Bovine Spongiform Encephalopathy and Variant Creutzfeldt-Jakob Disease

Description

Since 1996, strong evidence has accumulated for a causal relationship between ongoing outbreaks in Europe of a disease in cattle called bovine spongiform encephalopathy (BSE, or “mad cow disease”) and a disease in humans called variant Creutzfeldt-Jakob disease (vCJD). Both disorders, which are caused by an unconventional transmissible agent, are invariably fatal brain diseases with unusually long incubation periods, measured in years. The specific foods associated with transmission of the agent from cattle to humans are unknown; however, bioassays have identified the BSE agent in the brain, spinal cord, retina, dorsal root ganglia, distal ileum, and bone marrow of cattle experimentally infected by the oral route.

Occurrence

From 1995 through June 2002, 124 human cases of vCJD were reported in the United Kingdom (UK), 6 in France, and 1 case each in Ireland, Italy, and the United States. The patients from Ireland and the United States had both lived in the UK for >5 years during the BSE epidemic there. From 1986 through 2001, >98% of BSE cases worldwide were reported from the UK, where the disease was first described. During this same period, the number of countries reporting at least one indigenous BSE case increased: 4 such European countries through 1993, 8 through 1998, and 18 through 2001. As of July 2002, two countries outside Europe—Japan and Israel—reported their first indigenous BSE cases. The proportion of the annual total number of BSE cases worldwide reported outside the UK increased to >25% in 2000 and >45% in 2001. This increase reflected the declining large (>182,000 total cases) epidemic of BSE in the UK and the increasing number of other countries with improved surveillance and higher rates of BSE.

In 2001, only two countries, the UK and Portugal, reported a BSE incidence rate of >100 indigenous cases per million cattle >24 months of age. The reported BSE rates for other countries were Republic of Ireland, 62 cases per million; Switzerland, 49 per million; Belgium, 28; Spain, 24; Germany, 20; France, 20; Slovakia, 18; Italy, 14; and the Netherlands, 10. The reported rates for Denmark, Slovenia, Greece, the Czech Republic, Finland, Japan, and Austria ranged from 1 to 7 cases per million. The numbers of reported BSE cases and incidence rates, by country, are available on the Internet website of the Office International des Epizooties, at http://www.oie.int/eng/info/en_esb.htm.

In addition to the countries with confirmed BSE in 2001, by June 2002 the European Union's Scientific Steering Committee had classified 14 other countries as likely to have BSE or confirmed as having BSE at a lower level. These countries were Albania, Bulgaria, Croatia, Cyprus, Estonia, Hungary, Latvia, Lithuania, Luxembourg, Poland, Romania, San Marino, Slovenia, and Turkey.

In January 2002, the U.S. Food and Drug Administration published guidance to reduce the theoretical risk of transmission to humans by blood and blood products. This document included a list of European countries with BSE or possible increased risk of BSE that could be used for determining blood donor referrals. One deferral criterion was living cumulatively for 5 years or more in Europe from 1980 to the present. Among European countries listed were Bosnia-Herzegovina, Liechtenstein, Macedonia, Norway, Sweden, and Yugoslavia. Information is being generated rapidly on BSE issues, and updated sources should be consulted

Risk for Travelers

The current risk of acquiring vCJD from eating beef (muscle meat) and beef products produced from cattle in countries with at least possibly increased risk of BSE cannot be determined precisely. Among many uncertainties affecting this determination are the incubation period between exposure to the infective agent and onset of illness, the ultimate number and age distribution of vCJD cases in the UK that will result from earlier BSE exposures, the sensitivities of each country's surveillance for BSE and vCJD, the compliance with and effectiveness of public health measures instituted in each country to prevent BSE contamination of human food, and details about cattle products from one country distributed and consumed elsewhere. Nevertheless, in the UK, the current risk of acquiring vCJD from eating beef and beef products appears to be extremely small, approximately 1 case per 10 billion servings. In the other countries of the world, this current risk, if it exists at all, would not likely be any higher than that in the UK, particularly if BSE-related, public health control measures are being well implemented. Such measures would be especially important in a country such as Portugal, which like the UK has a relatively high incidence rate of reported BSE. Despite the exceedingly low risk, the U.S. blood donor deferral criteria focus on the time (cumulatively 3 months or more) that a person lived in the UK from 1980 through 1996, whereas for the rest of Europe the criteria focus on the time (cumulatively 5 years or more) that a person lived in these countries from 1980 through the present. The lack of a time limit on the latter criterion reflects uncertainties related to human food chain protection in some countries outside the UK, as well as concerns about overly complicating the screening questions posed to prospective U.S. blood donors.

Prevention

Public health control measures, such as surveillance, culling sick animals, or banning specified risk materials, have been instituted in European countries to prevent potentially BSE-infected tissues from entering the human food chain. The most stringent of these control measures, including a program that excludes all animals >30 months of age from the human food and animal feed chains, have been applied in the UK and appear to be highly effective. In June 2000, the European Union Commission on Food Safety and Animal Welfare strengthened the European Union's BSE control measures by requiring all member states to remove specified risk materials from animal feed and human food chains as of October 1, 2000; such bans had already been instituted in most member states. These control measures include banning the use of mechanically recovered meat from the vertebral column of cattle, sheep, and goats for human food and BSE testing of all cattle >30 months of age destined for human consumption.

To reduce any risk of acquiring vCJD from food, travelers to Europe or other areas with indigenous cases of BSE may consider either avoiding beef and beef products altogether or selecting beef or beef products, such as solid pieces of muscle meat (rather than brains or beef products such as burgers and sausages), that might have a reduced opportunity for contamination with tissues that may harbor the BSE agent. Milk and milk products from cows are not believed to pose any risk for transmitting the BSE agent.

— Lawrence Schonberger, Ermias Belay