What Factors Promote Or Inhibit Viruses
Throughout the article, the author(s) focuses on analyzing questions like: what factors promote or inhibit viruses from switching hosts? "Are viral intermediates with lower fitness involved in host switching?" (Parrish 464), and how can viruses be stopped from spreading new diseases and infections that lead to epidemics? The article explains the switching between different hosts (species) by viral cells to create new strains of viral diseases. Some examples mentioned in the text of viruses that are transferred between hosts to cause outbreaks are as followed: Measles, Smallpox, Influenza, CPV, HIV-1, SARS CoV, Dengue, Nipah, Marbury, Ebola, Myxoma, Hendra, and Canine Influenza (Parrish 458-Table1).
Often times, the animals serve as a reservoir for the virus to be spread to human or other animal populations.A viral disease will emerge if host switching is successful and the following conditions permit: Infecting a new host that is able to spread the initial infection, the newly infected host has to cause local chains of transmission in the new host population before outbreaks occurs, and a sustained endemic host-to-host disease transmission in the new host population (Parrish 457). Influenza viruses commonly use this method of host switching to cause new epidemics, and in some cases pandemics, each year.Viruses use recombination and reassortment to their advantage when undergoing genetic mutations (Parrish 463). These processes allow them to quickly alter genomic material to create an unfamiliar disease to the host's immune system.
The virus begins by infecting one host and incorporating its genetic information into the host's genome or replicating more viral cells to be transferred to a new host. The new host will undergo the same processes as the original host and this chain-like reaction will allow the virus to successfully infect numerous individuals. Viral mutations occur, causing the genetic material from animal and human genomes to combine to create new strains of influenza viruses, for example, H1N1 and H5N1. Other devastating viral infections due to animal-human transmission include HIV/AIDS and SARS CoV.
SARS CoV is transferred from bats to humans.Based on the information provided in Figure 3 of the document, host-switching viruses emerge through the following steps: 1. Exposure - virus is able to successfully enter host, 2. Infection - virus replicates in the host, 3. Spread - host transfers virus to new host, and 4. Adaptation - new virus must be able to successfully cause disease and repeat the previous steps in the new host. (Parrish 459). There are various factors that hinder virus transfer between hosts. They are as followed: geological barriers, ecological barriers, and behavioral separations. Alterations to any of these factors can either limit the amount of transfer taking place or promote new diseases to emerge.The HIV/AIDS virus has been a "deadly, infectious disease that humans acquired from primates approximately 70 years ago".
It infects a range of "1.8-4.1 million" yearly (Parrish 458). The geological barrier between the infected primates in the jungles of Africa and humans lessened the chances to the virus being spread to small populations, therefore the virus met a "dead-end". On the other hand, the virus required, "genetic changes to confer adaptations, and behavioral changes" amongst humans to strike (Parrish 460).According to the article, "to infect a new host, a virus must be able to efficiently infect the appropriate cells of the new host, and that process can be restricted at many different levels, including receptor binding, entry or fusion, trafficking within the cell, genome replication, and gene expression" (Parrish 460).
Humans have various defense mechanisms against viruses, for example: unbroken skin preventing virus from entering the blood stream, mucosal surfaces to trap pathogens, and lymphatic circulation to fight infections. The cellular attachment/receptor binding of the virus and new host cell is critical for host switching success. Every strain of viral infection that enters the host has a specific receptor-binding site located at a target region of the host. For example, "avian and mammalian viruses infect cells of different tissues and must recognize sialic acids [acts as a receptor for influenza viruses to allow attachment to mucous cells and is added to dietary supplements] (New Zealand Pharmaceuticals Limited) found on cells of the intestinal tracts of waterfowl or in the respiratory tracts of humans or other mammalsâ€¦" (Parrish 461).
This allows the binding site to change rapidly and the virus can adjust to the host. There are several intracellular proteins that block the progress of viral replication and infection of nearby cells in the host."Viral host switches between closely related species (e.g., between species within genera) may also be limited by cross-immunity to related pathogens or by innate immune resistance to related viral groups" (Parrish 460). The researchers have not been able to find any correlations between the spreading of viruses through only closely related species, mammals, by comparing HIV, Canine Influenza, and SARS CoV.
The virus now faces a barrier of host switching between closely related species."If several changes are required to allow host switching, then intermediate viruses would likely be less fit in either the donor or recipient hosts than the parental or descendant virusesâ€¦" (Parrish 464). To further prove this point, in the transmission of canine influenza virus, "FPV adapted to dogs through at least one lower-fitness intermediate. An experiment provided data that showed FPV in dogs were both less fit in cats than the FPV from which they were derived and less well adapted in dogs than the CPV variants that replaced them" (Parrish 464).
Viruses that could only partially adapt to their host would go extinct because they would not be able to fully function in replicating and spreading. On the other hand, their ability to cross any evolutionary "low-fitness valley" for partially adapted viruses is important for virus host switching (Parrish, 464).A major source of human infectious diseases arises from the newly developed technique of viruses switching from animal to human hosts. The best way to control the virus from rapidly spreading is to continue monitoring health and resorting to quarantine before a large number of people become infected. It is important for people in regions that are highly susceptible to diseases to get vaccinated to reduce the likelihood of new diseases emerging and prevent eradicated infections from reappearing.
Researchers have made a lot of progress in indentifying the numerous factors that influence and facilitate viral host switching. With all the progress made, they are still unable to predict which strains of viruses will combine or when they will combine to create new diseases. Understanding the pathways in which viruses switch hosts and replicated allows scientist to create ways of preventing the virus from spreading.In some instances, a virus can easily infect humans or it could be somewhat of a challenge due to non-permitting factors. As discussed, viruses need the right circumstances to successfully create an epidemic.
One of the most important requirements for a virus to successfully cause an outbreak is, to have a host that will successfully transfer the virus to other hosts. Once the virus causes an outbreak to emerge, it can lead to formation of epidemics and pandemics that could potentially "wipe-out" a large number of organisms, especially humans. Although there are antiviral currently available, an extensive amount of effort is being applied to researching and understanding how viruses operate so that new treatments can be made to eradicate viral diseases rather than treat the symptoms.
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