Temporal aspects of the epidemic of bovine spongiform encephalopathy in Great Britain: holding-associated risk factors for the disease

The objectives of this study were first to describe the pattern of the epid emic of bovine spongiform encephalopathy (BSE) in Great Britain in terms of the temporal change in the proportion of all cattle holdings that had expe rienced at least one confirmed case of ass to lune 30, 1997, and secondly t o identify risk factors that influenced the date of onset of a holding's fi rst confirmed BSE case. The analyses were based on the population of Britis h cattle at risk, derived from agricultural census data collected between 1 986 and 1996, and the BSE case data collected up to lune 30, 1997 The unit of interest was the cattle holding and included all those recorded at least once on annual agricultural censuses conducted between lune 30, 1986, and lune 30, 1996. The outcome of interest was the date on which clinical signs were recorded in a holding's first confirmed case of BSE, termed the BSE o nset date. Univariate and multivariate survival analysis techniques were us ed to describe the temporal pattern of the epidemic. The BSE epidemic in Gr eat Britain started in November 1986, with the majority of affected holding s having their BSE onset date after February 1992. After adjusting for the effect of the size and type of holding; holdings in the south of England (s pecifically those in the Eastern, South east and South west regions) had 2. 22 to 2.43 (95 per cent confidence interval [CI] 2.07 to 2.58) times as gre at a monthly hazard of having a BSE index case as holdings in Scotland. Aft er adjusting for the effect of region and type of holding, holdings with mo re than 53 adult cattle had 5.91 (95 per cent CI 5.62 to 6.21) times as gre at a monthly hazard of having a BSE index case as holdings with seven to 21 adult cattle. Dairy holdings had 3.06 (95 per cent CI 2.96 to 3.16) times as great a monthly hazard of having a BSE index case as beef suckler holdin gs. These analyses show that there were different rates of onset in differe nt regions and in holdings of different sizes and types, that the epidemic was propagated most strongly in the south of the country, and that the grow th of the epidemic followed essentially the same pattern in each region of the country, with modest temporal lags between them. The control measures i mposed in 1988 and 1990 brought the expansion of the epidemic under control , although the rate of progress was slowed by those regions where the effec tiveness of the control methods took some time to take full effect.