 likely to have been a more important source of infection than indigenous BSE (6). Thus, a proportion of cases observed to date outside the UK may have been acquired through imports from the UK rather than by the patients’ exposure to indigenous BSE. The inclusion in the analysis of the 6 non-UK case-patients thought to be infected in the UK reduced all 3 correlation coefficients, as one would expect if the supposition that they were infected in the UK is correct.
These findings come with several important caveats. First, they are based on small numbers of vCJD cases. Even a small number of additional non-UK cases in the future could alter the findings substantially. Furthermore, we have not performed multivariable analyses to determine which of the 3 exposures of interest were correlated with vCJD incidence because we were concerned that the small number of cases might lead to unreliable results. Second, the analyses of imports are based on UK Customs and Excise data, not all of which been validated by importing countries. Even if these data are reasonably accurate, the actual level of BSE infection entering the human food chain in importing countries cannot be estimated because many important unknown variables exist, such as the age distribution of imported live bovines, the age at slaughter of these animals, the culinary habits in each country, and the possibility that some of the UK imports may have been re-exported to other countries. Third, indigenous BSE-infected cattle entering the food supply will have gone undetected until the introduction throughout the EU of the active abattoir testing program for BSE in 2000/2001; even now, cattle in the early stages of infection are unlikely to be detected. Furthermore, the efficiency of BSE surveillance varies from country to country: underascertainment is likely in those countries with limited or no active testing programs.
It is noteworthy that none of the 162 UK patients with vCJD identified up to September 2006 were born after 1989, the year in which the specified bovine offal ban was introduced to minimize human exposure to BSE; however, 2 of the 34 non-UK vCJD case-patients were born after 1989. Measures equivalent to the UK ban on specified bovine offals were not introduced in many continental European countries until 2000.
Despite these caveats, our results suggest that, globally, imports from the UK may have been an important source of infection and for some countries may even have been the main source. If this is so, our findings have several implications. Past UK exports may be the major determinant of the current incidence of vCJD outside the UK. The greatest volume of these exports was to France, the Netherlands, and Ireland. Thus, initially at least, we might expect the largest number of vCJD cases to occur in these countries. However, exposure to BSE through imports from the UK ceased in 1996, and exposure to indigenous BSE is likely to have continued at some level until the measures introduced in 2000. Thus, the proportion of vCJD cases due to exposure to indigenous BSE may increase with time.
We thank our colleagues from the European and Allied Countries Collaborative Study Group of CJD (EUROCJD) and our international colleagues for the data on vCJD incidence included in this article.
The EUROCJD surveillance system is funded by the Directorate General for Health and Consumer Affairs (2003201) and the Neuroprion Network of Excellence (FOOD-CT-2004-506579). National CJD surveillance is supported in the Netherlands by the Dutch Ministry of Health Welfare and Sports. P.S.-J. was supported by the grant Wenceslao Lopez Albo from the Marques de Valdecilla Institute for Formation and Research of the Fundación Pública Marqués de Valdecilla.
Dr Sánchez-Juan is a consultant neurologist. Since 2004 he has been coordinating the Dutch CJD surveillance system at the Department of Epidemiology and Biostatistics, Erasmus MC, Rotterdam, the Netherlands. His main research interests are prion diseases and the genetic epidemiology of neurodegenerative diseases.
- Will RG, Iro