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 f rom 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 c ell counts of >1 million cells per mi. A second method was based upon agree ment between cases reported through the regular disease-reporting system an d cases reported on a management questionnaire. Depending upon the method o f 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 b ecause of documented non-reporting behavior (screening) resulted in a prepo nderance 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. (C) 2001 Elsevier Science B.V. All rights reserved.