An infectious disease that has newly appeared in a population or that has been known for some time but is rapidly increasing in incident or geographic range.
Why do some viruses start to infect human, when it does not usually occur in the past?
Reasons behind emerging viruses:
Spontaneous evolution of a new virus entity
For RNA viruses, as a consequence of the lack of proof-reading activity of RNA virus polymerases, new viral genetic variants are constantly created. RNA viruses readily adapt to changing environmental conditions (Pressure exerted by natural selection). Also virus population are heterogeneous, so there is a vast variation to the viral genome. Therefore, the high mutation rate of RNA viruses helps the virus to infect new species which it is unable to before the virus mutates.
Random assortment (only in virus with segmented genome)
Picture below describe the whole process of random assortment of virus which causes the virus to evolve and become an emerging virus.
https://sites.google.com/a/luther.edu/genetics/_/rsrc/1241888676464/students/andy-eisenberg/swine-influenza-h1n1/Reassortment.jpg
Both Virus A and Virus B have segmented genomes. They infect the host cell at the same time (co-infection). When both viral genomes of A and B are transcript and expressed, they are present and mixed in the host cells in large amounts, thus the virus are released out of the cells, re-assorted and many new strains of virus are produced.
Example of a virus with segmented genomes will be the influenza virion. Below is a picture to show the timeline of how the new strain of influenza virus, H1N1, evolved.
Concentration of people with shared lifestyle
Concentration of people with shared lifestyle is one of the reasons for emerging virus. For example, drug addicts who share the same needle for consumption of the drug, by using the same needle over and over again for the whole group, there is a chance for the rest of the group to get infected by the virus if one of the drug addict is infected by an emerging virus. Thus, if the concentration of people with shared lifestyle increases, the chances that more people will get infected increases proportionally.
Breakdown in public health
Breakdown in public health is not likely in developed countries. In less developed countries like Africa, where the resources are limited, poor sanitation is very common, this in turn, lead to the promiscuous distribution of human and animal waste. These wastes are one of the highest contributors to successful parasitism.
Climate Change
Climate change is also one of the causes of emerging viruses. “Global warming -- with an accompanying rise in floods and droughts -- is fueling the spread of epidemics in areas unprepared for the diseases, say many health experts worldwide. Mosquitoes, ticks, mice and other carriers are surviving warmer winters and expanding their range, bringing health threats with them.”
Taken from http://www.washingtonpost.com/wp-dyn/content/article/2006/05/04/AR2006050401931.html
Man invading natural habitat of animal
Man invading animal natural habitast is also one of the reasons behind emerging viruses. For agricultural needs, man deforests. By deforesting, man got closer to wild animals as they invade the animal natural habitat. As the contact between animal and human becomes closer, the possibility of diseases being transmitted from animal to humans increases.
Thus, from the above few points, when human came into close proximity with these animals who are carriers of the virus, or eating the animals who are carriers of the virus that are not prepared properly, the chances of them of them getting infected by these viruses increases.
Examples of emerging infectious diseases
Why do some viruses start to infect human, when it does not usually occur in the past?
Reasons behind emerging viruses:
- Virus Factors
- Human Factors
- Spontaneous evolution of a new virus entity.
- Generation of a novel strain due to co-infection of different strains in an individual (Random assortment)
Spontaneous evolution of a new virus entity
For RNA viruses, as a consequence of the lack of proof-reading activity of RNA virus polymerases, new viral genetic variants are constantly created. RNA viruses readily adapt to changing environmental conditions (Pressure exerted by natural selection). Also virus population are heterogeneous, so there is a vast variation to the viral genome. Therefore, the high mutation rate of RNA viruses helps the virus to infect new species which it is unable to before the virus mutates.
Random assortment (only in virus with segmented genome)
Picture below describe the whole process of random assortment of virus which causes the virus to evolve and become an emerging virus.
https://sites.google.com/a/luther.edu/genetics/_/rsrc/1241888676464/students/andy-eisenberg/swine-influenza-h1n1/Reassortment.jpg
Both Virus A and Virus B have segmented genomes. They infect the host cell at the same time (co-infection). When both viral genomes of A and B are transcript and expressed, they are present and mixed in the host cells in large amounts, thus the virus are released out of the cells, re-assorted and many new strains of virus are produced.
Example of a virus with segmented genomes will be the influenza virion. Below is a picture to show the timeline of how the new strain of influenza virus, H1N1, evolved.
Taken from Times magazine August 24,2009.
Under human factors, they include- Concentration of people with shared lifestyle.
- Breakdown in public health.
- Climate change
- Man invading natural habitat of animal
Concentration of people with shared lifestyle
Concentration of people with shared lifestyle is one of the reasons for emerging virus. For example, drug addicts who share the same needle for consumption of the drug, by using the same needle over and over again for the whole group, there is a chance for the rest of the group to get infected by the virus if one of the drug addict is infected by an emerging virus. Thus, if the concentration of people with shared lifestyle increases, the chances that more people will get infected increases proportionally.
Breakdown in public health
Breakdown in public health is not likely in developed countries. In less developed countries like Africa, where the resources are limited, poor sanitation is very common, this in turn, lead to the promiscuous distribution of human and animal waste. These wastes are one of the highest contributors to successful parasitism.
Climate Change
Climate change is also one of the causes of emerging viruses. “Global warming -- with an accompanying rise in floods and droughts -- is fueling the spread of epidemics in areas unprepared for the diseases, say many health experts worldwide. Mosquitoes, ticks, mice and other carriers are surviving warmer winters and expanding their range, bringing health threats with them.”
Taken from http://www.washingtonpost.com/wp-dyn/content/article/2006/05/04/AR2006050401931.html
Man invading natural habitat of animal
Man invading animal natural habitast is also one of the reasons behind emerging viruses. For agricultural needs, man deforests. By deforesting, man got closer to wild animals as they invade the animal natural habitat. As the contact between animal and human becomes closer, the possibility of diseases being transmitted from animal to humans increases.
Thus, from the above few points, when human came into close proximity with these animals who are carriers of the virus, or eating the animals who are carriers of the virus that are not prepared properly, the chances of them of them getting infected by these viruses increases.
Examples of emerging infectious diseases
- Ebola virus (first outbreaks in 1976 and the discovery of the virus in 1977)
- HIV/AIDS (virus first isolated in 1983),
- Hepatitis C (first identified in 1989, now known to be the most common cause of post-transfusion hepatitis worldwide)
- Influenza A (H5N1) virus (well known pathogen in birds but first isolated from humans in 1997)
- Legionella pneumophila (first outbreak in 1976 as Legionnaire disease and since associated with similar outbreaks linked to poorly maintained air conditioning systems)
- E. coli O157:H7 (first detected in 1982, often transmitted through contaminated food, has caused outbreaks of hemolytic uremic syndrome)
- Borrelia burgdorferi (first detected in 1982 and identified as the cause of Lyme disease).
- New variant of Creutzfeldt-Jakob disease, which was first described in 1996. The agent is considered to be the same as that causing bovine spongiform encephalitis, a disease which emerged in the 1980s and affected thousands of cattle in the UK and Europe.
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