Prion Diseases

Additional scientific description

These diseases affect the nervous system of mammals and, although the exact mechanism for transmission is as yet unknown, it is thought to be due to a putative infectious protein or protein-like substance called a prion, a ubiquitously expressed protein termed PrP or PrPc which undergoes conversion to an abnormal prion protein termed PrPsc (WHO, no date).

These diseases occur when proteins normally in the body misfold and cause illness. The misfolding leads to brain damage and other symptoms. Symptoms may take years to develop. But once they do, the disease rapidly progresses and leads to death. One of the reasons people may get sick with prion diseases is through eating or handling meat contaminated with prions. People may also become infected if they get prions in their bodies. This may occur through organ transplant or being exposed to contaminated equipment during surgery (CDC, 2024).

The term Transmissible Spongiform Encephalopathies (TSE) includes:

• Creutzfeldt-Jakob disease (CJD) in humans
• scrapie in sheep and goats
• chronic wasting disease in deer
• bovine spongiform encephalopathies (BSE) in cattle (WHO, no date)

Human

Many different prion diseases affect people, but some are more common than others. The most common prion disease in people is Creutzfeldt-Jakob disease (CJD). CJD is a prion disease that causes a person's brain to break down or stop working normally. Once symptoms start, the disease progresses quickly. It is always fatal, usually within a year. CJD is sometimes called "classic CJD" to avoid confusion with variant CJD. Most cases of classic CJD are in older people. They appear sporadically. This means there is no known reason that the cases begin.

A smaller number of cases occur in people who inherited genetic changes that make them prone to CJD. Occasionally, there have also been illnesses in people who were exposed to surgical equipment or other products contaminated with prions (CDC, 2024).

Another prion disease, variant Creutzfeldt-Jakob disease (vCJD), has a similar name but is a different, much rarer, disease (CDC, 2024). vCJD is a rare and fatal human neurodegenerative condition first described in March 1996. As with CJD, vCJD is classified as a TSE because of characteristic spongy degeneration of the brain and its ability to be transmitted. The development of vCJD is strongly linked with exposure, probably through the consumption of meat and meat products, to the agent causing BSE which was first reported in the UK in 1986. In contrast to the traditional forms of CJD (sporadic, familial, and iatrogenic), vCJD has affected younger patients, average age of 29 years, compared to >65 years for the other forms. vCJD has a relatively longer duration of illness, median of 14 months, compared to 4.5 months for CJD (WHO, no date).

Creutzfeldt-Jakob disease (vCJD) is tied to eating beef from a cow that had a prion disease called Bovine Spongiform Encephalopathy (BSE). It was discovered in 1996 in the United Kingdom. The disease affected much younger people than CJD. vCJD often infected people in their 20s or younger. It caused mental symptoms and behaviours, as well as pain when touching things (dysesthesias), that CJD does not (CDC, 2024).

Other prion diseases include kuru, variably protease-sensitive prionopathy, sporadic fatal insomnia, and inherited forms caused by genetic mutations (CDC, 2024). It is a non-inflammatory neurodegenerative disease and is a form of transmissible spongiform encephalopathy transmitted through the act of cannibalism (Asher, 2018). The symptoms of kuru progress rapidly with cerebellar and extrapyramidal signs and symptoms, with death occurring within one to two years of the onset of symptoms (Gajdusek, 1957).

The World Health Organization (WHO) has published guidance on case classification and surveillance standards for vCJD and other human-transmissible prion diseases (WHO, 2003).

Animal

There are a number of known prion diseases in animals:

Bovine spongiform encephalopathy (BSE):

BSE is a type of prion disease that affects cows. The first cases were identified in the mid-1980s, but BSE didn't gain widespread attention until a major outbreak in the United Kingdom later that decade into the 1990s. Millions of cows were infected.

By 2005, 24 countries had reported BSE among native cattle. (20 of the 24 were in Europe).

The spread of infection in cattle was likely tied to feeding practices. It is speculated that it is related to cows being fed meat and bone meal from other cows that had prion disease (CDC, 2024).

Chronic Wasting Disease (CWD):

CWD is a prion disease that affects animals like deer, elk, moose, and reindeer. It has been found in the United States, Canada, Norway, Finland, and Sweden. Imported cases have also been reported in South Korea. Though there have been no cases of CWD in people, experts think it poses a theoretical risk to people. This is because CWD is related to BSE, which has caused disease in people. If CWD could spread to people, hunters and people who eat meat from CWD-infected animals would likely be most at risk (CDC, 2024).

Scrapie:

The term scrapie encompasses two different pathological entities: classical and atypical scrapie. Both conditions are chronic, neurodegenerative, fatal diseases of sheep and goats and are caused by similar agents. However, they differ in clinical presentation, pathogenesis, pathology, biochemical properties and epidemiology. Classical scrapie is characterised by vacuolar changes in the central nervous system (CNS). It has been recognised as a clinical disorder for more than 250 years and is classified as a transmissible spongiform encephalopathy (TSE), or prion disease, as defined by the accumulation of an abnormal form of a host cell membrane-bound glycoprotein (prion protein or PrP) referred to as PrPSc, in the CNS. Polymorphisms of the PrP gene are associated with susceptibility to scrapie. The more recently identified condition known as atypical scrapie has some clinical and pathological features similar to classical scrapie but is not considered to be transmitted in field situations. The epidemiology is consistent with a non-contagious condition that occurs sporadically. Classical scrapie may be transmitted from dam to offspring in the period from parturition to weaning, and in utero. It can also be transmitted horizontally to unrelated sheep or goats. The infectious material can persist for several years in pastures and in buildings. Foetal membranes are a source of infection, and milk from clinically affected animals can transmit disease. The incubation time between primary infection and clinical disease is usually longer than 1 year and may sometimes exceed the commercial lifespan of the animal. The majority of cases occur between 2 and 5 years of age. Clinical disease develops only if the agent enters the CNS. Atypical scrapie, where it presents clinically, is reported mostly in older animals, and occurs with a geographical distribution suggestive of a spontaneous disease, although it has been transmitted experimentally. There are no vaccines available. There is no evidence of a causal link between classical or atypical scrapie and human TSEs (WOAH, no date).

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