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As time passes, humans evolve and viruses are evolving simultaneously with us. The serious issue of human viral infections is not due to the formation of brand new viral species, but actual animal viruses that have gained the ability, throughout the passing decades, to switch hosts and infect humans. Though there are now many human viral infections that originally belonged to animals, the process of that host switching and the creation of epidemics has specific steps that must be made in order to do so. There are many intrinsic characteristics of these viruses that can possibly lead to a prediction in their abilities of cross-species transmission. Regardless of those characteristics, all viruses must pass through many different environmental, demographic and cellular barriers in order to switch hosts successfully.
When looking at the different emerging viral infections of today, they typically originated from animals and were unknown to man before the host switch. Diseases such as HIV, SARS and Ebola are all such examples, and have consumed millions of dollars in research and medications. How were these viruses even able to make that initial leap to a different host? In order for viruses do this, they have to have multiple exposures to the new host candidate. Because this step is absolutely essential in cross-species transmission, it is believed that many other viral host switches have been stopped because of the lack of viral-new host interactions. Multiple interactions must also occur along with different adaptive changes of the virus. The limited opportunities that animal viruses have to interact with humans, and the adaptive changes they must make, is the initial barrier they must pass in order to fully switch hosts. Those opportunities to interact are all dependent on geographical, ecological and behavioral factors of the environment and humans. When those factors facilitate the occurrence of viral-human interactions, the likelihood of a cross-species virus transmission increases.
Cross-species virus transmission probability can sometimes be related to population density. Some viruses, for instance, influenza virus, use birds as vectors which gives them the ability to infect a wide range of people in different geographic areas. These viruses donââ‚¬â„¢t necessarily depend on a high population density in a single area. However, other viruses, for instance HIV, depend on a high human population density for sustainability due to its usual mode of sexual transmission. Cross-species virus transmission susceptibility may also correlate to the relativity of the initial and new host. Meaning, if the new host is genetically similar to the natural host that already belongs to the virus, then host switching may occur with less difficulty. Depending on the type of virus, there are some that can infect distant host relatives. For instance, SARS was transferred from bats to humans and Feline Panleukopenia virus was transferred from cats to dogs. Others can only cross-species transmit to hosts that are closely related to their natural host. For instance, the SIV transfer from the chimpanzee to HIV in humans.
For certain viruses that depend on vectors for transmission, genetic variations may be needed also in order to survive intermediary vectors before reaching the new host. Trying to adapt to these different vectors can increase or decrease the difficulty in infecting the new hosts. Also different intermediate hosts can act as amplifiers for the virus, increasing the amount of virus production, thereby increasing infections among humans. This happened with the Nipah virus and SARS by the farming of certain animals, which gave these animal viruses closer contact to humans. Recombination of genetic information inside of an intermediary animal reservoir tends to decrease the difficulty for influenza viruses to include humans in their host range. The virusââ‚¬â„¢s segmented genome can mix and recombine forming brand new strains. When this happens, outbreaks are bound to occur, causing its emergence and spread in human population.
Cellular barriers must also be crossed for a virus to successfully switch hosts. The multiple number of steps needed to infect a cell makes this a very strong barrier. In order for a virus to infect a cell, it must bind to a receptor and then enter the cell by such processes as fusion, endocytosis, or translocation. If they cannot attach, then they have to genetically adapt to being able to bind to a specific receptor. Since this is a new environment for the virus, they will also have to blindly steer themselves through this cell that they have never experienced before, successfully replicate their genome and then have those genes expressed. Dealing with a new environment and new host machinery, along with experiencing new forms of attack from the host can increase the difficulty in infection.
As humans, we have a many different levels of personal immunity, which normally starts with the outer most skin, or internal surfaces (Respiratory/GI tract). These mechanical defenses are barriers that viruses may have to physically adapt to in order to continue with the infection. There are also other immunity cells patrolling around the body as a surveillant for the entrance of viruses or the production of non-self proteins. Once viruses pass this barrier, the fight to infect is far from over. Now they must deal with intracellular mechanisms put in place specifically to kill organisms such as viruses that do not belong. Interestingly, human cells can make different types of cytidine deaminases that will be packaged into the newly made virions and will prevent them from further infecting other cells of the body. While making progeny virions, different host specific factors and interferons can also restrict viruses at different levels of viral reproduction before it gets released.
Some believe that the host range belonging to the virus at the moment can predict its ability to increase its host range. However, both viruses with a very wide range of hosts and others that infect a small range of hosts have been found to be able to successfully infect new species. The tendency a type of virus has toward evolution can, however, be important for some and not for others, depending on their present host range. Certain viruses that have a very wide host range have very few barriers that restrict them from co-speciation. When you look directly at the genetic makeup of different viruses, whether it is RNA, DNA, single or double stranded, the rate at which their genetic information can be replicated is very important for co-speciation to occur. When virus first enters a new host, the mechanisms used to replicate their genome in this new environment is far from polished and is still vulnerable to attack by intracellular mechanisms. Therefore, viruses that endure high amounts of genetic variation (like RNA viruses) give the virus a better chance of adapting. Though these changes could be profitable for the goal of infecting a new host, those same mutations could easily decrease their ability to thrive in their initial host depending on what type of expressions occur from the mutations.
The genetic changes that must occur for a virus to fully gain the ability to infect and "thrive" in a new host usually takes many intermediate viral stages. As a strain of viruses goes through generations of more and more genetic changes, their ability to infect the new host increases. It is in this stage that these viruses are genetically in the middle of being fit to infect the natural host and fit to infect the new host. They are now "partially adapted". Being able to make it pass this stage can be very difficult and also may be considered a reason for why new host emergences do not happen often. In this intermediary stage the virus is not fully efficient in its transmission and therefore this becomes an optimal point for outbreak control. Once they finally make it inside of the body and are able to replicate, they normally induce humans to act in ways that optimize their chances of release and transmission. For instance, virus can induce sneezing, increase their own proliferation within the body to be released in natural bodily functions (oral-fecal transmission), or cause intense viremia in hopes of vector transmission.
With all of these barriers viruses must pass through to achieve a successful cross-species transmission, it is not surprising that this usually entails months to years before they are fully adapt and the transmission is complete. There are many animal viruses that have been documented and also many viruses that we are completely unaware of. Because of this, it is somewhat impossible to predict which ones will eventually infect humans or when this will even happen. Different strategies have been put in place in the case of stopping epidemics such as vector control, vaccine storage, and human disease surveillance of specific countries. Being able to understand the barriers viruses must cross and how different viruses have already crossed, can help us in the future to figure out ways to stop their emergence.
Viral cross-species transmission to humans from animals is undoubtedly a very serious issue. Some of the most devastating microbial induced human deaths are due to viruses that have switched natural hosts from animals to humans. The more these switches happen, the more we are able to learn and the more money institutions will put forth to research this. This article thoroughly explained the processes that viruses must go through to move from animals to humans. It is scary to realize that there are so many viruses out in there right now that are in that intermediary step, working toward gaining the ability to infect mankind. However, I also realize that this is something to be expected. There are only so many barriers that we can put up to protect ourselves from viruses. As they adapt and evolve in hopes of infecting us, we must do the same in hope of protecting ourselves.