Prions as Novel Infectious Agent

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Table of Contents

Definition and Discovery of Prions
Effects of Prions on Animals and Humans
Diseases Caused by Prions
Means by which Prions Arise and Propagate
Works Cited

Definition and Discovery of Prions

Prions are characterized by folded protein molecules that do not contain any genetic information. “Prions are composed of amino acids, which are the essential building blocks of proteins”. The word prion, which is also called proteinaceous infectious particles, often refers to the abnormally folded proteins which can change other proteins from their normal state to an abnormal one (Prion Fact Sheet).

Unlike other infectious agents like viruses, bacteria, fungi, and parasites, prions do not have a genome. Prions are simple proteins that do not contain any genetic material. Though prions also cause diseases, the lack of genetic material distinguishes them from the other infectious agents. Prion proteins have two forms: the normal and undamaging protein or the PrPc and the harmful and disease-causing protein or the PrPSc (Prusiner). “A chain reaction is formed when a conversion from PrPc to PrPSc occurs”. Long slender strands aggregates are formed when an ample amount of PrPSc protein is made. Prions convert the normal proteins, which are similarly composed of amino acids, into prions like themselves by entering the cells. Prions cause diseases that are neurodegenerative by extracellularly accumulating within the central nervous system and forming plaques known as amyloids which messes up the normal tissue structure by creating holes in the tissue (Prion disease).

Prions were discovered by Stanley B. Prusiner, an American neurologist from the University of California. He was awarded the 1997 Nobel Prize in Medicine or Physiology for discovering a new genre of disease-causing agents which do not have any genes or genetic material (Weiss). The Nobel Prize came after two decades of trying to prove that strange infectious proteins could cause diseases in the brain like the mad cow disease which affects both animals and people, while he was ignored and even ridiculed by his peers. His peers assumed that the diseases were caused by undiscovered and slow-growing viruses and not infectious proteins.

The first recorded disease caused by a prion, but which was not recognized then as being a prison-related infection, was Kuru. The disease Kuru was found in the isolated New Guinea Fore tribe. The disease spread or was transmitted because of the tribe’s religious practice of eating the brain tissue of their dead relatives, which they do by grinding the brain into a pale grey soup and then heating it (Prion Diseases). It was speculated that the brain tissue that was ingested by some members of the tribe was infected. When the practice of ingesting brain tissue was stopped, the occurrence of the disease declined, and eventually, the disease never arose within the tribe again.

Another significant event relating to prion infection was the 1986 Mad Cow epidemic in England This, however, still did not drive scientists to suspect harmful proteins as the cause of infection, instead they suspected a possibility that slow viruses may be the cause as these viruses spend decades inflicting harm and disorder in their hosts and are hard to isolate (Guyer).

Effects of Prions on Animals and Humans

Prions infect both animals and humans. They cause fatal diseases. There is no natural protection in the body of both animals and humans against prions because they are not recognized by the immune system as foreign (Prions infect animals and humans). Prions cause transmissible spongiform encephalopathy or TSE diseases in humans and animals. These TSE diseases affect brain tissues which develop tiny holes that make them look like sponges when viewed using a microscope. Aside from being fatal, TSE diseases are untreatable. The most common TSE diseases include the Mad Cow Disease in cattle, Scrapie in sheep, and Chronic Wasting Disease in deer, elk, and moose (Prion Fact Sheet).

“Different regions of the brain are affected by different prions”. There are different symptoms per affected brain region. The regions of the brain comprise the cerebral cortex, thalamus, cerebellum, and brain stem. The symptoms manifested if the cerebral cortex is affected include loss of mental acuity and memory, and at times visual impairment, while the symptom for thalamus damage is insomnia (Different prions affect different regions of the brain). Problems in body movement coordination and walking difficulties are associated with damage to the cerebellum. An accurate diagnosis of a prion disease reflected on the images of the brain may only be achieved upon autopsy.

Diseases Caused by Prions

Prion diseases are progressive neurodegenerative disorders affecting humans and animals, characterized by distinct spongiform changes or holes in the brain tissue related to neuronal loss, long incubation periods, and failure to produce an inflammatory response (Prion Diseases). Many regions of the brain were affected and amyloid plaques or insoluble protein aggregates containing a high percentage of beta-sheet conformation were observed (Cann).

The most common prion disease is Scrapie which is found in sheep and goats. The afflicted animals become very irritable which in some cases leads to intense itchiness of the body that makes them scrape their hair or wool off (Prusiner). Another commonly known prion disease is Bovine Spongiform Encephalopathy or more often referred to as Mad Cow Disease. Mad Cow Disease is an infectious disease found in the brain of cattle. Prions attack the brain and create sponge-like holes which cause slow deterioration of the cattle brain and affect the whole body eventually leading to death (Mad Cow Disease Overview). Humans who ate tissue from diseased cattle and became infected will die of the same brain disease which may grow over many years.

“Other prion diseases include Transmissible Mink Encephalopathy, Feline Spongiform Encephalopathy, and Chronic Wasting Disease of mule deer and elk”. (Transmissible Mink Encephalopathy ). Transmissible Mink Encephalopathy or TME is a disease that affects the central nervous system of mink that is ranch raised. Symptoms of TME include nest soiling, spreading of droppings all over the cage, difficulty in eating and stepping into their food (Transmissible Mink Encephalopathy ). Advanced cases of the disease in animals include actions like compulsive chewing tail chewing and jaw clenching. Feline Spongiform Encephalopathy is similar to Mad Cow Disease but the animals affected are cats. “Chronic Wasting Disease affects elks and mule deer”. The manifestations of the disease include weight loss for over weeks or months, excessive salivation, behavioral changes, difficulty swallowing, and head tremors, and ataxia in some animals (Belay, Maddox, and Williams). Most animals infected with the disease die from aspiration pneumonia within several months after acquiring the disease. Transmission through direct contact between animals including water sources and contaminated feed has been confirmed with supporting evidence.

Kuru, Creutzfeldt-Jakob disease, Gerstmann-Sträussler-Scheinker and fatal familial insomnia are human diseases caused by prions. Kuru originated from the Fore highlanders of New Guinea. It was called laughing death. Many highlanders were afflicted with a strange and fatal disease that resulted in ataxia or a loss of coordination and dementia. Those who were infected acquired the disease after eating the brain tissue of a dead highlander. “Creutzfeldt-Jakob disease or CJD is a degenerative and fatal brain disorder”. (Creutzfeldt-Jakob Disease Fact Sheet). CJD in most cases appears later in life and develops very rapidly. The onset of symptoms happens around age sixty and an estimate of ninety percent of infected individuals die within a year. “Patients may have behavioral changes, failing memory, visual disturbances and lack of coordination in the early stages of CJD and as the illness advances, patients experience involuntary movements, mental deterioration, weakness of extremities, blindness, and even coma” (Creutzfeldt-Jakob Disease Fact Sheet).

Gerstmann-Sträussler-Scheinker which is manifested as ataxia and other cerebellum damage signs, and fatal familial insomnia which is characterized by difficulty in sleeping followed by dementia, are both in most cases inherited and appear in middle life.

Means by which Prions Arise and Propagate

Prions are both infectious and hereditary diseases. They are also considered sporadic in that there are known cases where there is no known risk factor in the environment of those afflicted with the disease, but the likelihood is high that infection was contracted either through infection or heredity. Infection of humans may stem from diet or the food and water ingested, and medical procedures like surgery and corneal transplants. The source of food and water should be carefully inspected to avoid ingesting infected water or food item. In medical procedures, transmission is possible only by invasive methods like injection or inoculation and not by casual patient contact as what occurs during routine patient care (Agamanolis). An autopsy can also be safely done without fear of transmission by using added chemical treatment with formic acid. Hereditary transmission occurs through mendelian transmission where the dominant trait is acquired. In this case, the parent-child transmission of the human prion diseases is possible.

Works Cited

Belay, ED, et al. “Chronic Wasting Disease and Potential Transmission to Humans.” 2004.

Centers for Disease Control and Prevention. 2007. Web.

Cann, A.J. Principles of Molecular Virology, 2nd Edition. New York: Academic Press, 1997.

“Creutzfeldt-Jakob Disease Fact Sheet.” 2003. National Institute of Neurological Disorders and Stroke.

“Different prions affect different regions of the brain.” 1997.

Guyer, Ruth Levy. “Prions: Puzzling Infectious Proteins.” National Institutes of Health. 2007. Web.

“Mad Cow Disease Overview.” 2005. Web.

“Prion.” Wikipedia. 2007.

“Prion disease.” Wikipedia. 2007.

“Prion Diseases.” 2006. Centers for Disease Control and Prevention. Web.

“Prion Diseases.” 1999. Tulane University. Web.

“Prion Fact Sheet.” 2006. Water Environment Federation. Web.

“Prions infect animals and humans.” 1997. Nobelprize.

Prusiner, Stanley B. “The Nobel Prize in Physiology or Medicine 1997.” Nobel Prize. Web.

“The Prion Diseases.” Microsoft Encarta2006 [CD]. Redmond, WA: Microsoft Corporation, 2005.

“Transmissible Mink Encephalopathy.” Feb 2002. United States Department of Agriculture. Web.

Weiss, Rick. “U.S. Neurologist Wins Nobel Prize for Discovery of Prions.” 1997. Tech.

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