This article focuses on the transmission of viruses between different species and how epidemic diseases emerge. There are many questions that are raised throughout the article. These questions focus on the sources of new epidemic viruses in humans and animals, host switching, and evolutionary mechanisms that leads to the emergence of diseases.
According to the studies done by Parrish and fellow scientists, there are many sources of new epidemic viruses, but the main sources are epizootic and enzootic viruses that come from animals. Epizootic viruses are diseases that arise in an animal population at a given time and spread faster than what was expected. Unrecognized viruses also cause new epidemics of diseases to arise in the human and animal population. Diseases such as SARS, Ebola, Marburg viruses, and HIV-1 and 2, just to name a few, are examples enzootic viruses. According to the authors, host switching is also a major cause for the rise of new epidemic viruses in the human population (457-470). Host switching is when a virus or disease can cross from one species into another and remain viable. "HIV/AIDS is an important recent example of viral emergence by host switching. Following its emergence into humans from primates an estimated 70 years ago, HIV has infected hundreds of millions of people" (Parrish, et al. 457-470). HIV originated from the primates and has undergone host switching with the human population. Now HIV is one of the largest viruses within the human population. Some of these host switching viruses can cause major losses of life and money in the economy. "A recent example of viral disease emergence by host switching is the CoV causing SARS, which infected thousands of persons and spread worldwide in 2002 and 2003â€¦SARS CoV caused hundreds of deaths and economic disruption amounting to $40 billion" (Parrish, et al. 457-470).
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Another question raised in this article deals with environmental and demographic barriers to host switching. "Cross-host exposures are an important step in transference to new hosts, and some host-switching events are likely prevented because of limited contact between the viruses and the potential new hosts" (Parrish et al. 457-470). This idea shows that in order for a virus to switch between hosts, the virus and the host must come in contact with one another. If there is no contact in some form, host switching is unsuccessful. "For example, both HIV-1 and -2 have transferred to humans multiple times since approximately 1920 to create new epidemic virus clades" (Parrish et al. 457-470).
This article also expounds upon some of the environmental and demographic barriers that prevent host switching from occurring. For example, barriers between humans and primates was limited, therefore viruses could not spread between humans and primates. If the environments and demographics are not appropriate, host switching will not occur. "Contact between donor and recipient hosts is a precondition for virus transfer and is therefore, affected by the geographical ecological and behavioral separation of the donor and recipient host" (Parrish et al. 457-470). The authors also speak about the cause of some of the changes in viral infections and host switching. Changes such as IV drug usage, traveling, and farming, just to name a few, have promoted viral emergence. If these human- encouraged practices can be eliminated or greatly reduced, then there may be a change in the promotion of viral host switching.
"The density of the recipient host population is important in the onward transmission and epidemic potential of any transferred virus" (Parrish et al. 457-470). Having a large recipient host population will aid in the transmission of viruses between hosts. Viruses that may have been spread due to a high recipient host population are H5N1 flu virus, Aedes albopictus, and SARS CoV. The study also illustrates that there are three ways in which disease emergence can be affected. The first way is the emergence of diseases may be affected by host contact patterns and density. For example, HIV began as SIV in the primate population. In order for SIV to affect human populations, HIV had to undergo changes on the genetic level and in human behavior to infect humans. Another effect on disease emergence is through animal viruses coming in contact with other species that could never be infected by these viruses until they came into close contact with the virus (Parrish et al. 457-470). Thirdly, disease emergence has been affected through ecological changes such a farming of virus- infected animals. "Human infection with H5N1 influenza viruses most often occurs after the infection of poultry on farms or in live bird markets, allowing viruses of wild birds to gain access to human populations" (Parrish et al. 457-470).
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The article also discusses virus transfer and how this can be interrupted if there are complications in genome replication and related processes. Therefore, it is in advantage for the virus that genome replication and related processes occur correctly in order to transfer itself into a new cell. The author furthermore states that the production and shedding of infectious virus may also be host specific (Parrish et al. 457- 470).
"Spillover or epidemic infections have occurred between hosts that are closely or distantly related and no rule appears to predict the susceptibility of a new hosts" (Parrish et al. 457-470). According to the authors, viruses can be transferred between related species. For example, primates and humans can both be infected by HIV and cats and dogs can both be infected by FPV and bats and some carnivores, share the SARS CoV-like virus. Another point raised is that although some viruses are transferred between related hosts, this is not the only deciding factor in host switching. How much and how often the hosts come in contact with each other matters more than anything.
Another important point raised is the protection against viruses infecting in cells. There are many defense mechanisms set up to prevent infection. Some of these defense mechanisms include glycans, lectins, and glycoproteins. Therefore viruses can be blocked from infecting a host cell. Receptor binding can also affect the host that the virus will infect. If the virus binds to a different type of receptor of the new host, the virus will then be able to infect that new host. This is the case with FPV and mammalian and avian influenza viruses and HIV-1. According to the author, the virus must undergo a complex process and go through many changes to be able to bind to new receptors in new hosts to cause infection. But in the study it was discovered that once binding of the virus and the receptor take place, there may also be complications that furthermore prevent infection. If there are problems with the protein blockage, then the virus cannot infect the cell.
According to authors, the existing host range of a virus is a factor in host switching because viruses can be divided into two categories, generalists and specialist (457-470). Generalists have a wide range of host and can switch between varieties of hosts. Specialist, on the other hand, usually only affect a few host and usually does not undergo host switching to unrelated species. But despite the host range and if the virus is generalists or a specialists, the virus can still infect any host.
There are also, according to the author, viral evolutionary mechanisms that lead to virus emergence. But according to the authors, these evolutionary changes are not necessarily needed all the time for a virus to emerge in a new host (457-470). This is important because there are other important ways for a virus to emerge in new hosts, meaning that evolutionary changes are not the only way. According to authors there are also viral fitness trade-offs. The major challenge with trade-offs is the rise of mutations that affect the virus and the virus may not be able to infect a host cell appropriately. Mode of transmission also plays a major role in the emergence and transfer of viruses. "For example, arthropod vectors that feed on a range of mammalian hosts can facilitate cross-species viral exposures" (Parrish et al. 457-470). Recombination and re-assortment also are important in host switching and viral emergence. Recombination and re-assortment can cause mutations such as deletions and change genetic information in viruses. The 1957 H2N2 and 1968 H3N2 influenza A viruses are examples of re-assortment. A question is also raised by the author. Are viral intermediates with lower fitness involved in host switching? The answer to this is that "The process of virus transfer to a new hosts is rarely observed directly but can be inferred by comparing viral ancestors in donor hosts with emergent viruses from recipient host" (Parrish et al. 457-470).
To conclude, this article focused on the transmission of viruses and the emergence of new epidemic diseases. There are still some things that are uncertain in the viral community. According to the authors, the way that viruses allow adaptation to new hosts, the nature of trade-offs and how that affect cross-species transmission, and which viruses in the animal kingdom will cause infections in humans are still uncertain. Though further study and experimentation, there may be answers to these uncertainties. From the studies completed to gain the knowledge in the article, studies and experiments can be done to further understand one of the major viruses affecting the human population today, HIV. I believe that since a lot is known about how viruses are transmitted and can affect cells, we should now look for new ways to stop the infection of HIV. If the receptor binding of HIV in a host cell can be inhibited, then the virus will not be able to successfully transfer to the new host. But this is not the only way to stop HIV from infecting cells. There are other ways that are soon to be discovered. I believe that this article is relevant and important because knowing that viruses can be transmitted across different species is important, especially for research purposes. Also knowing that some animal viruses can also cause viruses in humans is also important. It is very relevant today to know how new epidemic diseases arise and how to control the disease. If it is known where the disease emerged from it may be easier to find a cure to further stop the spread of the disease.
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