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Prions

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| Prion Disease | VET 140 Mr. Bell |

Roger Meadows
5/7/2012
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A prion is a protein that has been mutated. Prion means proteinaceous infectious particle. Proteinaceous infectious particle can be broken down into:” proteinaceous which is relating to, resembling, or being protein, Infectious which is capable of causing infection, and particle which is a minute quantity or fragment” (Merriam – Webster 2012). So a proteinaceous infectious partial would be a very small protein that can cause an infection. The protein called PrP can be found in nerve cells all through the brain. The prion does not have a nucleus like other infectious dieses, and this makes it rare. Without the nucleus, it is not a virus or bacteria; it is only a protein (Hinz, A., Doremus, C., 1997). Prions cause a wide range of neurologic diseases in sheep, cows, and humans and were identified by Stanley Prusiner at UCSF in 1981 (Immunity and Disease, 2001). Prions are responsible for transmissible spongiform encephalopathies or TSE’s. Spongiform encephalopathies can develop three different ways, either by a normal prion changing into an abnormal one, or the PRNP gene that targets the creation of a protein which mutates into an abnormal protein, or consumption of contaminated items (Hinz, A., Doremus, C., 1997).
TSE’s are very rare and affect around one person in every million each year over the world. TSE’s include bovine spongiform encephalopathy, or mad cow disease in cattle, scrapie in sheep, chronic wasting disease in deer, and Creutzfeldt-Jakob disease in people. The most well know TSE is mad cow disease. From 1984 through 2000, two hundred thousand cattle with bovine spongiform encephalopathy died, with another four million slaughtered that potentially had the infection. All the infections have been in Eastern Europe, with the majority in England. Bovine spongiform encephalopathy is thought to be spread from the practice of including parts of carcasses that are unusable for human consumption into high protein animal feed. The practice of adding meat and bone meal is now illegal in cattle feed. Deer with chronic wasting disease leave droppings behind that the sheep eat giving them scrapie. When the infected sheep are slaughtered and their meat and bone tissue is added to cattle feed, the cattle then become infected with mad cow disease (Immunity and Disease, 2001).
Chronic wasting disease is a transmissible spongiform encephalopathy of mule deer, white tailed deer, elk, and moose. To date, chronic wasting disease has been found mainly in members of the deer family. Most cases of chronic wasting disease occur in adult animals. The disease is progressive and always fatal. The most clear and constant clinical sign of chronic wasting disease is weight loss over time. Behavioral changes additionally occur in the majority of cases, including decreased interactions with other animals, listlessness, lowering of the head, blank facial expression, repetitive walking in set patterns, and a smell like meat starting to rot (Turkington, C. & Harris, J., 2009).
Scrapie is a fatal, degenerative disease that affects the nervous systems of sheep and goats. It is one of several transmissible spongiform encephalopathies, like chronic wasting disease of deer. Like other spongiform encephalopathies, scrapie is caused by a prion. Scrapie has been known since the 18th century, though it was thought to be viral and bacterial at first. The name scrapie comes from one of the clinical signs of the condition, in which affected animals will impulsively scrape off their hide against rocks, trees or fences. The disease causes an itching sensation in the animals. Other clinical signs include excessive lip-smacking, altered gaits, and convulsive collapse (Turkington, C. & Harris, J., 2009).
Bovine spongiform encephalopathy incubation period is very long, four to five years. It is always fatal, causing death within weeks to months of symptom onset. The brain of a deceased cow infected with this disease appears sponge-like; it has many small holes in the tissue caused by the death of many brain cells. As the brain tissue slowly dies, the animal loses control over its movements and behavior looking crazed or mad. During the last stages, infected animals become aggressive, lack co-ordination, and are unsteady on their hooves (Ridgway, t., 2002).
Creutzfeldt-Jakob disease is a deteriorating neurological illness that is incurable and always fatal. Creutzfeldt-Jakob disease is sometimes called a human form of mad cow disease, because bovine spongiform encephalopathy is believed to be the cause of Creutzfeldt-Jakob disease. Creutzfeldt-Jakob disease is the most common type of transmissible spongiform encephalopathy found in humans. With Creutzfeldt-Jakob disease, the brain tissue gets holes and takes on a sponge-like texture. The first symptom of Creutzfeldt-Jakob disease is quickly increasing dementia, leading to memory loss, personality changes and hallucinations. This goes along with physical problems such as speech impairment, jerky movements, balance and coordination dysfunction, changes in gait, rigid posture, and seizures (Turkington, C. & Harris, J., 2009).
The human disease Kuru is a transmissible spongiform encephalopathy and first appeared in the early 1900’s in New Guinea. It reached widespread proportions in the 1950’s among the natives of the Southern region of the country. Over 1,100 people died of the disease between 1957 and 1968. Most of these cases were women. Anthropologists concluded that the cause was the ritual acts of cannibalism of the dead which was practiced in the area. Upon the death of an individual, the women would cut up the corpse, strip the limbs of muscle, and remove the brains and internal organs. The organs were lightly cooked and eaten or smeared on the bodies of the funeral participants. After this practice stopped, the incidence of Kuru declined to almost non-existent levels. The symptoms of Kuru are depression, tremors, unsteadiness, inappropriate laughter, and ultimately death. Kuru was first thought to be genetic, then viral, and then a prion disease after Prusiner identified and defined TSE’s in the 1980’s (Turkington, C. & Harris, J., 2009).
Scientists at the National Institute of Allergy and Infectious Diseases Rocky Mountain Laboratories are studying abnormal prion protein molecules and why they cause transmissible spongiform encephalopathy diseases. Some research has proven when prion proteins are modified to remove a membrane anchor in laboratory animals, abnormal prion proteins were created due to scrapie infection, but not the disease. Without the membrane anchor, researchers believe, the abnormal prion protein is incapable of damage to the brain cells. Drugs aimed at blocking contacts between normal and abnormal prion protein might be able to stop the progress of disease. (National Institute of Allergy and Infectious Diseases. 2011)
At National Institute of Allergy and Infectious Diseases Rocky Mountain Laboratories, studies are continuing to better understand the ways that transmissible spongiform encephalopathy infections cross species. Experiments have found that species once thought to be resistant to certain TSE strains can be carriers of the infection for life without ever becoming sick. (National Institute of Allergy and Infectious Diseases. 2011)
In closing, there is a lot to be learned about this strange, infectious abnormal protein called a prion. Scientist have been able learn a lot about prions in the last 20 odd years, but have really only scratched the surface. Cross-species infections have been proven, through the consumption of infected products. With the actions and regulations that have been enacted for BSE containment, it appears to have been effective in reducing the incidence of this disease.

References

Ridgway, T. (2002). Mad cow disease: bovine spongiform encephalopathy. NY, NY: The Rosen Publishing group
Turkington, C. & Harris, J. (2009).The Encyclopedia of the brain and brain disorders. NY, NY: Facts on file
Hinz, A., Doremus, C. (1997). The Prion Protein. Retrieved April 17, 2012 from http://www.msu.edu/- hinzalia/
Immunity and Disease. (2001, February 28) Prions. Retrieved on April 17, 2012 from http://mcb.berkeley.edu/courses/mcb50/Prions.pdf
National Institute of Allergy and Infectious Diseases. (2011, July 05). Prion Diseases. Retrieved April 17, 2012 from http://www.niaid.nih.gov/topics/prion/pages/default.aspx
Merriam - Webster. (2012). Medical. Retrieved on April 17, 2012 from http://www.merriam- webster.com/medical/

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