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

Abstract

The objectives of this study were first to describe the pattern of the epidemic of bovine spongiform encephalopathy (BSE) in Great Britain in terms of the temporal change in the proportion of all cattle holdings that had experienced at least one confirmed case of BSE to June 30, 1997, and secondly to identify risk factors that influenced the date of onset of a holding's first confirmed BSE case. The analyses were based on the population of British cattle at risk, derived from agricultural census data collected between 1986 and 1996, and the BSE case data collected up to June 30, 1997. The unit of interest was the cattle holding and included all those recorded at least once on annual agricultural censuses conducted between June 30, 1986, and June 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 onset date. Univariate and multivariate survival analysis techniques were used to describe the temporal pattern of the epidemic. The BSE epidemic in Great Britain started in November 1986, with the majority of affected holdings 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 (specifically 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 great a monthly hazard of having a BSE index case as holdings in Scotland. After adjusting for the effect of region and type of holding, holdings with more than 53 adult cattle had 5.91 (95 per cent CI 5.62 to 6.21) times as great 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 holdings. These analyses show that there were different rates of onset in different 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 growth of the epidemic followed essentially the same pattern in each region of the country, with modest temporal lags between them. The control measures imposed in 1988 and 1990 brought the expansion of the epidemic under control, although the rate of progress was slowed by those regions where the effectiveness of the control methods took some time to take full effect.

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

“…Secondly, no significant difference was found in the prevalence of Prionics positive cattle between cattle from dairy herds and those from beef suckler herds in the tested population [11]. This finding was in contradiction with British data since Wilesmith [18] found that the hazard ratio of being an affected herd was three times higher for dairy herds than for beef suckler herds, and 1.7 times higher for mixed herds, in agreement with previous studies [14,17].…”

Case-control study on feed risk factors for BSE cases born after the feed ban in France

“…So, in order to define a study sample in which cases and controls have been submitted to a global comparable level of risk, they were matched on their birth cohort. The production type of cattle, dairy or beef, was also considered as a potential confounding factor [13,23]. Since the farmer practices concerning the cattle feeding were related 512 N. Jarrige et al to the production type, it was decided not to pair cases and controls on this factor to avoid a possible overmatching bias but to consider it as a potential confounder in the model.…”

“…This tends to prove that the effect of the production type was due to the factors related to feedstuff consumption. Concerning the herd size, it was introduced in the final model as an offset, considering that large herds have a statistically greater chance to have a case than small ones [23] and that farmer practices also depend on the herd size. The process of inclusion of the bovines in the sample study was made so as to know the reason of each exclusion.…”

Estimating the trend of the French BSE epidemic over six birth cohorts through the analysis of abattoir screening in 2001 and 2002

“…It helped to keep the study as powerful as possible. However, we made an adjustment for the production type of cattle which is highly related to the BSE risk [15,26], first because the culling rates differ between the two types, second because we cannot exclude that the BSE incubation period varies between dairy and beef cows. Another modelling approach might have been to build a single model with the entire slaughtered population and incorporating interaction variables between the year of test of birth cohorts to account for the postulate that the effect of the year of test depended on the birth cohorts being tested; comparing the odds of being a cow of a given birth cohort slaughtered in 2001 to those of a cow of the following birth cohort slaughtered in 2002 would have given the estimates of the risk increase/ reduction between cohorts.…”