Bovine Spongiform Encephalopathy (BSE) and Variant Creutzfeldt-Jakob (CJD) 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 incubation periods typically measured in years. Transmission of the BSE agent to humans, leading to vCJD, is believed to occur via ingestion of cattle products contaminated with the BSE agent; however, the specific foods associated with this transmission are unknown. 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, suggesting that these tissues represent the highest risk of transmission.

Occurrence

From 1995 through August 2004, 147 human cases of vCJD were reported in the United Kingdom (UK), 7 in France, and 1 each in Canada, Ireland, Italy, and the United States. The patients from Canada, Ireland, and the United States had lived in the UK during a key exposure period of the UK population to the BSE agent. By year of onset, the incidence of vCJD in the UK appears to have peaked in 1999 and to have been declining thereafter. However, the future pattern of this epidemic remains uncertain.

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 European countries reporting at least one indigenous BSE case increased from 4 to18 through 2001. During 2001-2003, three countries outside Europe (Canada, Japan, and Israel) reported their first indigenous BSE cases, and others followed.

The reported BSE incidence rates, by country and year, are available on the Internet website of the Office International des Epizooties and new information is being generated on a regular basis. (http://www.oie.int/eng/info/en_esbincidence.htm).

The identification in 2003 of a BSE case in Canada, and the subsequent identification later that year of a BSE case in the United States that had been imported from Canada led to the concern that indigenous transmission of BSE may be occurring in North America. Safeguards to minimize the risk for human exposure to BSE have been implemented in the United States by the Department of Agriculture (http://aphis.usda.gov/lpa/issues/bse_testing/plan.html).

Transfusion of blood contaminated with the vCJD agent is believed responsible for a few cases of disease in the UK. This prompted the US Food and Drug Administration to publish guidance outlining a geography-based donor deferral policy to reduce the risk of bloodborne transmission of vCJD in the United States. This guidance document included an appendix that listed European countries with BSE or a possible increased risk of BSE for use in determining blood donor deferrals. One deferral criterion was living cumulatively for 5 or more years in continental Europe from 1980 to the present. http://www.aphis.usda.gov/NCIE/country.html#BSE.

Risk for Travellers

The current risk of acquiring vCJD from eating beef (muscle meat) and beef products produced from cattle in countries with at least a possibly increased risk of BSE cannot be determined precisely. Nevertheless, in the UK, the current risk of acquiring vCJD from eating beef and beef products appears to be extremely small, perhaps about 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 if BSE-related, public health control measures are being well implemented. Among many uncertainties affecting this determination are the incubation period between exposure to the infective agent and onset of illness, 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. Despite the exceedingly low risk, the US blood donor deferral criteria in effect as of September 2004 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.

Prevention

Public health control measures, such as surveillance, culling sick animals, or banning specified risk materials, have been instituted in many countries, particularly in those with indigenous cases of confirmed BSE, in order to prevent potentially BSE-infected tissues from entering the human food supply. The most stringent of these control measures, including a programme that excludes all animals >30 months of age from the human food and animal feed supplies, 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.

To reduce any risk of acquiring vCJD from food, travellers 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 like burgers and sausages), that might have a reduced opportunity for contamination with tissues that may harbor the BSE agent. These measures, however, should be taken with the knowledge of the very low risk of transmission as defined above. Milk and milk products from cows are not believed to pose any risk for transmitting the BSE agent.

Treatment

As of September 2004, treatment of prion diseases remains supportive; no specific therapy has been shown to stop the progression of these diseases.

Bibliography
  • Belay ED, Schonberger LB. The public health impact of prion diseases. Annu Rev Public Health 2005; 26:in press.
  • Centers for Disease Control and Prevention. Bovine spongiform encephalopathy in a dairy cow — Washington state, 2003. MMWR 2003;52:1280-1285.
  • Llewelyn CA, Hewitt PE, Knight RSG, et al. Possible transmission of variant Creutzfeldt-Jakob disease by blood transfusion. Lancet 2004;363:417-421.
  • Peden AH, Head MW, Ritchie DL, et al. Preclinical vCJD after blood transfusion in a PRNP codon 129 heterozygous patient. Lancet 2004;264:527-529.
  • Will RG, Alpers MP, Dormont D, Schonberger LB. Infectious and sporadic prion diseases. In: Prusiner SB, ed. Prion Biology and Diseases, 2nd Ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, New York, 2004: 629-671.

-Lawrence B. Schonberger, Ermias D. Belay, James J. Sejvar

© Medic8 ® All Rights Reserved.