Prions are infectious proteins, responsible for a number of diseases in different animals, including humans. The word prion is a portmanteau of “proteinaceous” and “infectious”. They were first theorized in the 1960s, and their existence and nature was confirmed in the early 1980s.
There are many different types of disease caused by prions. All prion diseases affect the brain of the recipient, and none are curable, ultimately resulting in death. Recent developments may eventually lead to a vaccine in the future.
In animals, prions usually attack one species, or two closely-related species. The sickness known as scrapie is a prion disease that attacks sheep and goats. Prions are responsible for mink encephalopathy in minks. Chronic wasting disease affects certain species of deer and elk, and feline encephalopathy and feline spongiform encephalopathy attack cats.
Perhaps the most famous disease caused by prions is what is known as bovine spongiform encephalopathy — more commonly referred to as mad cow disease. Mad cow disease was unknown until the latter part of the 20th century, and its discovery came as something of a shock to most people. Although not as serious in cattle as other livestock diseases — most notably hoof and mouth disease — it received an enormous amount of attention when it was discovered that it could apparently jump species to infect humans.
Most of these diseases caused by prions have as their vector of transmission the ingestion of contaminated meat — especially the brains — of the same species. Since modern practices often use ground-up bones and waste meat as a protein supplement for livestock, many cattle were ingesting other cattle, including brain tissue. It appears that the lowering of certain temperature guidelines for sterilization in the United Kingdom allowed the prions to move freely, and a small-scale epidemic was soon underway.
In humans, there are four primary diseases caused by prions. The first, Creutzfield-Jakob disease, has a variant, known as vCJD, which appears to be transmitted by ingesting contaminated beef. vCJD is the most common of the prion diseases in humans, but is still relatively rare — infecting only about one in every million people. Like all prion diseases, vCJD attacks the brain. Symptoms include dementia, which progresses very rapidly, followed by hallucinations, severe memory loss, seizures, loss of motor function, and ultimately death. The time from onset to final death from vCJD is quite short — rarely more than a few months, sometimes less than a few weeks.
Gerstmann-Straussler-Scheinker syndrome (GSS) is another disease in humans caused by prions, though much less common than vCJD. GSS is thought to affect only about one in every hundred-million people, making it incredibly rare. The cause of GSS is thought to be a genetic change, and the exact vector is unknown. Victims of GSS experience dementia and loss of coordination, ultimately resulting in death. Life expectancy for victims of GSS is better than for those with vCJD, but is rarely more than a few years.
Fatal familial insomnia is another disease in humans which was recently discovered to be caused by prions. The disease has been known and documented since the late 1970s, but it was not until the 1990s that the prion responsible was discovered. FFI is usually characterized at onset by chronic insomnia, which in turn leads to many psychological dysfunctions. Later, delusion and hallucination sets in, after which the ability to sleep disappears entirely. Finally, after about a year from onset, the patient dies.
Kuru is the last and earliest discovered of the common human diseases known to be caused by prions. Kuru is also sometimes known as the laughing sickness, as a result of the delusional laughing which often accompanies it. It was first discovered in parts of New Guinea at the beginning of the 20th century, particularly in the region of South Fore. This region had a high prevalence of ritual cannibalism, and it is now known that it was the consumption of human brain tissue that was responsible for the transmission of the disease.