Subclinical Mastitis in Norwegian Dairy Cows

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Abstract
An epidemiological assessment of subclinical mastitis in a randomed cow population is described based on semiannual examinations of quarter samples from 328 commercial dairy herds in a 3-yr period. Each 3rd cow and 9th quarter in average suffered from subclinical mastitis. Almost 50% of the cases were nonspecific mastitis. Staphylococcus aureus was the main pathogen isolated from infectious mastitis. The average prevalence of S. aureus mastitis on quarter basis was 3.3%, while prevalence of all infectious mastitis on quarter basis was 6.5%. The mastitis prevalence varied related to lactation number (age), lactation stage and geographical origin of the herds. The distribution of all types of subclinical mastitis in the herds varied from 7%-56% on cow basis and the rates of S. aureus mastitis on quarter basis varied from < 1% to 14%. S. aureus mastitis was found more frequently on hind quarters than on front quarters. Approximately 81% of the cows that had S. aureus mastitis, suffered on 1 quarter, 15% on 2, 3% on 3 and 1% on all 4 quarters. When cows with 1 healthy front quarter were selected, the risk of finding S. aureus mastitis was 2.2% on the other front quarter and 3.2% on 1 of the hind quarters. Having cows with S. aureus mastitis on 1 front quarter, the possibility of finding S. aureus on 1 of the others increased .apprx. 7 times compared with the cows just mentioned. This trend was also unveiled for other types of infectious mastitis or for nonspecific mastitis.