Volume 13, Number 8—August 2007
Source of Variant Creutzfeldt-Jakob Disease outside United Kingdom
We studied the occurrence of variant Creutzfeldt-Jakob disease (vCJD) outside the United Kingdom in relation to the incidence of indigenous bovine spongiform encephalopathy (BSE) and to the level of live bovines and bovine products imported from the UK during the 1980s and the first half of the 1990s. Our study provides evidence that a country’s number of vCJD cases correlates with the number of live bovines it imported from the UK from 1980 to 1990 (Spearman rank correlation coefficient [rs] 0.73, 95% confidence interval [CI] 0.42–0.89, p<0.001). Similar correlations were observed with the number of indigenous BSE cases (rs 0.70, 95% CI 0.37–0.87, p = 0.001) and carcass meat imported from the UK from 1980 to 1996 (rs 0.75, 95% CI 0.45–0.89; p<0.001) Bovine imports from the UK may have been an important source of human exposure to BSE and may have contributed to the global risk for disease.
In 1996 a new variant of Creutzfeldt-Jakob disease (vCJD) was described in the United Kingdom (1). By September 2006, 196 cases had been reported worldwide; most (162, 83%) occurred in the UK. Laboratory and epidemiologic studies provide strong circumstantial evidence for a causal link between vCJD and the bovine spongiform encephalopathy (BSE) epizootic in cattle (2,3) with the most likely route of primary human infection being through dietary exposure to highly infected bovine tissues (3).
In recent years, vCJD has been identified in a number of European countries with indigenous outbreaks of BSE, including 20 cases of vCJD in France, 4 in Ireland, 2 in the Netherlands, and single cases in Portugal, Italy, and Spain. A growing number of cases of vCJD have also been identified in countries outside Europe that have minimal incidence of BSE, including Japan, the United States, and Canada, and also in Saudi Arabia, a country in which BSE has not been identified (4). Several of the non-UK patients, notably 2 Irish, 1 Canadian, 2 American, and possibly the Japanese patient, may have been infected during periods of residence in the UK, but most cases (28 of 34) occurring outside the UK were in persons who had never visited the UK. Although vCJD cases have occurred in countries with very low incidence of BSE, no cases have been reported in countries with higher incidence of BSE, such as Switzerland (460 reported cases of BSE) and Germany (395 cases) (5). This fact raises questions as to the source of infection in the vCJD cases outside the UK. Were the patients exposed to indigenous cases of BSE or to infected bovine material imported from the UK? An analysis of infection risk in France suggests that the most likely source of vCJD in that country is imported infected material from the United Kingdom (6). In the Republic of Ireland, the transmission of BSE to humans was estimated to be equally likely from indigenous BSE or from UK imports (7). Given the apparently weak association between the occurrence of indigenous BSE and vCJD in some countries, we studied the occurrence of vCJD cases outside the UK in relation to the level of imported bovines and bovine products from the UK during the 1980s and the first half of the 1990s.
A European Union (EU) surveillance network, established in 1993, ensures prospective surveillance for CJD by using standard methods (8) in 18 European countries and the United States, Canada, Israel, and Australia. By August 24, 2006, 32 cases of vCJD had been identified in these countries (excluding the UK) (Table 1). Two more cases had been identified in countries outside the surveillance network, Japan and Saudi Arabia. Case-patients in Canada (n = 1), the United States (n = 2), Ireland (n = 2), and possibly Japan (n = 1) were considered as likely to have been infected during periods of residence in the UK.
Data on indigenous BSE cases detected by both passive and active surveillance were obtained from the World Organization for Animal Health webpage (www.oie.int/eng/info/en_esbmonde.htm) (5). The number of live cattle