Transmissible spongiform encephalopathies—prion diseases—are human and animal diseases affecting the central nervous system and more specifically the brain and spinal cord. Some can be transmitted through food. They are caused by unconventional transmissible agents known as prions, particularly resistant to conventional inactivation processes. These diseases are always fatal and there is currently no treatment.
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Updated on 23/08/2016
Transmissible Spongiform Encephalopathies (TSEs), also known as prion diseases
General information on TSEs
What are TSEs?
Transmissible spongiform encephalopathies—prion diseases—are neurodegenerative diseases affecting the central nervous system and more specifically the brain and spinal cord. They affect certain animal species—cattle, sheep, goats, felines, mink and deer—in addition to humans. They are caused by unconventional transmissible agents (UTAs) known as prions, rather than conventional biological agents such as bacteria, viruses, parasites or mould. UTAs are particularly resistant to conventional inactivation processes. They are purely protein-based according to the prion hypothesis (PRION being the acronym for PRoteinaceous Infectious ONly particle). The diseases are always fatal and there is currently no therapeutic treatment.
These diseases are characterised by a long incubation period ranging from several years to around 40 years in humans. The lesions observed in the brain form cavities within the neurons, giving the disease its “sponge-like” character. An abnormal three-dimensional protein accumulates in certain tissues. It is difficult for the body to break it down due to its unusual shape, distinct from the host’s normal proteins. This abnormal PrPres protein is specific to these diseases and is therefore the marker sought by TSE screening tests (surveillance programmes in rendering plants and in animals presenting clinical signs or subject to emergency slaughter).
Some of these diseases can be transmitted through food. Also known as “mad cow disease”, BSE in its classical form was caused by cattle eating contaminated meat-and-bone meal used as feed. Human variant Creutzfeldt-Jakob disease (vCJD) is caused by the BSE agent, transmitted to humans through the consumption of tissue, especially nervous tissue, from affected animals.
Different types of TSE
In humans, TSEs include:
- Creutzfeldt-Jakob disease
Known since 1920, this disease occurs most often in isolated cases, with no apparent cause, although some cases appear genetic (mutations in the prion protein gene sequence) and others iatrogenic, such as contamination following a transplant or surgical operation affecting the central nervous system, or through use of a contaminated human growth hormone, now banned.
- Variant Creutzfeldt-Jakob disease (vCJD).
Identified in 1996 among abnormally young patients whose clinical signs differed from the classical Creutzfeldt-Jakob disease, this TSE is the result of human contamination by the agent causing bovine spongiform encephalopathy (BSE).
- Other human TSEs
Other known TSEs include kuru (Papua New Guinea), Gerstmann-Sträussler-Scheinker disease and fatal familial insomnia.
In animals, TSEs include:
Bovine spongiform encephalopathy (BSE)
- Classical BSE
Identified for the first time in 1986, the number of infected cattle rose drastically in the United Kingdom, reaching a peak of 37,280 identified cases in 1992 alone. The disease affected the rest of Europe and some other countries—more especially Japan, the United States and Canada—but to a much lesser extent than the UK outbreak. Epidemiological research quickly determined that the causal agent was transmitted through contaminated meat-and-bone meal probably not heated to a high enough temperature before being fed to cattle.
- Atypical BSE
From 1986 to 2004, it was thought that there was only one BSE prion, with the same biochemical signature in all detected cases. However, in 2004 two new biochemical profiles were discovered, subsequently named atypical L- or H-type BSE. There are as yet few scientific data on the distribution of this prion in animal tissues or the epidemiology of these forms of BSE. The regular occurrence of such cases at very low frequencies suggests that they may be spontaneous.
- Scrapie in sheep and goats
Scrapie is a TSE common to both sheep and goats. It was first described in the 18th century and is widespread. Historically, it was the first TSE proven in 1936 to be transmissible among animals. There are two distinct forms:
- Classical scrapie
Classical scrapie has thus been named following the discovery of an atypical scrapie. It covers several distinct strains, differentiated through biochemical techniques—screening for PrPres and analysing its properties—or through inoculation of a laboratory animal (different incubation times, variation in the distribution of lesions within the central nervous system). Some sheep are naturally resistant to classical scrapie, an advantage exploited under a national programme of genetic selection to combat classical scrapie.
Transmission in livestock farming conditions may be horizontal, between animals, or vertical, passing from one generation to another.
- Atypical scrapie
Identified some ten years ago among sheep and goats, atypical scrapie is now described in several countries since active surveillance programmes have been initiated.
The biochemical properties of the abnormal protein associated with this disease appear identical in all the animals found positive and quite distinct from those found with classical scrapie strains.
The agent behind this disease appears only slightly or not contagious in livestock farming conditions.
- BSE among small ruminants
Like cattle, small ruminants were also fed contaminated meat-and-bone meal. The BSE agent has been shown experimentally to be orally transmissible to sheep and goats. In France, a single case was described in a goat in 2005. Despite intensive screening of goats in 2005 and sheep in 2006, no other case has been detected in France. The prevalence of the disease appears to be very low among small ruminants.
- Other animal TSEs
There is also feline spongiform encephalopathy (transmission of the BSE agent through contaminated food), mink spongiform encephalopathy and chronic wasting disease affecting certain deer species in North America.