Incidence of clinical mastitis in Danish dairy cattle and screening for non-reporting in a passively collected national surveillance system

Dairy herds from three counties in Denmark (n=2144) were monitored for a 1-year period to measure the incidence rate (incidence-density) of clinical mastitis. Two different screening methods were evaluated to remove herds from the database for which the herdsman and veterinarian were suspected of under-reporting the mastitis cases which had occurred. One method was based upon agreement between clinical mastitis reports and measures of somatic cell counts of >1 million cells per ml. A second method was based upon agreement between cases reported through the regular disease-reporting system and cases reported on a management questionnaire. Depending upon the method of calculation and screening method used, rates of clinical mastitis varied between 36 and 48 cases per 100 cow-years at risk. The exclusion of herds because of documented non-reporting behavior (screening) resulted in a preponderance of herds with zero or very low incidence of mastitis being removed from the database. Following screening, the frequency distribution of herd mastitis incidence rates became decidedly more symmetrically and normally distributed.