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Review: Cross-Species Virus Transmission and the Emergence of New Epidemic Diseases
A severe problem that we face today is the switching of animal viruses to human hosts. Diseases that have come from animal hosts such as human immunodeficiency virus (HIV) and Ebola fever have become a major concern for human health today. Another virus that has caused many deaths is the H5N1 influenza A virus. The concern is that this virus could gain efficiency in human transmission and at the same time remain a human pathogen. There are three stages for the emergence of a viral disease that can lead to successful host switching. First is when a single host is infected without any further transmission. Second is when a spillovers causes transmission in a local area. Third is epidemic host to host transmission in a novel host population. Factors that determine the rate of transmission from the above stages include contact between host and virus, host immune defenses at the molecular level, viral factors that enable efficient host transfer, and determinants of the efficient viral spread within the new host population.
An important example of a viral disease resulting from host switching is HIV. This virus emerged from primates about 70 years ago. Even though there is so much information available regarding this virus, there are still 1.8-4.1 million HIV infections that occur each year. Another example is Severe Acute Respiratory Syndrome (SARS), which is caused by a corona virus. Before being controlled by aggressive public health initiatives, SARS caused thousands of deaths. Two other examples of human viruses that could have originated from prehistoric animals are measles and smallpox. With all that said, it is important to understand the mechanisms of entry and spread in new hosts, including factors such as demographics, host and cellular properties, and the controls of virus transmission.
Through the prevention of host to host contact, new host transmission can be limited to some extent. An important barrier to host transmitted diseases in the past was limited opportunity for primate human contact. Contact between donor and recipient hosts is a precondition for virus transfer. Thus, it is affected by the geographical, ecological, and behavioral separation of the donor and recipient hosts. Factors that affect the geographical distribution of host species and decrease the behavioral separation of virus and host tend to increase the chances of viral transmission. Anthropogenic changes can also be responsible for transmission of viruses from animal to human hosts. These may include changes in demographic factors such as an increase in population or change in behaviors such as increased use of intravenous drugs. Finally, changes in the environment caused by human activities, such as deforestation can also lead to increase viral transmission from animals to humans.
The process of transmission of a virus to a new host cell can be regulated at many different levels. Viral transmission from one host to another is affected by the similarity of the two species. Even though infections can be spread to distant or closely related species, it is easier to transmit the disease to a distant host because closely related species may have developed cross immunity towards related pathogens. Viral transmission is also affected by physical barriers to the virus along with host factors that bind to the virus and render it inactive to cause an infection. Mutations in receptor binding can play a big role in the host switching of a disease. Besides receptor barriers viruses face host intracellular defenses as well. These include interferon release, macrophages, and other cell mediated and humoral immune responses.
The preexisting ability of viruses to transmit disease is thought to play a big role in determining the effectiveness of a virus to infect new host. Cross species transmission is more common in rapidly evolving viruses. RNA viruses have a more error prone replication rate and therefore a higher variability. Rapid variability helps improve the ability to adapt to the new host mechanisms. The mode of transmission of a virus can help determine the ability of the virus to spread as well. For example, viruses that use an insect vector have a very good chance to cause cross species infections. Reassortment of segmented RNA viruses to remove harmful mutations or to create beneficial genes will improve the ability of the virus for host to host transmission. Recombination events in viruses can lead to the virus obtaining efficiency to infect new hosts and thus causing outbreaks. For example, the SARS coronavirus seems to be a recombination between the bat coronavirus and another virus which lead to the acquisition of the ability to use human Ace 2 receptors effectively and infect humans.
Although it is difficult to directly observe virus transmission from one host to another, the parent virus can be determined by looking at the fitness of the virus in the donor or the recipient. Since, several changes are required in a virus to be able to switch hosts the virus should be less fit in the donor or the recipient after transmission than the original strain of the virus. Factors other than mutation in genes that are of concern are the ability of viruses to adapt to overcome physical barriers of the hosts and to make efficient their spread by the induction of sneezing or increasing viral concentration in blood of host so it can be easier for a viral vector to pick up and transmit the virus.
Complete adaptation to a new host takes a lot of time for the virus and it involves a number of mutations. This was illustrated by looking at isolates from nipah virus from the beginning of an outbreak and comparing them to isolates towards the end of the outbreak. The two isolates were significantly different from each other suggesting mutations in the virus for the purpose of adaptation to the new host or multiple insertions of the virus after gaining better suited adaptations to survive in human host.
Although considerable progress has been made on our knowledge of zoonotic diseases caused by host switching, it is not easy to predict when another disease may strike. However, with better information of the origins of viruses it may be easier to potentially curb the virus in its original host before spread or transmission to human hosts. Reducing contact between reservoirs and hosts can also be effective in reducing transmission rates. Vaccination of animals is another strategy that can be employed to reduce transmission. Anti-viral drugs may also be used depending on the situation and side effects of the drug. A generic strategy has to be developed in order to combat emergence of new diseases because it is impossible to predict which virus will switch to human hosts and how it would affect humans.
The article brings to light an important issue that we are dealing with today. The issue of outbreaks caused due to the host switching of viruses from animal to human hosts. Many of the diseases responsible for the large number of the deaths in the past few decades were caused by viruses that had switched hosts from animals to humans. The information presented in the article is pretty alarming. Even though we are fortunate that host switching is rare and virus adaptability to a new host takes time, it is not impossible for a virus like the H5N1 Influenza A virus to gain mutations for efficient human to human transmission and at the same time retain its deadly pathogenicity. In order to prevent this, animal viruses should be studied more closely than they have been in the past, effective ways to prevent host switching should be employed to prevent outbreaks, and more effective strategies should be designed to combat the emergence of host switch viral diseases. Another important issue that the article brings up is that neglecting our ecological conditions can also lead to improve the ability of the host switching in viruses. This is why it is important to keep in mind this parameter when trying to deal with ways to prevent the spread of host switching viral diseases.