The largest threats to public health are the viruses that have been most recently introduced into the population. A transfer across the species barrier of certain viral diseases creates new fatal infections that are not understood enough to treat when the viruses are young and at their strongest and largest threat to the host because of their genetic variation often required to adapt to new hosts. An article, "Cross-species virus transmission and the emergence of new epidemic diseases" by C.R. Parrish, et al, states that significant viruses such as measles and smallpox "may have originated in wildlife or domesticated animals in prehistoric times." Recent viruses to cross the species barrier include SARS, influenza, HIV, and Ebola fever. As time goes on after the species cross over, and if the conditions allow the infection to spread to be an epidemic, more becomes known about the virus and how to battle its effects. This process is a difficult one, and for many diseases is a constant struggle. Even though HIV is becoming more and more well understood, "1.8 to 4.1 million new human HIV infections still occur each year (Parrish, et al)." With the recent deaths caused by the H5N5 bird flu it has become apparent that the fight to keep people healthy and prevent death from infection is important to our survival, "the toll on human populations would be enormous if the H5N1 virus acquired efficient human-to-human transmissibility while retaining high human pathogenicity (Parrish, et al)." How viruses evolve to suit new hosts from across the species barrier is not yet well understood, and further study is important to the prevention of spread and protection of human hosts.
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Parrish, et al stresses the risks of these viruses that have crossed the species barrier and emphasizes the importance of understanding how this transfer happens. Transfer from one species to another may be able to be prevented; it is known that viral transfer is "affected by the geographical, ecological, and behavioral separation of the donor and recipient hosts (Parrish, et al)." and likely crossing the species barrier is due to human behavioral changes that increase its likelihood. Increased and multiple contacts allow for viral transfer, "HIV-1 and -2 have transferred to humans multiple times since 1920 to create new epidemic virus clades" and this is likely due to the increase in human and primate exposure during that time. Transfer multiple times allowed for "multiple and complex adaptive virus changes (Parrish, et al)." that allowed for spread of the virus further from human to human in the population. The avian flu however is not due to all the same factors, it increases human exposure because it is "carried long distances by migratory birds, allowing them to become widely dispersed geographically (Parrish, et al)." These birds then spread the virus to birds in close contact with humans, such as farm animals or bird markets, and the virus is able cross species to human hosts.
The transfer from animal to human has several factors that when inhibited keeps the virus from infecting a new host, these include "receptor binding, entry or fusion, trafficking within the cell, genome replication, and gene expression (Parish, et al)." Even though HIV transfer was from the closely related chimpanzees to humans, other virus transfer such as SARS from bats to human shows that there "no rule seems to predict the susceptibility of a new host (Parish, et al)," not even host genetic separation. It seems though that genetics does play an important role, viruses with a high level of evolution are more likely to cross the species barrier. Most new viruses are "poorly adapted, replicate poorly, and are inefficiently transmitted (Parish, et al)." This means the viruses that change the most, RNA viruses, should have the highest chance to adapt to a new species of host, but the rates of evolutionary change is not always predictable. Some viruses, some DNA, some RNA, are capable of recombination which allows for multiple genetic changes all at once to create a virus with totally new benefits or remove harmful mutations and a very high rate of evolution. A recombination of genes was associated with the emergence of both HIV and SARS. Many times recombination is important for host switching due to "incremental host adaptation... [and] secondary reassortments... after transfer, which may have facilitated its further adaptation (Parrish, et al)."
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The major question posed by Parrish, et al in "Cross species..." is whether viral intermediates with a lower fitness involved with host switching or not. Since several changes are required to adapt to a new species of host, "intermediate viruses would likely be less fit in either the donor or recipient hosts than the parental or descendant viruses (Parrish, et al)." An example would be the FPV virus from cats to dogs that required an intermediate that was "less fit in cats than the FPV from which they were derived and less well adapted in dogs than the CPV variants that replaced them (Parrish, et al)." This need for a less fit intermediate would be a barrier for any virus because "partially adapted viruses would quickly go extinct, as they would be unfit in the donor host and also insufficiently adapted to... spread in the recipient host (Parrish, et al)." A weak intermediate step, if caught in this stage, could mean early control of emerging infection. The mechanism for viral spread is not yet understood and "[h]ow viruses gain the ability to spread efficiently... is a key question in viral emergence (Parrish, et al)."
Understanding viruses is incredibly important for the future well being of humans. How viruses are able to spread and then evolve to fit a new species of host is not yet well understood and a new way to target a virus before it has even matured into a harmful infection could stop the spread before it starts to show symptoms in humans. The need to understand viral switch between species is growing. The increase of population and population density means a higher food demand and higher density of animals raised on farms. The recent avian flu followed by the swine flu outbreak are indications that study needs to be done to prevent viruses capable of recombination from jumping the species barrier and spreading new virus infections across the population. If an intermediate can be isolated of potentially fatal viruses, it could prevent new pandemics, infections and deaths through the isolation and active control of the new virus, possibly even before it reaches a strong fully adapted form.