Vet. Res. 34 (2003) 185-192
DOI: 10.1051/vetres:2002065

How the surveillance system may bias the results of analytical epidemiological studies on BSE: prevalence among dairy versus beef suckler cattle breeds in France

Christian Ducrot^a, Pascal Roy^b, Eric Morignat^c, Thierry Baron^c and Didier Calavas<sup>c

a</sup>  Unité d'Épidémiologie Animale, INRA Theix, 63122 Saint-Genès-Champanelle, France
^b  Service de Biostatistique des Hospices Civils de Lyon, Centre Hospitalo-Universitaire Lyon-Sud, 69495 Pierre-Bénite Cedex, France
^c  Unité d'Épidémiologie, AFSSA Lyon, 31 av. T. Garnier, 69364 Lyon Cedex 07, France

(Received 20 June 2002; accepted 10 October 2002)

Abstract
Until recently, epidemiological studies on Bovine Spongiform Encephalopathy (BSE) were based on Mandatory Reporting Systems (MRS) of clinically suspect bovines only, but rapid diagnostic tests were validated in 1999 and are used for targeted surveillance in Switzerland, France and other countries, as a complementary and secondary tool. Data on 30491 cattle issued from a French pilot program targeted at cattle having died on the farm, subjected to euthanasia or sent for emergency slaughter, did not show any significant difference in BSE risk between dairy and beef suckler breeds. The data also revealed that part of the clinical cases of BSE escaped the MRS, which permitted to detect more dairy than beef suckler affected cattle compared to the targeted surveillance in the same period (from August to December 2000) and region (Bretagne, Pays de la Loire and Basse Normandie regions). Analyzing together the data of the targeted surveillance and mandatory reporting system programs with a non-conditional logistic regression, we found that the odds of a dead cow being a BSE case among all dead cattle was 3.2 times higher for dairy breeds compared to beef suckler breeds. This confirmed British findings but points out to the fact that considering either MRS or targeted surveillance data alone may possibly create biases in epidemiological studies on BSE.

Key words: BSE / epidemiology / bias / logistic regression / breed

Correspondence and reprints: Christian Ducrot Tel.: (33) 4 73 62 42 63; fax: (33) 4 73 62 45 48;
    e-mail: ducrot@clermont.inra.fr

© INRA, EDP Sciences 2003