Salmonella dublin in Danish dairy herds: Frequency of change to positive serological status in bulk tank milk ELISA in relation to serostatus of neighbouring farms

Bulk tank milk from 1,429 herds were collected in 3 rounds from 19 differen t geographic areas. The milk samples were tested by use of indirect LPS-ELI SA procedure to detect Salmonella dublin antibodies. From the obtained OD-v alues herd seroprevalence in the given area was determined and GR-scores ca lculated for each herd by addition of the number of positive sampling round s by the 5 geographically closest neighbour herds. In the 19 different area s the calculated prevalence ranged from 0.01 to 0.41. Totally 3,697 GR-scor es were given. The mean GR-scores in the areas ranged from 0.0 to 6.5. High er GR-scores were found in herds changing to seropositive status compared w ith herds seronegative throughout the study period. The results indicate th at the risk for a dairy herd to receive S. dublin infection increases with the disease status among the nearest neighbours and with the prevalence of seropositive herds in the geographic area.