17. Viruses
Viruses
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There are several human diseases known to be caused by prions. Creutzfeldt-Jakob disease is rare, occurring in about 1 out of 1 million people. It usually appears in midlife, between ages 20 and 70. The average age of onset is about 50. Recently, a form of the disease called variant CJD (vCJD) has emerged. It was first detected in the United Kingdom in people who had eaten beef from cattle infected with bovine spongiform encephalopathy, so-called “mad cow disease”. Once the first symptoms are seen, both variants of CJD progress very rapidly from onset to disability to death. CJD affects the cerebrum, where it causes holes called vacuoles and amyloid plaques— clusters of prion proteins and infected cells. Gerstmann-Straussler-Scheinker syndrome (GSSS) is a rare human prion disease. GSSS is caused by a single mutation in the gene encoding the prion protein, and is characterized by amyloid plaques in the brain in several areas, mostly in the cerebellum. The disease causes eye-body coordination difficulties, intellectual disturbances, and eventually dementia and death. Fatal familial insomnia is a genetic disease caused by a mutation in the gene encoding the prion protein. The onset of the disease usually occurs around age 50. The structure in the brain that is most affected is the thalamus, which is responsible for the regulation of circadian rhythms. The first symptom of fatal familial insomnia is a progressive insomnia, which eventually leads to complete lack of sleep, and coma and death a year and a half after the onset of symptoms. Kuru was a prion disease found among the South Fore tribes of New Guinea starting around 1900. Kuru was also known as laughing disease, because the victims would have uncontrollable laughing fits as their symptoms progressed. Kuru was found to be transmitted by brain matter of deceased relatives, which was handled and eaten during mourning rituals. When cannibalism was outlawed in the region in the late 1950s, the disease sharply declined, and cases of kuru have since vanished. Kuru mostly affected the cerebellum, and the most pronounced symptoms were related to motor skills. Several animal diseases are also known to be caused by prions. Scrapie is a fatal, degenerative disease that affects the central nervous system of sheep and goats. Scrapie has been known since the 18th century, and it is believed that bovine spongiform encephalopathy was transmitted to cattle through the practice of using contaminated sheep bone meal in cattle feed. Infected sheep often scrape themselves against objects until their skin is raw, giving the disease its name. Eventually, the infected sheep lose motor control and die. Transmissible mink encephalopathy (TME) affects the central nervous system of ranch-raised mink. TME has an average incubation period of more than 7 months before the onset of symptoms. Symptoms can last from 3 days to 6 weeks. Early symptoms include an increase in nest soiling and spreading of droppings in the cage. Microscopic examination of the brains of infected minks shows that the disease is limited to the central nervous system, causing sponge-like changes in specific areas of the brain. Epidemiologic studies suggest that animals contract the disease by external exposure to the infectious agent, such as by eating contaminated feed. Chronic wasting disease is a fatal neurological disease found in deer and elk, caused by prions. The disease attacks the brains of infected deer and elk, causing the animals to become emaciated, display abnormal behavior, lose bodily functions, and die. This disease has been found in both wild and captive deer and elk. Bovine spongiform encephalopathy (BSE) is the so-called “mad cow disease” that brought prion diseases to popular attention. BSE is a transmissible, neuro-degenerative, fatal brain disease of cattle. The disease has a long incubation period of 4-5 years. BSE is transmitted through the consumption of BSE- contaminated meat and bone meal supplements in cattle feed. Affected cattle behave erratically and begin to lose motor control, and eventually become unable to stand up. Infected cattle are sometimes referred to as “downer cattle”. It is believed that some humans acquired the variant Creutzfeldt-Jakob Disease from eating contaminated meat from downer cattle.
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![Plaque Assay: Assay for estimating viral titre or viral concentration [plaque forming units / mL]](https://img.youtube.com/vi/zFqyaoLQbxQ/mqdefault.jpg)
