Many of the recent pandemic diseases have been caused by viruses that have crossed the species barrier such as HIV and influenza. These zoonotic viruses started in animal hosts and have started infecting humans. This paper's main question is to understand how viruses undergo change to infect humans as the virus has to undergo many obstacles and barriers to initiate infection. To find out more how this is done, molecular and biological processes were looked at. Looking at how the virus is transmitted and then transferred from animal to human host can better provide treatments, cures, and hopefully how to control and prevent zoonotic diseases.
One important factor to look at in zoonotic diseases is the cross-host relationship. In a disease such as HIV, the average recipient does not come in contact with a primate however the disease has killed millions of people. Geographical, ecological, and behavioral separation of the host and recipient is key; people travelling to different countries and bringing back diseases as well as high risks behaviors such as intravenous needle use are a couple of examples. The density of a population has profound effects on transmission rates as well; the denser a population the faster the virus is spread. The recipient must be exposed to the virus, infected with the virus where the virus is transmitted or spread, and finally adapted to the human biological system (Parrish p. 459 Fig 3).
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Viral recombination and genetic reassortment in segmented viruses allow the virus to gain genetic changes in a single step rather than multiple steps. Depending on whether the virus is DNA or RNA will effect how much recombination will occur. A great example of recombination is HIV which is a retrovirus. SARS CoV seems to have developed from a recombination event between the bat virus Cov and some other bat virus which then crossed species barriers to humans and carnivores. When the bat was eaten, the receptor binding sequence of the virus might have been acquired by recombining with the human CoV. Recombination events interrupt optimal proteins that have important structural and biological functions. Influenza A virus is replicates by reassortment. HA and NA proteins on the surface of the virus acts on the host cells sialic acid receptors; HA is needed for binding and NA is used to cleavage away from the host cell.
Evolutionary changes are not always required for viruses to emerge in new hosts. Canine distemper virus has a large range of hosts in mammals but its emergence is limited to contact primarily. In other cases, the evolutionary emergence of viruses allows for the virus to be infected and transmitted efficiently with the new recipient host. Genetic variation plays a key role; the greater the rate of genetic variation, the greater the rate of adaptation to the new host recipient. RNA viruses lack proofreading capabilities and are thus more error prone, have fast replication, and short virus generation time whereas DNS viruses have proofreading capabilities and fast generation times. Thus, DNA viruses usually exhibit cospeciation with their host. Because evolutionary emergence of viruses to allow adaptation to new recipient hosts is not well understood there is some evidence that indicates RNA viruses coevolving with their host over long periods of time as; such an example would be the hantavirus which shows a high degree of specificity for a particular host.
As a virus crosses the species barrier, it must complete a viral trade-off. Viruses that require adaptation to the new recipient host decrease their fitness due to mutations that increase its ability to infect. Once inside the new host, the virus usually will not exhibit fitness levels as seen in its usual animal reservoir, only a small portion of the mutation will exhibit increased fitness. Like most trade-offs, there will be some benefits and some losses. The virus may have decreased fitness, but it has gained new genetic material from the host cell as well as the ability to infect a new host.
The mode by which a virus chooses to transmit is very important constraint. Infuenza A virus infects birds via the enteric route whereas in mammals, Influenza A uses the respiratory route. While it is not clear as to why a virus chooses a particular mode of transmission, it is probably due to choosing the most efficient mode to spread as quickly as possible. Insects are great vectors for cross-species viral exposures due to interacting with multiple environments. An example of a vector virus is the arboviruses which have three modes of transmission: droplets, fecal-oral, and sexual. These different modes of infection give the virus a greater rate of transmission and a greater range of hosts to infect. An experiment was experiment executed using the genome of the bat SARS-like CoV and the human CoV. Figure 6 indicates that there is one breakpoint in the genome of both the human Cov and bat SARS-like CoV.
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While it is not possible to identify which viruses will cross the species barrier, if studies are done to learn common pathways of zoonotic viruses, it may be possible to predict and prevent future epidemics. Controlling insect populations as well as staying abreast of areas of known viral infections should help to decrease emerging diseases. Strategies to decrease epidemics should be to catch the zoonotic before it is transmitted to large quantities of people or stop the change in natural host. The emergence of new viral diseases by animal-to-human host switching has been, and will likely continue to be, a major source of new human infectious diseases (Parrish p. 467). A better understanding of the many complex variables that underlie
such emergences is of utmost importance to public health (Parrish p. 467).
This paper was written well and easy to understand. It gave a nice overview of what it known about how viruses cross the species barrier to infect new hosts. Although there was not a lot of experimentation done in this paper, studies indicate a close relationship between viral DNA and human DNA. In some instances, only one amino acid separates different species from one another. The next line of investigation should be to produce more effective vaccines that take less time to create as well as monitor people's travels to places that have known emergent diseases.