Prions and transmissible spongiform encephalopathies (TSEs)

Prions are proteins thought to come from neurological tissues in animals. They are not cellular organisms or viruses. In their normal noninfectious state, these proteins may be involved in cell-to-cell communication. When these proteins become abnormally shaped (i.e., prions), they are able to transform molecules of the normally shaped protein to the abnormal prion configuration. This process is repeated numerous times until the number of abnormally shaped molecules causes overt illness. When consumed in the diet, prions are thought to be absorbed into the body where, again, they begin the process of changing their normal protein counterparts into prions. The specific prions of interest in disease and their normally configured proteins are those found in mammals. However, similar proteins occur in other organisms, from chickens to yeasts.

Prions are associated with a group of diseases called Transmissible Spongiform Encephalopathies (TSEs). In humans, the illness suspected of being foodborne is variant Creutzfeldt-Jakob disease (vCJD) . The human disease vCJD and the cattle disease, bovine spongiform encephalopathy (BSE), also known as "mad cow" disease , appear to be caused by the same agent. Other similar but not identical TSE diseases exist in animals, but there is no known transmission of these to humans. Included among these is chronic wasting disease (CWD) of deer and elk, and the oldest known of these diseases-scrapie-which occurs in sheep and goats . No early acute clinical indications for TSEs have been described. After an extended incubation period of years, these diseases result in irreversible neurodegeneration that becomes the cause of death.

How are TSEs diagnosed?

The most reliable means for diagnosing any TSE is the microscopic examination of brain tissue-a post-mortem procedure. Preliminary diagnoses of vCJD are based on patient history, clinical symptoms, electroencephalograms, and magnetic resonance imaging of the brain.

What foods are associated with TSEs?

The major concern for consumers is the potential contamination of meat products by central nervous system tissue (brain and spinal cord) during routine slaughter. This indirect intake of high-risk tissues may have been the source of human illnesses in the United Kingdom and elsewhere. Bovine meat (if free of central nervous system tissue) and milk have shown infective potential in test animals. Gelatin, derived from the hides and bones of cattle, appears to be very low risk , especially when produced from materials originating in countries free of BSE. Based upon many studies, scientists have concluded that forms of CJD other than vCJD are not associated with food consumption.

How common are TSEs?

There are no reported human cases of vCJD or bovine cases of BSE in the United States.. In the United Kingdom, there have been 94 human cases of suspected or confirmed vCJD from 1993, when the illness was first recognized, through February 2001. Since 1986, more than 180,000 cases of BSE have occurred there in cattle, particularly dairy herds. The feeding of rendered TSE-infected animal by-products to cattle is believed to have caused the epidemic of BSE. Practices such as this have now been prohibited, resulting in a dramatic decline in the number of cases.

What are the symptoms vCJD?

Cases of vCJD usually present with psychiatric problems, such as depression. As the disease progresses, neurologic signs appear-unpleasant sensations in the limbs and/or face. There are problems with walking and muscle coordination. Sometimes, late in the course of the disease, victims become forgetful and then experience severe problems with processing information and speaking. Uncontrolled muscle twitching and death follow.

Who is susceptible to vCJD?

All cases of vCJD to date have occurred in individuals of a single human genotype that is methionine homozygous at codon 139. About 40% of the total human population belongs to this methionine-methionine homozygous state. The susceptibility of other genotypes is not yet known.

How are prions detected in food?

No practical detection or inactivation methods exist, at present. The abnormally shaped prions are resistant to most heat and chemical treatments. Decontamination methods are so drastic that food subjected to these processes generally becomes inedible. Consequently, the key to food protection is obtaining meat from animals not infected with BSE and protecting against contamination of food with brain and spinal cord tissue.