7 Prions: A Mysterious Infectious Agent 病原性蛋白顆粒： 神秘的致病元兇 By Daria Zaitseva What could possibly link a sheep illness called scrapie, a disease associated with cannibalism called Kuru, and a neurodegenerative disease called Creutzfeldt-Jakob disease? These fatal diseases occur in different species, and yet share a chilling commonality. More mysteriously, scientists could not identify the pathogen – neither virus nor bacteria were the culprit. Human’s knowledge at that time simply could not break the mystical curse. Let’s explore the mystery and see how one of the most obscure killers was discovered. Scrapie: Affected Sheep Uncontrollably Scratching Their Backs We begin in 18th-century England, where sheep farming was a cornerstone of the economy. But farmers soon faced a disturbing problem: Some sheep began scratching their backs against posts uncontrollably, then stopped feeding and became lame, and eventually turned emaciated and died [1]. The only way to prevent the spread of the disease was to isolate the sick animals from the flock. Nevertheless, without the ability to investigate further, this eerie disease was soon forgotten [1]. By the middle 20th century, scientists took a closer look [1]. They tried to identify the underlying pathogen. As the first step, they succeeded in experimentally transmitting the disease by inoculating the brain or spinal cord tissue from a diseased animal to a healthy one. The onset of disease can take as long as one to two years, so it left the scientists confused about the results of the experiment at first. Then, scientists attempted to identify the pathogen by treating tissue samples with different standard inactivation methods at the time, such as using a bacterial exclusion filter to remove, if any, bacteria. They also applied a dose of ionizing radiation that could disrupt, if any, nucleic acid (including DNA and RNA) in a separate experiment. However, the tissues remained infectious, so they realized the pathogen could be unusual this time. The only clues from brain dissections are the signature vacuoles – described as “soap bubbles” – in the cytoplasm of nerve cells, and the strange holes – called spongiform – in the sheep’s brains, giving the brain a sponge-like appearance. Kuru: A Mystical Curse Associated with Cannibalism As scrapie returned to the limelight, two medical doctors, Daniel Gajdusek and Vincent Zigas, reported firsthand in 1957 from Papua New Guinea a mysterious disease that
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