As the title indicates, this article discussed the spread of diseases and viruses throughout different species of animals. In order for a virus to successfully infect new species, there are certain steps that must take place. These steps include: infection of the new host, an outbreak in the new host, and a sustained disease transmission in the new host (Parrish, et al., 2008). These steps will be more specifically addressed throughout this paper.
Most of the new viral diseases emerging in humans have originated from other animals. HIV/AIDS was given as an example of this event. Since HIV/AIDS was first transmitted from primates to humans, millions of people have contracted it. Another example of this can be seen in the 2002 outbreak of SARS which originated in bats. Even though it seems as if viral disease transmission between species is fairly easy, it is not. There are many factors that play a role in inter-species transmission.
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There must be contact between the donor and recipient host in order for the virus to be transmitted. There are geographical, ecological, and behavioral separation factors that affect this. Examples of these factors are wildlife trade, agricultural expansion, and bush meat hunting respectively. Humans unknowingly permit the spread of viruses to new areas through trade and travel. Another example of the importance of contact was given in the transformation of SIV into HIV. HIV wasn't able to flourish in humans until it reached a densely populated area that would sustain onward transmission.
Amplifier and intermediate hosts also help increase the possibility of the emergence of a new disease by introducing viruses to new alternative hosts that it normally would not come into contact with. An example of this was given by the Nipah virus in Malaysia. Ther was intense pig farming and along these piggeries were fruit orchards which attracted fruit bats. Fruit bats, the reservoirs of the Nipah virus, introduced the virus to the pigs and caused a massive outbreak. This shows that host contact and density play an important role in the emergence of a new disease.
In order to infect a new host, a virus must be able to efficiently infect the appropriate cells of the new host (Parrish, et al., 2008). The many host barriers makes the virus have to complete one or more changes to penetrate themm. Also the virus must be able to evade the innate antiviral responses of the host. The first level of protection of the host against viruses occurs at the level of entry into the mucosal surfaces, within the blood, or within the tissues. The human flu was the given example of this. Glycans or lectins may bind to the receptors and eliminate the incoming virus.
Viruses can however change their structures to attach to different receptors. An example of this can be found in the way that the feline parvovirus gained two mutations, allowing it to bind to the canine transferrin receptor, thereby becoming the canine parvovirus.
The existing host range is also an important part of a virus switching hosts. There are two types of viruses. There are the generalist and the specialist viruses. The generalists infect a broader category of host, while the specialists only affect a few hosts. This in turn makes the generalists better at infecting new hosts because they are more adapted for it and can use the host cells mechanisms of the other hosts to infect the new one.
The topic of evolution was discussed also. Genetic variation is important to the emergence of a virus in a new host. The higher the rate of variation, the more likely it is for adaptation of the virus to the new host. A challenge for host-switching viruses that require adaptation however is, the new mutations that help it to thrive in the new host often decrease its fitness in the original host.
Another important factor infecting cross-species infections is the way the virus is transmitted. Different hosts and environments require different modes of transmission.
Recombination is important to infecting new hosts as it allows the virus to make many genetic changes within one step, leading to the gain of helpful genes and deletions of harmful ones. An example of this was observed in the emergence of the 1957 H2N2 and 1968 H3N2 influenza A pandemic viruses (Parrish, et al., 2008). Another example was given in the way that the SARS CoV arose from a recombinant between the bat CoV and another virus before affecting humans (Parrish, et al., 2008).
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Another key in viral emergence is viral intermediates. Viral transfer very rarely occurs directly, but rather occurs gradually. This gradual change produces intermediates that are not fit enough to infect the new host but allow for more changes to be made to reach a more fit/equipped virus. Early detection of these intermediates in the new host would produce a chance for epidemic control.
Although there are new viruses constantly emerging, there are ways to decrease the spread of them. Health monitoring, quarantine and vaccination all play a critical role in the decrease, and even extinction of some viruses. Antivirals may also be used to decrease the occurrence of viruses even though they are not as helpful in large scale outbreaks.
Overall this article was very informative. It addressed many dimensions of the spread of cross-species viral transmission. It showed the process of infection from the host and the viral side. One thing that was noticed however, was that the article would touch on a topic and then say that they mechanism or relationship wasn't very well understood. An example of this was when the article stated that little was known about the evolution of viruses to allow adaptation to new hosts. In a way this was kind of a letdown because it was thought that the writer was about to go into more detail about the subject. This did however spark an interest and makes one want to find further research about this topic. As stated above, overall this was a very informative and supportive in helping the reader to understand the process of viral transmission between species and how it can be decreased.