A hierarchical trend model for bovine virus diarrhoea virus (BVDV) sero-conversion in Norwegian dairy herds from 1993 through 1997

The Norwegian bovine virus diarrhoea (BVD) control-and-eradication program performed annual testing of the national cattle population since its first screening in January 1993. A bulk-tank milk ELISA antibody test was used fo r the initial screening of dairy herds. Based on the annual bulk-tank milk- test result, a binary variable denoting herd sero-conversion (a surrogate m easure for incidence) was created. The count of herds with sero-conversion in each community was regressed on year and the initial herd-sero-prevalence for each community in a Poisson h ierarchical trend model - modelling the risk of sero-conversion. By using t his modelling approach, estimates of trend specific for each hierarchical l evel of organisation were included in the trend model (community, veterinar y district and county) could be estimated. The main BVD trend showed a steadily declining sero-conversion risk. The co mmunities in the highest herd-sero-prevalence quartile in 1993 continued to have the highest sero-conversion risk throughout the study period - decrea sing from an average predicted sero-conversion risk in 1993 of 0.12 (95% CI ; 0.10, 0.13) to 0.02 (0.007, 0.04) in 1997. There was an expressed variati on in the level of sero-conversion for all the three hierarchical levels, b ut the trend only varied at the lowest level (community). (C) 2000 Publishe d by Elsevier Science B.V.