The objective of this study was to determine the factors that were associated with the cure of subclinical mastitis caused by Staphylococcus aureus treated during lactation. One hundred forty-three quarters that were infected with S. aureus were available from a number of treatment trials. Analysis of these data showed that the most important factors associated with cure were age of the cow, somatic cell count at the time of treatment, presence of the infection in the front quarters, and stage of lactation. Other factors, such as number of infected quarters per cow and sensitivity or resistance of the strain to penicillin, were not significant. Because of the relatively low probability of cure, it is important to know risk factors for cure and, hence, to choose cows for treatment with great care. Using these data, a prediction equation was developed to determine the cure rate of subclinical mastitis caused by S. aureus when treated during lactation.

Data from five dry cow antibiotic therapy trials were analyzed. Records were only included for cows with > or = 1 culture-positive quarters that were subclinically infected with Staphylococcus aureus. Data for 406 quarters affected with S. aureus from 283 cows on 73 farms were analyzed for quarters, cows, and herds. The probability of cure of an infected quarter decreased when SCC increased, when another quarter was infected in the same cow, when the infection was in a hind quarter, and when the percentage of samples that were positive for S. aureus was higher before drying off. Variables predicting complete bacteriological cure of a cow were log SCC, age of the cow, and the number of infected quarters. The probability of a cure decreased as SCC increased. The probability of cure decreased as age increased, and cows with more than one quarter infected were .57 times less likely to be cured than cows with 1 infected quarter.

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Drying off cows with antibiotics is carried out to cure subclinical mastitis and to prevent new infections. The cure rate of pathogenic streptococci in the udder is about 90% in the Netherlands. For S. aureus this is much lower (average about 60%). Various factors, which affect this rather low cure rate are already discussed (4,5). In this paper the results are presented of a detailed statistical analysis of the cure rate of 406 S. aureus infected quarters from 283 cows on 73 farms. French and German summaries on PDF