Every few years there is an emergence of a new disease; this pathogen is often not previously seen in humans. These diseases have the ability to have a devastating affect. How come there is no previous knowledge of these diseases? Well in the case of viruses, the new viral disease is often nonexistent in humans and is transferred from animals. This transfer of an infectious disease from a host species to a new species is called a cross-species transmission. For this transmission to happen, the virus must overcome various barriers. These barriers can be a physical obstacle such as environmental or demographic barricades preventing the ability of the virus to switch hosts. This includes the amount of time the host has contact with recipient species; which is determined by behavior and social interactions. Another barrier is the specific recipient host barriers that prevent the pathogen to efficiently spread in new host species. This includes the type of tissue the pathogen interacts with, the host's receptors and virus's ability to bind, and the intracellular responses restricting the virus. Once a virus has overcome these factors, the virus's adaption to the new host after transmission is just as important. The virus's ability to infect and transmit from host to recipient species is directly related to its ability to overcome these factors.
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"The major sources of new human viral diseases are enzootic and epizootic viruses of animals" (Parrish). The viral pathogens such as Human Immunodeficiency Virus (HIV) and Swine Flu virus (H1N1) were viruses that were already present in animals transmitted to humans. For cross-species transmission of a virus to occur there must be contact between the organisms. This contact for humans includes domestication of animals, hunting, farming, traveling, sexual practices, etc. The contact with animals allows for a rise in for zoonotic disease or spread from animal to human. The impact of the viral disease on the new host can have various affects. A major factor in determining the level of on a new species is by density of recipient species. For example: a virus transmits to a human from a raccoon in a very desolate area, the human intraspecies contact is poor and the chance of spreading is not as likely compared to an urban area such as New York City. The higher rate of contact with animals is directly related to the chance of a disease to be spread. For example, the "Nipah virus in Malaysia was facilitated by intensive pig farming, which amplified epizootic virus transmission" (Parrish). This shows how human behavior and relationship with the environment can impact transmission.
For a virus to have a cross-species transmission the virus must infect target cells. The infecting virus can have difficulty in many specific steps in this process. The virus can be impeded at "receptor binding, entry or fusion, trafficking within the cell, genome replication, and gene expression" (Parrish). For a virus to infect a new host, the virus must find the specific tissue cells to bind to, such as mucus, epithelial, or blood cells. There are often precise sites viruses bind to. An enveloped virus such as the HIV virus has many proteins on its envelope that bind to specific receptors. Often, pathogens show specificity in organisms for example: In HIV, the virus binds to the "CD4 host receptor and the CCR5 or CXCR4 co(-)receptors" (Parrish). A host without these receptors will not become infected with that stand of HIV virus. Restrictions can also occur in the viral replication process with the use of enzymes and/or proteins. In the replication of the influenza A virus, when a segment of RNA genome is assorted in a new host and the new cell reduces the replication rate. For a virus to be efficient at infecting a new host species the virus must often have evolutionary changes.
Although Mutations in a virus to better adapt to a host are not always necessary, the emergence of changes in a virus may allow the virus to become efficient at infection and transmission. Often a virus that has undergone transmission to a new host species is not well adjusted to the new host cell environment. The evolution of viral genome can only benefit the virus. The variation in genetic content in the virus is vital for survival. The variation gives the virus a higher probability to adapt to the new host cell. Mutations in viruses depend if the viral genome is composed of DNA or RNA. RNA viruses have quick replications that are prone to errors. RNA viruses lack a proofreading protein thus mutations are either beneficial or hurtful. This allows for large populations of the virus with genetic variation. DNA virus's can affect a broad number of host species. However, they tend to replicate slower than RNA viruses. The RNA viruses tend to co-evolve with a specific species thus cross-species transmission is not as likely as DNA viruses. Besides genetic variation there is another way for viruses to have changed genetic material.
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Recombination of viral genome in a host can produce highly beneficial genetic changes for a virus. Recombination is when two viruses of the same type infect the same host cell and exchange genetic material. The progeny of the resulting virus are a hybrid of the parent viruses. These viruses often have a different virulence than the parent viruses. The influenza A virus has three hosts; pigs, humans and birds. Each host has different strands that affect that particular host. The H1N1 swine flu virus was a result of recombination of the influenza A virus that transmitted to humans. The H1 receptor had little to no immunity in humans thus spread and caused an epidemic. The HA and NA receptors on the influenza A virus are for binding and cleaving respectively. The influenza A virus has adapted or multiple occurrence of transmission to humans. After a cross-species transmission of a virus has the ability to infect and transmit, then "full host adaptation may take months or even years to complete" (Parrish).
Studying the pattern of virulence in animals and watching zoonotic viruses is a necessity for microbiologists to understand and predict the chances of specific viruses crossing into a new host. Not watching the pattern of viruses will result in the same reoccurring instances of outbreaks and epidemics such as SARS and the H1N1 virus. To better protect individuals and prevent the rate of harmful cross-species transmission to humans, the next step is to begin to document the pattern of viruses and the host cell's defense mechanisms. Without research like this the fight against infectious pathogens will be lost. The rate of mutations in pathogens against antivirals and antibiotics is far greater than production of new alternative medicines. This article, although summarizing a meeting from 2005, is pointing in the right direction for the next generation for microbiologists.
Parrish. Colin. R. Edward C. Holmes. "Cross-species virus transmission and the emergence of
new epidemic diseases". Microbiology and Molecular Biology Reviews. 2008. Vol.