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BSE and vCJD

Bovine spongiform encephalopathy (BSE), also known as mad cow disease, is a transmissible spongiform encephalopathy that is responsible for the human neurodegenerative disorder known as variant Creutzfeldt-Jakob disease (vCJD). BSE first appeared in cattle in the United Kingdom (UK) in 1986. The origin of BSE is thought to be cattle feed containing meat & bone meal supplements contaminated by scrapie infected sheep carcasses. Similarly, BSE was transmitted to humans through consumption of beef products contaminated by infected neural tissue, such as hot dogs, sausages, lunchmeat, meat pies and various canned meat goods. BSE infections in the UK have decreased substantially since 1992 due to slaughter of infected animals and changes in animal feed production. However, the incidence of new cases appears to be increasing in other European countries. Spain has detected 15 cases since it began testing in November 2000. BSE has not occurred in the US or other countries that do not import live cattle, beef products, or livestock nutritional supplements from the UK.

Nearly a decade after the first recognized case of mad cow disease, the first cases of vCJD appeared in the UK. Characteristics atypical for CJD included young age of onset (16 - 29 years) as well as the neuropathologic finding of amyloid plaques in addition to spongiform encephalopathy. Molecular methods have confirmed the link between BSE and vCJD. The following table lists the year of onset of vCJD as of May 2001.

Year

UK

France

Ireland

1994

8

1

 

1995

10

 

 

1996

11

 

 

1997

14

 

 

1998

17

 

 

1999

24

2

1

2000

28

0

0

2001

16

1

0

Total

128

4

1

 

The amount of infected tissue ingested appears to play a role in transmission, however, genetic predisposition may also affect disease susceptibility. To date, all vCJD patients tested have been homozygous for methionine at codon 129 of the prion protein (PRNP) gene. Forty percent of the population has this genotype. Heterozygotes may become ill after longer incubation periods.

The incubation period of vCJD is not known. If it were 10 to 15 years, the earliest patients would have been infected in the early 1980’s, when BSE was still silently incubating in a small number of cattle. The large increase in human exposure to contaminated beef during the late 1980’s could lead to a parallel increase in vCJD cases during the next few years. If large numbers of infected persons are silently incubating the disease, the potential for human to human iatrogenic spread in the near future is very great. The disease could unknowingly be spread during invasive medical and surgical procedures and donation of organs, tissues and blood. No sensitive screening tests or effective instrument sterilization procedures currently exist.

The following examples illustrate the potential for worldwide spread of vCJD. The blood of three British donors who were subsequently diagnosed with vCJD has been distributed to 11 countries. Irish authorities announced in December 2000 that a polio vaccine administered in 1998 and 1999 contained albumin from a British vCJD victim. Most of the 83,500 doses of the vaccine had already been administered to infants. Brazil received 45,000 doses of contaminated albumin. Britain’s National Health Service has provided blood products to Dubai, India, Turkey, Brunei, Egypt, Morocco, Oman, Singapore and Russia. Turkish health officials believe that 840 vials of immunoglobulin were made from the blood of a British vCJD infected donor. The FDA recently discovered that five major drug companies were still making vaccines using fetal calf serum and meat broth from cattle in countries that have reported BSE outbreaks. The vaccines included polio, diphtheria, tetanus and anthrax. The drug companies have agreed to stop using these materials, but estimated that it would take at least one year to replace the potentially contaminated vaccines with reformulated products. Some dietary supplements have also been found to contain bovine byproducts imported from BSE-endemic countries. Eighty-seven cases of mad cat disease have been confirmed among UK felines, presumably caused by ingestion of cat food containing BSE-infected beef.

So far, no cases of vCJD have been diagnosed in the United States and the FDA has taken several measures to prevent future transmission. In June 1997, the use of most mammalian protein in animal feeds for cows, sheep and goats was banned. However, FDA still permits the feeding of mammalian protein to poultry, swine and domestic pets and cow blood can be added to all types of animal feed. In 1999, FDA mandated that blood centers must defer donors who spent greater than 6 months in the UK from 1980 through 1996. The 6 month period was adopted so that >80% of total US person-years in the UK would be excluded. The resulting 2 to 3% decrease in eligible blood donors was absorbed by blood centers without causing serious blood shortages. The FDA is currently considering issuing an expanded deferral to include all potential blood donors who have lived in the UK for 3 months or any European country for 6 months since 1980. This policy will eliminate approximately 8% of all current blood donors. If this occurs, severe blood component shortages, especially during summer months and holidays, are inevitable.

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