Emerging viral diseases in humans are a matter of grave concern: new viruses within the population can often cause widespread epidemics (p. 457). These new viruses in the human population are often the results of cross species transmission (p. 458). SARS, for example, originally infected bat hosts but moved on to human hosts in 2003-2004 (p. 458). Additionally, HIV transmitted from primates to humans, around seventy years ago (p. 458). Smallpox, measles and influenza are also thought to have originated from animal hosts (p.458). In order to prevent the emergence of new viruses, it is important to understand how viruses are transmitted from one species to another and how they adapt to their new host. A research article, "Cross Species Virus Transmission and the Emergence of New Epidemic Diseases", by Parrish, Colin R. Parrish, Edward C. Holmes, David M. Morens, Eun-Chung Park, Donald S. Burke, Charles H. Calisher, Catherine A. Laughlin, Linda J. Saif, and Peter Daszak, addresses the factors leading and aiding to cross-species transmission. The article examines the animal sources of emerging viruses, environmental and demographic factors influencing host to host transmission, the mechanisms by which cross species transmission occurs, and how viruses may evolve and adapt to a new host (458-466).
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In the article "Cross Species Virus Transmission and the Emergence of New Epidemic Diseases" many factors are discussed concerning the transmission of viruses to new hosts. The article examines how HIV previously had transmitted from primates to humans many times, but did not cause widespread epidemics (p. 459). The conclusion is reached that in these first cases of primate to human transmission, the virus may have only infected one person or a small group of people (p. 459). The article discusses how HIV was only able to cause an epidemic when it reached a high-density population of humans, and was able to further spread throughout the globe due to behavioral patterns of travel and human expansion (p. 459).
Another factor that contributes to host to host transmission is environmental factors, an example of which is the Nipah virus (p.460). The article examines how Nipah virus in Malaysia spread by increased pig farming (p.460). Fruit orchards planted around piggeries brought bats, the carriers of Nipah virus to the piggeries, increasing human and pig's exposure to Nipah virus (p.460). From analyzing past viral diseases, the conclusion is made that factors such as an increase in the distribution of species geographically, the decrease in separation between species due to behavioral changes, environmental changes such as deforestation and agriculture, and high-density populations all promote and increase transmission of viruses between species (p.459-460).
The article "Cross Species Virus Transmission and the Emergence of New Epidemic Diseases," examines the mechanisms by which viruses are able to cross the species barrier. By looking at past examples in which viruses have crossed the species barrier, it is concluded that viruses can transmit between closely related species and distantly related species (p. 460). It also seems that how often two species come in contact with one another may be more of a determining factor for cross-species transmission, than the relatedness between two species (p. 460).
To infect a new host, a virus must be able to infect the cells of the new host and use the host cells for replication. This is analyzed step by step in the article. First it is recognized that a virus must enter the new host through the skin, mucosal areas, or enter the blood (p. 460). After entry, a virus must be able to evade the innate immune response of the new species host, either by defensive mechanisms already assumed by the virus or by evolving defensive mechanisms against the host immune system (p. 460). The next step discussed, is how a virus binds to a receptor of a new host cell, and it is assumed that the virus must undergo some changes to recognize a new receptor site of a new host cell (p. 460- 461). Other changes that must occur for some viruses to gain entry and successfully infect a new host cell are also analyzed. For example some viruses may have to change capsid structure, develop mechanisms of defense against interferons, and evolve slightly to be able to successfully utilize the new host's cells for gene replication and gene expression (p. 461). The host ranges of viruses are also looked at, and it is concluded that viruses that are already able to infect many different host are more likely to cross species barriers to new host than viruses that are "specialist" and only infect one species (p. 462). From looking at all the factors needed for cross-species transmission it is assumed that a virus capable of infecting a new host must have some plasticity and be able to adapt to a new host (p. 460- 462).
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It is clear that there are many barriers preventing some viruses from infecting a new host, so evolving and adapting to a new host is important for infecting a new species. The article examines how viruses may evolve and adapt to a new host, and concludes that the level of genetic variation is an important factor (p. 462). The article also claims that cross species transmission must then occur more often in rapidly evolving viruses, which are prone to error in the replication process (p. 462). The article analyzes viruses that undergo genetic recombination and conclude from looking at viruses that have successfully transmitted to new hosts in the past, that genetic recombination is advantageous for cross species transmission (p. 463). For example, the emergence of HIV in humans occurred simultaneously with genetic recombination between viruses of different primate species (p. 463).
Most viruses that transmit to a new host species also have to go through a "low fitness step", an intermediate step in which the virus has undergone partial changes and is not completely fitted for the old host or the new host (p. 464). The article examines this phenomenon and concludes that the low fitness step is essential in the process of viral host switching (p. 464). The low fitness stage of a newly emerging virus is also thought to be a stage at which a virus can be easily eradicated (p. 464). SARS was not transmitted between humans very successfully when it first emerged, so the virus was identified and contained before it could become fully adapted to humans and spread effectively (p. 464-465). From this example, it is concluded that eradicating a virus at this stage can be quite successful (p. 464-465).
"Cross Species Virus Transmission and the Emergence of New Epidemic Diseases" encompasses a lot of information on the process of viral transmission from animal host to human host, by looking at past examples. The article is of much importance as many new viral epidemics in humans and other animals are the results of cross species transmission (p.458). The article helps highlight the main points but also points out where further research can go in this field of study. Further research in this area should investigate how viruses evolve and adapt to new hosts, and research on how transmission from animals to humans can be limited and controlled.
I feel this article brought to light some important aspects of how cross-species transmission occurs. The article reached accurate conclusions on the factors and mechanism that influence this process by looking at past examples of viruses that have crossed species. This article also provides good information on how vulnerable a virus can be during the process of adaptation to a new host. I feel this article explains how an emerging virus can be eradicated fast if identified quickly. This is important for preventing the emergence of new viruses in the human population, and new research in this field should focus on identifying and monitoring possible viruses that may transmit to the human population from other species populations.