Viral Agents"A filterable agent..."
Viruses are acellular organisms that contain nucleic acid in the form of RNA or DNA (either double or single-stranded) surrounded by a protein coat known as a capsid. Some viruses are further protected by a lipid bilayer (or membrane) with proteins planted in them known as an envelope. Viruses are 20 to 100 times smaller than bacteria, and unlike bacteria, they are not alive. Because they lack the machinery with which to replicate their nucleic acid, viruses function parasitically and besiege living cells in order to thrive.
Invasion and Defense
A virus first approaches its host cell and uses its surface proteins to bind to the host's cell surface receptor (viruses target specific hosts with the correct surface receptor). The virus may enter the cell through a process called endocytosis in which the virus is enveloped by a section of the cell's plasma membrane. Otherwise, nucleic acid is injected into the cell as the virus melds its lipid envelope with the cell membrane. The virus then appropriates the cell's ribosomes, enzymes, and reproductive machinery to assemble replica viruses. After reproduction, the new viruses exit the cell either by leaving the cell a few at a time through reverse endocytosis (budding) or by bursting forth from the cell (lysis). The process often leads to cell death due to cell lysis or cell suicide.
In response to viral invasion, the infected cells produce interferons or cytokines. Interferons are proteins that are released into the bloodstream, and they signal for other cells to produce enzymes to counter the infection. Cytokines are proteins released by infected cells to urge adjacent cells to mount their defenses against the viral invasion. Because it is difficult for treatments to distinguish between the viral processes and a cell's natural processes, most treatments address only viral symptoms. Viral infections generally do not respond to antibiotics but may be responsive to antiviral compounds, of which there are few available, and those that are available are of limited use.
Prevention against viruses consists of vaccinations. Killed or inactivated viruses are a major form of vaccinations. This method bears little risk of infection but produces a weak immune response. Live or attenuated viruses are also used, which induces a strong response, but increases the risk of causing the disease as well. Acellular, subunit, toxins, and conjugated vaccines are also used to elicit the appropriate immune response.
History of Viruses
Viruses have beleaguered the human immune system since the dawn of time; the mummified body of Egyptian pharaoh Rameses V from the 2nd century BCE bare signs of smallpox infection. Edward Jenner in 1796 is credited as the first to vaccinate by using antigen from cowpox to elicit an immune system response to smallpox. The Latin root of the word vaccine, vacca, means cow. Viral agents have eluded scientists until the late 19th century because their small size meant they were not observable through a light microscope. The discovery of the first actual virus was credited to Russian botanist Dmitri Iwanowski in 1892, who was studying the tobacco mosaic disease when he discovered that the agent which caused the disease slipped through filters that caught all known bacteria. The first virus was seen by the human eye in 1930s through an electron microscope.
There are three theories regarding the origin of viruses. The first theory is that they are intracellular parasites that lost the ability to replicate. The second theory states that viruses are ordinary cellular material that gained the ability to replicate. The third theory is that they come from self-replicated RNA. Unlike bacteria, fungi and other living organisms, viruses are not categorized by their evolutionary development but rather are categorized by morphology, size, RNA/DNA structure and antigens produced. There are nineteen families of viruses. Certain viruses such as Hepatitis C, D and Astroviruses remain unclassified.
Viruses as Biological Weapons
Viral agents were first used as weapons by British General Jeffrey Amherst in 1763 against the Native Americans during the French and Indian War. As a biological weapon, viral agents would most likely be distributed in aerosolized form. Unlike bacteria, antibiotic treatments are not effective therapies against viral infections. Inside a host, viruses may remain dormant for long periods before reviving to infect other hosts.
In a 1998 statement before the Joint Economic Committee of the United States Congress, Dr. Ken Alibek (formerly Kanatjan Alikbekov), the former First Deputy Director of Biopreparat, the Soviet biological weapons program, discussed the development of chimera viruses. To produce a chimera virus, genetic material from other viruses were injected into smallpox (or any other) virus. This process allows the new virus to retain smallpox's virulence and its appearance under a microscope while spreading an entirely different infection. This feat of genetic engineering allows viral agents to evade known vaccines and treatments. According to Dr. Alibek, in the 1980s, the Soviet biological weapons program developed a chimera virus with the genes of Venezuelan equine encephalomyelitis virus. Reports suggested that research on an Ebola and smallpox chimera virus was also conducted. Through genetic manipulations, new viral agents may be introduced into biological warfare.
In addition to being used as possible biological weapons agents, viruses can also be harnessed for positive research; genetically engineered viral agents can produce effective medical therapies against a variety of ailments.
|Arenaviridae Family||Venezuelan Hemorrhagic Fever||Argentine/ Bolivian/ Sabia-associated hemorrhagic fever||Lassa fever||Lymphocytic choriomeningitis (LCM)|
|Bunyaviridae Family||Crimean-Congo Hemorrhagic Fever (CCHF)||Rift Valley Fever||Hantavirus pulmonary syndrome (HPS)||Hemorrhagic fever with renal syndrome|
|Filoviridae Family||Ebola Hemorrhagic Fever||Marburg Hemorrhagic Fever|
|Flaviviridae Family||Kyasanur Forest disease||Omsk hemorrhagic fever||Tick-borne encephalitis|
|Paramyxoviridae Family||Hendra virus disease||Nipah virus encephalitis|
|Orthopoxvirus Family||Smallpox||Cowpox||Monkeypox||White pox|
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