Viruses Arise from Genetic Recombination and Mutation

Viruses Arise from Genetic Recombination and Mutation

Almost every year, a newly emerging influenza virus descends upon the human population. Other viruses not even heard of a few decades ago, such as HIV, Ebolavirus, Hantavirus, West Nile virus, and Zika virus, are often in the news. Where are these viruses coming from?

Emerging Viruses usually arise Through natural Phenomena

The human population is at greater risk than ever from zoonotic diseases, which are animal diseases capable of being transmitted from animals to humans. In fact, 70% of human infections come from pathogens originally found in animals. Many of these emerging infectious diseases are the result of viruses appearing for the first time in a population or rapidly expanding their host range with a corresponding increase in detectable disease.

Many are transmitted by insect vectors, such as ticks, fleas, or mosquitoes, as well as bats. One example is the West Nile virus that spread across the United States between 1999 and 2009. It now is endemic (prevalent) across the continental United States. As of this writing, another, the Zika virus, has appeared in parts of Florida, and many health experts expect this virus also will eventually become endemic in parts of the United States.

Examples of Emerging Zoonotic Viruses
Virus Genome Emergence Factor
HIV +ssRNA Contact with infected apes/monkeys in Africa
MERS +ssRNA Possibly spread by infected Egyptian tomb bats (via camels?)
SARS-associated +ssRNA Contact with infected horseshoe bats
West Nile +ssRNA Infected mosquitos transported unknowingly by global travel
Zika +ssRNA Contact with infected mosquitoes carried from Africa and South Pacific
Chikungunya –ssRNA Spread through new mosquito vectors and global travel
Ebola/Marburg –ssRNA Human contact with infected fruit bats
Influenza –ssRNA Mixed pig and duck agriculture, mobile population
Lassa –ssRNA Human contact with infected rodents
Nipah/Hendra –ssRNA Human contact with infected flying foxes (bats)
Sin Nombre (Hantavirus) –ssRNA Large, infected deer mice population and contact with humans

But no matter how these viruses are transmitted, what caused their emergence? One way “new” viruses arise is through genetic recombination. Take, for example, influenza. The mixing of genes between different influenza viruses generates new flu strains every season in both the Northern Hemisphere and in the Southern Hemisphere. The “swine flu” that broke out in Mexico in 2009 was the result of the reassortment of genome segments from a strain of avian flu virus, a human flu virus, and a swine flu virus.

Viruses also arise from a second force driving evolution—mutation. For example, when a single nucleotide is altered (point mutation) in a replicating RNA virus genome, the virus has no way to “proofread” and correct the mistake. Occasionally, one of these mutations might be advantageous. In the case of HIV and influenza viruses, a beneficial mutation could generate a new virus strain resistant to an antiviral drug or to a vaccine. With a rapid replication rate and burst size, it does not take long for a beneficial mutation to establish itself within a population.

Even if a new virus has emerged, it must encounter an appropriate host in which to replicate and spread. It is believed that smallpox and measles both evolved from cattle viruses, whereas flu probably originated in ducks and pigs. HIV almost certainly evolved from a monkey (simian) immunodeficiency virus. Consequently, at some time such viruses had to make a species jump. What could facilitate such a jump?

Our proximity to wild and domestic animals and their pathogens makes such a jump possible. Today, population pressure is pushing the human population into more remote areas of the world where potentially virulent viruses might be endemic. Evidence shows the Machupo and Junin viruses (Arenaviridae viruses that cause hemorrhagic fevers in South America) jumped from rodents to humans because of increased agricultural practices that, for the first time, brought infected rodents into contact with humans.

An increase in the size of the animal host population carrying a viral disease also can “explode” as an emerging viral disease. The spring of 1993 in the American Southwest was a wet season, providing ample food for deer mice. The expanding deer mouse population brought them into closer contact with humans. Leaving behind mouse feces and dried urine containing the hantavirus made infection in humans likely. The deaths of 14 people with a mysterious respiratory illness in the Four Corners area that spring eventually were attributed to this newly recognized virus.

Therefore, emerging viruses are not new. They are simply evolving from existing viruses, and, through human changes to the environment, these viruses are given the “opportunity” to spread or to increase their host range.

Reference and Sources


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