Transmissible spongiform encephalopathies and food
Transmissible spongiform encephalopathies (TSEs) are fatal degenerative brain diseases which include BSE in cattle; scrapie in sheep; kuru, Creutzfeldt– Jakob disease (CJD), and new variant CJD (vCJD) in humans. They are characterized by the appearance in the brain of vacuoles – clear holes that give the brain a sponge-like appearance – from which the condi-tions derive their name.
Several theories have been proposed to explain the nature of the agents that cause TSE. Prusiner was awarded the Nobel Prize in 1997 for the prion theory, which postulates that the agent is a proteinaceous infectious particle (PrP) that is capable of replication without the need of an agent-specific nucleic acid. The disease-associated prion (PrPSc) has been shown to have a different helical shape to normal cellular prion protein (PrPC) found on neuronal cells and some other cells, for example lymphoid cells. However, while it is widely acknowledged that PrPSc is very closely associated with the causative agent, there is a reluctance by some to accept PrPSc as the sole agent responsible for transmission. Another hypothesis suggests the agent is an unconventional virus, while a third suggests that it is a virion that has similar prop-erties to a virus but uses host proteins to coat its nucleic acid.
CJD is a fatal disease of humans, first described in the 1920s and found worldwide. CJD is predominantly a sporadic disease, but about 14% of cases are familial (inherited) and associated with genetic mutations. Less than 1% are iatrogenic (i.e., accidentally trans-mitted from person to person as a result of medical or surgical procedures). Classically, sporadic CJD occurs in those over 65 years of age and presents as a rapidly progressive dementia with myoclonus (shock-like contractions of isolated muscles), usually fatal within 6 months. Surveillance of CJD, a human neu-rological disease, was reinstituted in the UK in 1990 to evaluate any changes in the pattern of the disease that might be attributable to BSE. The overall inci-dence of CJD rose in the UK in the 1990s, although a portion of this increase was due to improved ascer-tainment of CJD in older people as a result of the reinstitution of surveillance.
New variant CJD, also referred to as variant CJD (vCJD), is a newly recognized disease in humans, which was first diagnosed in the UK in the mid-1990s. In contrast to the traditional forms of CJD, vCJD has affected younger patients (average age 29 years) and has a longer duration of illness (approximately 14 months). Early in the illness, patients usually experi-ence behavioral changes, which most commonly take the form of depression or, less often, a schizophrenia-like disorder. Neurological signs such as unsteadiness, difficulty walking, and involuntary movements develop as the illness progresses and, by the time of death, patients become completely immobile and mute.
A geographical association exists whereby the major-ity of BSE cases occurred in the UK and the majority of vCJD cases were also reported there. The emer-gence of BSE preceded vCJD, indicating a temporal association. Studies of stored human brain tissue internationally have not identified the histopatho-logical changes characteristic of vCJD before the current BSE epidemic. Incubation period and patho-logical lesion studies in mice and molecular typing studies demonstrate that vCJD is similar to BSE but different from other TSEs. It is now widely accepted that vCJD was transmitted to humans through the consumption of contaminated food.
Estimates of future prevalence of vCJD vary widely as too little is known about the disease, especially regarding the incubation period between exposure to the infective agent and the emergence of symptoms.