Disease and Evolution
The human body has been plagued with diseases since the beginning of time—pathogens like viruses and bacteria have made us privy to Mother Nature. As humans evolve, so do the diseases we are susceptible to. Some diseases that were once rare have become common, others have disappeared and newer, more daunting ones have emerged. Many of these changes have taken place in the wake of important transformations in human civilizations and ecology. It is therefore feasible to propose that diseases succeed and fail in response to humanity's advances. Natural selection is unable to provide us with perfect protection against all pathogens, because they tend to evolve much faster than humans do. E. coli, for example, with its rapid rates of reproduction, has as much opportunity for mutation and selection in one day as humanity gets in a millennium. And our defenses, whether natural or artificial, make for potent selection forces. Pathogens either quickly evolve a counter defense or become extinct. Diseases such as AIDS, Ebola, Polio have shown their wrath and humans have sought to find cures and treatment options. By definition, disease is essentially “a disorder of structure or function that produces specific signs or symptoms or that affect a specific location (not just from a physical injury)” (WHO, 2007). The true boundaries and limitations of disease remain elusive. Healthcare specialists and researchers use “normal” conditions as their basis in order to understand what disease is. By understanding what disease is, one can target and identify the causes of the “abnormal” condition. The truth about diseases is that they are all relative.
The question of how disease came to be remains unanswered though. Diseases are about as old as time—they have always plagued mankind. A vast majority of existing human infectious disease have their origin in other animal species. In order to be successful as a pathogen, there must be a balance of transmissibility and effect on host fitness.
Knowledge of microbial genomes, and the functions they encode, is severely limited. Among 40 phyla of bacteria, for example, most of the available genomic sequences were from only three phyla; sequencing of Archae and Eukaryote genomes has proceeded in a similarly sporadic manner (WHO, 2007). However, pathogens like virus, bacterium, prion, fungus, viroid or parasite are known to cause disease. Bacteria are typically 1- 5µm. In fact, a vast majority of bacteria are considered harmless or beneficial to humans. They are living prokaryotes and cause disease from parasitism or toxin production. Some common examples of bacteria causing disease are Tuberculosis or being exposed to Anthrax. On the other hand, viruses are also big players in the world of disease. Typically only 20-300 nm, they are interestingly the most abundant type of biological entity. They are acellular and cannot replicate on their own. They cause disease by hijacking a cell’s machinery. They have no energy metabolism, do not grow, produce no waste, and do not respond to stimuli—so are they living? That is a hot debate topic in the world of Science. What we do know is that viruses are behind some of the deadliest diseases in our world including Ebola, SARS, HIV/AIDS.
The Ebola Virus and Evolution:
The Ebola outbreak in West Africa has international medical organizations on high alert and people all around the world on the edge. Ebola is normally carried by animals like fruit bats, but occasionally makes the jump to humans. When it becomes apparent in humans, it is fatal, killing more than half of those infected. However, because it is only spread by direct contact with bodily fluids, most of the world need not fear for their lives. An important question to ask though is whether Ebola virus can evolve.
The Ebola virus is experiencing plenty of mutations. Ebola has RNA, not DNA, as its genetic material. When RNA is copied, many more mistakes are made than when DNA is copied. This gives viruses like Ebola a particularly high mutation rate when compared to DNA-based viruses like smallpox or chickenpox — though not as high as the rates at which HIV and the flu accumulate new mutations. This indicates that Ebola's high mutation rate and its rapid rate of replication may cause the virus to evolve quickly (Carol et. al, 2013). Ebola (at least the strains that we are most interested in) now live in humans, which have many physiological differences from bats. The quick pace of evolution in the recent Ebola outbreak may in part reflect the initial stages of the virus' adaptation to humans, as natural selection favors mutations that make the virus more successful in its new host (Gire et. al, 2014). It is difficult to diagnose Ebola infections and hampering the medical community's ability to treat and contain the virus. Such changes could also impact vaccine development and therapies for treating the disease. While Ebola's ongoing evolution is unlikely to lead to an airborne virus, it is likely to lead to other changes that will affect how we fight this deadly outbreak.
During an outbreak, the first individual to be infected with Ebola virus is thought to acquire the infection through direct contact with an animal that harbors the virus. Transmission between humans takes place through direct contact with patient’s body secretions including saliva, blood and semen. A person infected with Ebola virus does not spread the disease until s/he begins to manifest its symptoms; moreover, spread through the air does not take place. The typical course of Ebola virus infection spans over two to three weeks. The initial symptoms of the disease include myalgia, fever and malaise which are interpreted by the patient as ‘a flu-like illness’. As the disease progresses, patients manifest severe bleeding along with coagulation abnormalities. Neutrophilia (elevated neutrophil count), lymphopenia (low lymphocyte count) along with a skin rash and gastrointestinal bleeding are commonly observed in such patients (Suzuki et.al, 1997). The inflammatory response that develops is not protective and in most cases it is exaggerated. Worsening viremia (viral presence in blood) and the resulting liver damage may result in disseminated intravascular coagulopathy (DIC) – a disorder which results from over activity of proteins that control blood clotting (Gire et. al, 2014). The integrity of blood vessels is further compromised since the virus directly infects the endothelial cells of blood vessels at this stage. The ultimate outcome of this process is a hypotensive shock that results due to excessive bleeding.
The Ebola (EBO) virus is a member of the family filoviridae which is composed of non-segmented negative strand RNA viruses. There are four known subtypes of Ebola: Zaire, Sudan, Reston, and Ivory Coast (EBO-Z, EBO-S, EBO-R, and EBO-IC, respectively). EBO-Z is the most virulent subtype, with a 77% mortality rate in its most recent positively identified outbreak in Zaire in 1995 (Sanchez et al., 1996). However, not all subtypes are as virulent. EBO-IC is pathogenic in primates, but only caused a single nonfatal human infection. Likewise, EBO-R is only pathogenic in primates.
Virulent strains of Ebola cause a condition known as hemorrhagic fever. Ebola HF is a particularly severe form of HF, the symptoms of which are: high fever, severe hemorrhaging, and shock (Mühlberger et al., 1999). The exact means through which HF is achieved are highly diverse, quite impressive, and poorly understood. Fatal cases of Ebola HF are characterized by upregulation of gamma interferon (IFN-g). As the symptoms progressed massive amounts of IFN-g and secreted FasL were produced, indicating the activation of many cytotoxic (CD8+) T cells and other cytolytic cells (Mühlberger et al., 1999).
The responses of endothelial cells have been shown to be defective during an Ebola infection. Ebola’s effect on endothelial cells is surmised from the ability of Marburg, a related filovirus, to directly infect endothelial cells. Marburg has been clearly shown to replicate in endothelial cells during the infection. At the terminal phase of Marburg HF, shock is caused as a result of direct lysis of endothelial cells by the Marburg virus, massive release of inflammatory mediators by unspecific (phagocytosis) or specific (viral replication) activation of inflammatory cells, and oxidant injury or unspecific immune responses (Schnittler et al., 1993). In order for Ebola to cause infection, it must be capable of entering cells. Ebola fusion peptides have been shown to destabilize lipid vesicles which allows membrane fusion. The fusion protein is a glycoprotein (GP) that is not inert, but causes perturbations of the bilayer that results in membrane fusion. The action is thought to be analogous to the HIV fusion peptide. Ebola not only attacks the immune system by horribly disrupting its signaling pathways and inflammation response, but also destroys the tissues largely responsible for immune function (Carol et. al, 2013). Macrophages, stromal, and dendritic cells swell in the lymphoid tissue. Mitosis and involution of the lymphoid follicle are also blocked. Four to six days after infection, the severest damage is observed the red pulp of the spleen. The walls are completely destroyed and is accompanied by hemorrhaging. The stroma and macrophages have become necrotic. Similarly, in the white pulp and in the lymph follicles of the respiratory and GI tracts, macrophages and stromal cells have become necrotic and lymph tissue exhibits increased lymphoid depletion. Clearly, Ebola has developed many tricks to defeat our immune system.
Despite Ebola’s surge into the mainstream of virology, little is really known about this pathogen. The animal that serves as the viral reservoir is still unknown. Many species can serve as intermediate hosts, especially non-human primates, but Ebola is pathogenic to them as well. Some feel that the reservoir may be a species of solitary bat or it may be arthropod-borne (Monath, 1999). Bats experimentally infected with Ebola do not die, supporting the bat reservoir theory. Another theory is that filoviruses evolved from plant viruses (WHO, 2007). Traces of non-pathogenic or defective Ebola viral particles have been found in about 3% of the most common species in and around areas of outbreaks, but none of these are capable of starting an infection (Hagmann, 1999). Furthermore, in some human populations the number of Ebola seropositive individuals exceeds 30%. Clearly, non-pathogenic variants occur relatively frequently (Monath, 1999), but then outbreaks start usually from one infection, why? Although we are a long way from discovering all of Ebola’s secrets, our knowledge is already able to reap potential benefits. Because of Ebola GP’s ability to bind to endothelial cells, researchers are able to target therapeutic genes specifically at these cells. One such application could deliver growth factors to stimulate the growth of new blood vessels in place of damaged vessels, treating cardiovascular disease (Wickelgren, 1998).
Confirmation of diagnosis based on signs and symptoms is not a possibility. Therefore, tests like ELISA, Polymerase Chain Reaction (PCR), detection of IgM and IgG antibodies and Immunohistochemistry testing are used at various stages of the disease to confirm the diagnosis. No vaccine for protection against the disease is as yet available. Treatment is mainly supportive, which includes administration of intravenous fluids, maintaining balance of electrolytes, oxygen support, and maintenance of blood pressure and the treatment of secondary infections (Gire et.al, 2014). An antiviral drug named favipiravir was the first to demonstrate improvement in the condition of patients with low levels of Ebola virus in their blood. Ebola infection is associated with an unfavorable prognosis and the mortality rate may be as high as 90%; however, early detection and diagnosis of the disease may bring forth comparatively better outcomes.
Virophages
A striking question that we sought to answer was whether viruses get sick? This question has many implications- one of which being that the ability to “get sick” belongs to what we consider to be biological organisms. This brings up a debatable sub question- can viruses even be classified as organisms? One characteristic that all successful organisms share is that they utilize DNA and have a mechanism to pass this DNA on to a new generation. This characteristic is likewise shared by viruses- though their mechanism for passing on their DNA to create new viruses requires a host. Individuals may argue that the requirement for a host suggests that viruses cannot truly be an organism. To counter this argument, we have observed many other species of multicellular organisms that also require parasitism in order to reproduce. Another characteristic that viruses share with other organisms is the ability to interact with other organisms. Though they are incapable of moving from one location to another without the assistance of some vector, once in contact with a host they are able to interact with the “host environment” in order to begin the process of reproduction. Lastly, our argument concerning the status of viruses as simple organisms includes the fact that they are capable of evolving. We see that they do share many characteristics with other organisms- but is this enough to categorize viruses as organisms themselves? For the sake of discussion, we argue that at the very least, they are simple organisms.
Like organisms, viruses also come in many shapes and sizes. Research has shown that some viruses are so large that smaller viruses are capable of parasitizing them. It has also been suggested that these large viruses may be a sort of “transition” between viruses into more complex organisms like bacteria. For the first time, a group of scientists have discovered a virus that targets other viruses. It is so unique that they have classified it in an entirely new family – the “virophages”. These virophages share similarities with the bacteriophage— viruses that use bacteria as hosts. However, they are viruses that attack other viruses. The virophage Sputnik is a genetic chimera – a mish-mash of different genes from different sources. 13 of its genes have no equivalent in any other known virus, the other 8 genes share similarities to genes from other viruses, bacteria and even more complex cells (The Virophage—the virus eater, 2015. Sputnik has circular, ds DNA 3 genes are integrase complexes. Sputnik does not contain the necessary genes to replicate without co-infecting with mimivirus—it needs the larger virus in order to be a virus. Recent studies of Mimivirus have shown that Sputnik infection is not present when the Mimivirus is bald, so it thought that the fibers on the Mimivirus capsid are involved with adhesion of the virophage and host virus. There is a11% increase in Mimivirus capsid thickness, multiple Mimivirus capsid layers and asymmetric accumulation of capsid fibrils. Co-infection with Sputnik decreases the yield of infectious mimivirus particles by 70% and decreases cell lysis at 24 hours by 13% (The Virophage, 2015). This data indicates that yes, viruses can in fact infect other viruses and cause disease. This fascinating new topic shows the elusiveness and trivial nature of diseases and how they come about.

References
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