The Crucial Role Genetics Plays in the Mutation and Transmission of Infectious Diseases
written by: Finn Orfano•edited by: lrohner•updated: 2/28/2011
Does genetics play a critical role in the evolution of infectious diseases? Do genetics affect communicable disease? And if so, how and why? Take a glance at this article if you want to know the answers to that and whether or not humanity is fighting a losing battle against disease.
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A Serious Threat to Humanity
Infectious diseases (commonly referred to as communicable diseases)—those transmitted from an infected animal or human to another via airborne inhalation, bodily fluids, or food—is still a major threat to humanity. In developed countries with advanced medical care and strict hygiene, it is sometimes easy to forget this until a pandemic strikes. The H1N1 flu was a great testimony illustrating this fact.
To put in perspective, infectious diseases can wipe out thousands within days, weeks, even months, especially if one does not have the immunity to the respective disease. Back in 1875, a Fijian chieftain’s son visited Australia and unknowing contracted measles. When he returned back to his father, he passed the disease on through his coughing and sneezing. Within four months, 20,000 people on that island died from the measles.
Another good example centers around the time when Europeans settled in the Americas. Up until the 1400’s, Native Americans were never introduced to diseases such as influenza, smallpox, yellow fever, or the plague. Upon the Europeans’ arrival in the New World, the Native American population fell from 50 to 90 percent over the following years from contracting these diseases.
As long as creatures continue to thrive on this planet, infectious diseases will always be pose a serious risk to the well-being of those creatures, including humans. It is an arduous struggle, however, between man’s ability to combat infectious agents via science (antibiotics, vaccinations) and the ever evolving (and sometime elusive strains) of infectious agents out there. To understand why it is battle, one must understand the critical involvement that genetics plays in this ongoing war against disease.
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The Role of Genetics in the Evolution of an Infectious Agent
The forefront of the war against infectious disease is waged on a microscopic level, one where genetics plays a vital part for the invader as well as the host. Genetics can be the determining factor on whether a pathogenic bacteria or a virus evades the body’s immune system. Genetics can play a role in a infectious agent's evolution, whether a bacteria will become antibiotic resistant (as is the case with MRSA) or whether a virus will learn a new mode of transmission/ invasion of a new host (as is it is speculated with Ebola).
Genes are like a library, they house all the traits and developmental functions that a living organism will express. When genes are mutated via the the genomic sequencing for that particular gene, this can alter traits and developmental functions for that particular organism.
So what do genes actually do in regards to infectious agents such as bacteria and viruses?
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Bacterial Gene Transfer and Mutation
Bacteria share genetic material with eachother via conjugation, transduction, or transformation. The unique feature about bacteria isthat along with their bacterial chromosomal DNA, they also carry extra, circular genes known as plasmids. Plasmids naturally occur within bacteria. These plasmids can be transferred to other bacterial cells, thus giving other bacteria the same type of trait/function that the donor cell has. This type of sharing/transferring of genetic material is performed either through conjugation (direct cell to cell transfer) or transformation (plasmid released into environment and bacterial cell alters self so as to be able to uptake it).
Let's take an example of what this entails in regards to an infectious agent. Antibiotics are supposed to either kill or inhibit pathogenic bacteria. What if some bacterial cells are resistant to the antibiotic and survive? These survivor cells can pass that antibiotic resistant gene on to other bacterial cells that are naturally suspectible to that particular antibiotic. Through means such as conjugation or transformation, it can go from four or five antibiotic resistant cells to thousands in a matter of minutes. Science then has to face particularly virulent bacteria that is difficult to treat, like MRSA (methicillin-resistant staphylococcus aureus).
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Viral Gene Transfer and Mutation
Viruses, on the other hand, need a host in order to replicate. Viruses, unlike what people may presume, do not intentionally try to harm the host. They need the host for survival. In order to replicate, they use proteins attached at the surface of their cell to attach to the surface proteins of the host's cells. The virus, once attached to the host's cell, will inject its DNA (or RNA) into the cell so that it will use its resources to multiply and create more virus cells. Once there are enough virus cells, the host cell will burst and die, release hundreds of new virus cells into the surrounding environment.
How do they mutate then?
Viruses either have DNA or RNA within them. If a virus contains DNA, it "proofreads" its genetic code for errors during replication, thus making that kind of virus less likely to mutate. It's stable, in other words. Viruses that contain RNA, however, lack this "proofreading" ability during replication, which allows a greater chance of error in the genetic coding process. Influenza, for example, contains RNA. Influenza is a virus that rapidly mutates. One only has to think about the avian flu of 2009 or the swine flu of 2010. These particular influenza strains, which normally infected birds and pigs, were able to mutate enough so that they could infect a new host: humans.
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Bioterrorism: A Stark Reality
Natural mutations in the genetic code can be small, inconsequential changes and take a period of time to develop. It is relative based on what piece of genetic information is changed. However, viruses and pathogenic bacteria may have their DNA intentionally changed to fit the purposes of man. This is made possible through researchers who have mapped and published the genome of various pathogens, making it easily accessible to anyone else who is researching the same type of pathogen.
However, the threat of using the genomes of pathogens as a means of warfare, is real, intimate, and longstanding. Remember, the Native Americans who were not exposed to smallpox before the arrival of the Europeans? During the French-Indian war, at the siege of Fort Pitt, English soldiers gave blankets infected with smallpox to nearby Indian tribes. Another example of bioterrorism happened during the Middle Ages, when victims of the Black Plague were flung over castle walls by the encroaching enemy.
Lesson learned, bioterrorism is real. Except, instead of flinging bodies of infected people over castle walls, humans are using more advanced techniques.Ken Alibek, a well-renown Russian scientist who defected from the Soviet Union in 1990's and has published books on the advanced methods of biowarfare and the Russian Biological Weapons Program, in which he played an integral role.
What does bioterrorism have to do with genetic mutations? Knowing the genome of a pathogen and how it functions leaves open a lot of possibilities for altering traits or making "super bugs." Let's take smallpox, for example. It is a stable virus which lasts in the environment for a long time, has the ability to remain in people for a long period of time, and is highly infectious. Ebola, while an extremely virulent disease, cannot survive long outside of the host. It only thrives for a couple hours and only within bodily fluids. Who is to say that one cannot incorporate the ebola virus genome into the smallpox one, making a stable while still being highly virulent super virus (even airborne?). Smallpox has not been eradicted.
There are other theories out there concerning a "zombie virus." It is speculated that if rabies were genetically mingled with influenza, it could produce an airborne disease that would create the same kind of "rage virus" seen in popular zombie flicks and books. Will it happen? It's difficult to say for sure. Is it possible? Yes.
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Losing Battle with Disease?
Between man intentionally reconstructing the genome of pathogens, bioterrorism, newly emerging infectious diseases, and natural mutations that can occur in already existing ones--is humanity in a losing battle with infectious diseases?
Humanity will always struggle with disease. To define it possibly as a "losing battle," or wondering such, is misleading since there are so manyfactors out there (such as economy, chronic disease, and the genetics of the people affected) which play a daily important role. Unlike those in our past, however, we live in a globalized culture now. When SARs devastated China, America knew. Scientists are proactive in their research and vaccinations. The technology out there will continue to advance as well as education and awareness on emerging infectious diseases.