[The occurrence of hardjo-positive dairy cows in the Northern Netherlands. A prospective serological survey].

More than 5000 lactating cows from 82 farms, in the northern part of the Netherlands, were serologically examined for antibodies to Leptospira hardjo. Hardjo-positive cows were found on 52% of the farms with a mean incidence of 23% positive animals. Considerable differences were observed in the percentages hardjo-positive cows per region (7-30%), as well as in the percentages positive cows per positive farm (22-46%). With the exception of one region (14%) the percentages of hardjo-positive farms per region hardly differed (average 58%). In addition to the animal survey 488 sera of men working and/or living on cattle-farms were also examined. Twenty-six persons were hardjo-positive. Most of these persons (circa 90%) milked regularly. All seropositive persons were found on farms with hardjo-positive cattle. Results from an inquiry on circa 200 farms indicated, that hardjo-infections probably occur with a higher frequency on larger farms. Finally the results from this survey confirm the observation in the literature, that symptoms of hardjo-infections are more frequently found in the autumn.