Synopsis of 'Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases'. Viral diseases have been, and will be, one of the major health concerns in the human population for some time. The likelihood of total eradication of viral diseases is implausible, not only due to the sheer number of current known human viruses but the threat of new emerging diseases caused by zoonotic infection. The article addresses this threat along with difficulties and obstacles that virions must overcome to become successful in switching to a new host. Virulence factors that contribute to a virus's pathogenicity of their original target host must be altered to be able to survive in a different environment within the new host; these alterations may be dramatic as in viruses that infect across taxonomic classes as opposed to viruses that infect within the same order due to the difference in biological structure, but as the article suggests, "while the evolutionary relatedness of the hosts may be a factor in host switching, the rate and intensity of contact may be even more crucial" (Parrish et al. 460).
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The source of new emerging human diseases come from a large reservoir of enzootic and epizootic viral diseases in animals that have been always present but do not effect humans until given the opportunity to switch hosts. Interaction and contact between humans and animals have been limited due to geographical, ecological, and behavioral barriers to allow for sufficient exposure. This exposure, if obtained by the virus, must then be followed by a sufficient interhuman contact in order for the virus to avoid dead-end infections of that strain (Parrish et al. 458-460). As with many diseases, population density is an important factor in transmission. Large populations that are in constant contact with each other enables for more opportunities for viruses to spread, essential for a newly emerging viral disease. Since limited exposure may prevent host switching to the potential host, intermediate hosts are likely to take a penultimate role in the adaptation of the virus to it's new host. By using intermediate and amplifier hosts, viruses may bring animal diseases which would normally have little contact with alternative hosts into close contact with recipient hosts (Parrish et al. 460).
Once adequate exposure of the new host is established, entry into the host will follow. Preceding attachment to a specific cell, viruses must bypass primary boundaries, such as skin and normal flora, into more internal target cells that provide a more preferable environment for the virus to replicate. During this stage, viruses that have previously been foreign to the new host will contain elements that are recognized as foreign, thus eliciting a relatively quick immune response. Viruses must then adapt rapidly in their new host in order to successfully establish itself. Transferring to a new host requires changes in interaction from the previous target cell to the new host's cells. Unless the virus is host-switching to nearby relative of it's previous host, receptor binding mechanisms must change to accommodate the new host cell. Due to the complexity at which receptors bind their respective ligand, viruses must make multiple mutations in order for entry into the cell (Parrish et al. 460). Mode of transmission may be another complication involved in host-switching. The virus must either use existing modes of transmission or obtain more adaptive modes, such as droplet spread or fecal oral, increasing their chances of interhost transmission (Parrish et al. 642-643).
Despite these complications that viruses may have, viruses may remedy them through usage of recombination and reassortment. The current abilities of viruses that increase the chances of viral infection are also aiding viruses to obtain a new host. By using variant strains of viruses that are simultaneously infecting a single host, RNA and DNA viruses are able to alter multiple parts of their DNA to form a new, possibly more virulent, strain. Multiple viral information sharing enables portions of genetic information required by emerging viral disease to infect alternative hosts. This not only allows the virus to become more proclaimed in its original host but may lead to acquisition of machinery to which the virus may infect a new host (Parrish et al. 463).
However many difficulties that viruses must overcome to enable themselves to switch hosts, the possibility is certain given the opportunity. This provides humanity with an ever imposing threat of viral disease that is ever changing and an animal reservoir that is too large to ever realistically manage. With an immense number of different species of animals, some of which are still yet to be discovered, along with the yet unknown animal viruses, mankind can only set up few preventative measures. As the article suggests, along with conventional infection control procedures, continued close observation and research would be best to prevent any unanticipated outbreaks or epidemics (Parrish et al. 466-467). Although there is inadequate resources to research every virus and vaccinate every animal, focus on research of zoonotic viral diseases is essential to human health. Further understanding of mechanisms of viral transmission and adaptation allows for a quicker assessment and treatment of cases once an inevitable emerging viral disease breaks out. By understanding where likely sources of zoonotic diseases may come from, researchers may predict and aid major concern areas before the disease escalates by coming into contact with a larger population. Despite the importance of current illnesses that is afflicting the current population, the number of different diseases will only continue to grow as humans will unavoidably be in contact with animals (Parrish et al. 467). Thus, to ensure the health of future generations, research must not be fixed on current human pathogens but to study possible animal born illnesses as well.
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