The epidemiology of bovine spongiform encephalopathy in the Republic of Ireland before and after the reinforced feed ban

Abstract

Bovine spongiform encephalopathy (BSE) is a prion disease spread by the inclusion in cattle feed of meat and bone meal made from cattle infected with the BSE agent. In the Republic of Ireland, a reinforced feed ban on mammalian meat and bone meal (MMBM) was introduced on 17th October 1996 to stop further infection of cattle. Between then and July 2010, 44 cases of BSE from 40 herds have been born, termed "born after the reinforced ban" or "BARB" cases. The objectives of this project were: (a) to describe the epidemiology of these BARB cases, (b) to determine area-level risk factors for BSE herds and how they related to the stage of the BSE epidemic, and (c) to evaluate whether the spatial pattern of BSE cases was non-random and had changed over time. The BSE epidemic was divided into three phases: cases born prior to 1991, born 1991-October 1996 and BARB cases. To determine the area level risk factors for BSE herds, a case-control study was conducted for each phase of the epidemic. We selected four control herds for each herd with one or more BSE cases. A grid of hexagons of 10 km diameter was created covering the territory of the Republic of Ireland and BSE herds and control herds were assigned to a hexagon. The numbers of cattle herds, dairy herds, piggeries and poultry holdings within the hexagons containing these case and control herds were enumerated. To evaluate the spatial pattern of BSE cases, standardised mortality ratios were calculated for each hexagon, and Oden's Ipop was used to investigate clustering. The descriptive analysis showed "feeding of concentrates" as the only common factor to all BARB cases for which information existed. The case-control study identified being a dairy herd as a risk factor during the pre-1991 phase of the BSE epidemic. Dairy herd type, a large proportion of local herds which were dairy and large numbers of piggeries and poultry holdings locally were also risk factors during the 1991-1996 phase. For the post-October 1996 phase (BARBs), dairy herd type and large numbers of other herds locally were risk factors. The spatial pattern of BSE cases changed over the three phases of the epidemic and was non-random, with evidence of clustering. The evidence supports the hypothesis that BARB cases do not arise spontaneously but rather are caused by the same food-borne infectious route as other BSE cases.