aTINE Norwegian Dairies BA, PO Box 9051 Grønland, N-0133 Oslo, Norway bSection of Preventive Veterinary Medicine, Department of Large Animal Clinical Sciences, The Norwegian School of Veterinary Sciences, PO Box 8146, Dep., N-0033 Oslo, Norway cDepartment of Population Medicine, Ontario Veterinary College, University of Guelph, Guelph, Ont., Canada N1G 2W1 dSection of Food Hygiene, Department of Pharmacology, Microbiology and Food Hygiene, The Norwegian School of Veterinary Sciences, PO Box 8146, Dep., N-0033 Oslo, Norway
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 for the initial screening of dairy herds. Based on the annual bulk-tank milk-test result, a binary variable denoting herd sero-conversion (a surrogate measure 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 hierarchical trend model — modelling the risk of sero-conversion. By using this modelling approach, estimates of trend specific for each hierarchical level of organisation were included in the trend model (community, veterinary district and county) could be estimated.
The main BVD trend showed a steadily declining sero-conversion risk. The communities in the highest herd-sero-prevalence quartile in 1993 continued to have the highest sero-conversion risk throughout the study period — decreasing 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 variation in the level of sero-conversion for all the three hierarchical levels, but the trend only varied at the lowest level (community).
This article was published in Preventive Veterinary Medicine, 47, Paul S. Vallea,b, S. Wayne Martinc and Eystein Skjerved, A hierarchical trend model for bovine virus diarrhoea virus (BVDV) sero-conversion in Norwegian dairy herds from 1993 through 1997, 39-52, Copyright Elsevier 2000.