dairy herds. Cows with recently acquired streptococci or staphylococci subclinical mastitis were randomly assigned antimicrobial treatment. Treated quarters had a higher bacteriological cure and decrease in SCC than control quarters. Evaluating the effect of duration of infection revealed that early treatment of Staph. aureus subclinical mastitis tended to be more effective than later treatment. Staph. aureus isolates obtained from the field trial were genotyped to determine whether strains that are commonly found in multiple host species differed in bacteriological cure from strains primarily associated with bovine mastitis. Bacteriological cure of strains associated with multiple host species was around 2.5 times higher compared with strains primarily associated with bovine mastitis. Host adaptation of Staph. aureus was found to be associated with known predictors of cure. Staph. aureus strains associated with multiple host species were more often penicillin-resistant. Additionally, penicillin-resistant, mastitis associated Staph. aureus strains had a higher SCC than other strains. The direct and indirect epidemiologic and economic effects of lactational treatment of recently acquired subclinical mastitis caused by contagious pathogens were studied using a dynamic and stochastic bio-economic model. Lactational treatment of contagious pathogens resulted in less clinical flare-ups, less transmission within the herd and lower annual costs of intramammary infections. Lactational treatment of subclinical mastitis must be accompanied by other management measures to decrease transmission of contagious pathogens within the dairy herd to be economically beneficial. Antimicrobial treatment of subclinical mastitis caused by environmental pathogens did not seem to be economically beneficial. A decision tree was developed to select herds and cows to most likely benefit from lactational treatment of recently acquired subclinical mastitis with antimicrobials