Viruses, viroids, and prions

Seth D. Greenlaw

Biology101

Paper 1

June 8, 2002

Viruses, viroids, and prions are not technically living organisms. In this paper I will attempt to describe what makes them different from living organisms, why they are important, and what diseases they cause. However, before starting off on this whirlwind of biological information, it’s important to define what viruses, viroids, and prions are.

For example, Columbia University’s Columbia Electronic Encyclopedia states, “A virus is a parasite with a noncellular structure composed mainly of nucleic acid within a protein coat” [1]. According to the Columbia Electronic Encyclopedia, “A viroid is an microscopic infectious agent, much smaller than a virus, that infects higher plants such as potatoes, tomatoes, chrysanthemums, and cucumbers, causing stunted or distorted growth and sometimes death. Viroids are single strands of RNA and lack the protein coat of viruses” [1]. The Columbia Electronic Encyclopedia continues to explain a prion as an “Undefined infectious agent thought to cause a group of diseases known as prion diseases or transmissible spongiform encephalopathies” [1].

Infectious and parasite are the two words that most accurately describe all three of these organisms. These two words will also greatly help in understand how they are different from living organisms in that they lie somewhere in the grey area between living and non-living states. As Mr. John C. Brown states “A virus is not strictly alive, nor is it strictly dead” [2]. According to Brown, these organisms have certain important data, “genes made of DNA or RNA”, that permit them to replicate themselves, however, they can’t replicate on their own [2]. As Mr. Speer explains “When its comes into contact with a host cell, as virus can insert its genetic material into its host, literally taking over the host’s function. An infected cell produces more viral protein and genetic material instead of its usual products” [3]. It is this inability to replicate on their own, and their infectious/parasitic properties, that makes them not technically living organisms, as they must invade a living cell in order to replicate.

However, these organisms are important as Mr. Speer mentions “Because viruses can transfer genetic material between different species of host, they are extensively used in genetic engineering” [3]. In regards to genetic engineering, Speer also says that “Viruses also carry out natural genetic engineering: a virus may incorporate some genetic material from it’s host as it is replicating, and transfer this genetic information to a new host, even to a host unrelated to the previous host. This is know as transduction, and in some cases it may serve as a means of evolutionary change – although it is not clear ...

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It’s important to know that viruses can infect humans, animals, and plants in horrific proportions. However, I will only be focusing on viruses that are found in humans. For example, a retrovirus (HIV) is thought to cause AIDS, several viruses (e.g. Epstein-Barr virus, human papillomavirus) cause particular forms of cancer in humans, and many have been shown to cause tumors in animal. Other viruses that infect humans cause measles, mumps, smallpox, yellow fever, rabies, poliomyelitis, influenza, and the common cold [1]. Viruses through out history have devastated the human race as a whole. While HIV/AIDS grab the headlines these days back in 1918 the “flu” caused a global epidemic, or pandemic, that caused over 20 million deaths worldwide and 500,000 deaths in the United States [8].

Viroids, however, don’t cause disease in humans (with the exception of Hepatitis D) while instead they are a common plant pathogen that can cause serious economic problems. For example, in a report from Mr. Hanold to the Australian Quarantine Committee in 1996, one can easily see the economic applications of such pathogens. Mr. Hanold writes, “Horticulture is Australia's second largest rural industry with a gross value ex farm gate estimated at around $3,500 million in 1994. The combined gross value of horticultural production (excluding wine and table grapes) was estimated at $2,572 million for 1988-89, and the value of this sector's contribution to the export trade (both as fresh and processed produce) was $420 million. From 1985-86 to 1990-91, horticultural exports doubled in value to reach $694 million.” [6]. One can see the potential for economic disaster when such pathogens cause crop failure as they did in “1988 when an epidemic of celery mosaic virus in South Australia caused up to 70% crop infection with a large number of the infected plants unfit for marketing, even locally, and a celery-free period was recommended for disease control. Assuming a loss of production value, both in quantity and quality, over the whole sector of 5-10% would mean an annual loss of $175-350 million.”[6].

While the amount of diseases that viroids cause are too numerous to mention, according to Mr. Jones of the American Phytopathological Society the following are examples of diseases common to the Apple family [4]:

“Apple blister bark” (‘Delicious’) Apple fruit crinkle viroid
“Apple dimple fruit” Apple scar skin viroid
“Apple fruit crinkle” Apple fruit crinkle viroid

Prions, however, don’t invade plants but instead attack both animals and humans with equal devastation. According to Dr. Shaun Heaphy the following are specific examples of animal diseases caused by prions [5]:

Scrapie: sheep
TME (transmissible mink encephalopathy): mink
CWD (chronic wasting disease): muledeer, elk
BSE (bovine spongiform encephalopathy): cows

Specific examples of human diseases caused by prions include [4]:

CJD Creutzfeld-Jacob Disease
GSS: Gerstmann-Straussler-Scheinker syndrome
FFI: Fatal familial Insomnia

Of these diseases I will go a bit more in depth with BSE or commonly known as “mad cow disease”. This is a degenerative, transmissible and fatal neurological disease occurring in cattle. It affects the brain and the spinal cord. The disease is ultimately fatal for cattle within weeks to months after the symptoms become visible. This disease alone has had huge economic repercussions, as according to the London Observer Service, BSE “has cost Britain $6.4 billion, claimed the lives of 43 people and triggered fears that the death toll could eventually reach several million.” [7].

Quarantine and vaccinations have provided some help in combating the seemingly unbeatable army of viruses, viroids, and prions. However, medical science and human determination can only do so much in this war against mutable enemies. Just as a meteor brought the ice age for the dinosaurs, perhaps the next cataclysm will not be from a meteor but tiny organisms that will bring a new type of “ice age” for us. When will the next epidemic, outbreak, or pandemic occur and will we be able to survive? These are questions that must be left unanswered as the future is an unforeseeable place and yet one must be reminded that life seems to have an unstoppable desire to survive.