Biologically speaking, every species on the planet hosts unique viruses that can affect another species, but specifically adapted to infect it.
In recent years, we have seen some of these viruses jump from animals to humans, causing destruction. The diseases or infections caused by these viruses are known as zoonotic.
Currently, the human population is increasing rapidly, forcing people to live close to the animal habitats, bringing them into frequent contact with these wild animals they normally wouldn’t come into contact with.
Viruses can hop from animals to humans, just as a virus can pass from humans to humans, which is most often via close contact with bodily fluids such as blood, urine, feces, and mucus.
But over time, these viruses have mutated and evolved to target a specific animal species, making it difficult to hop between species. It is mostly by chance that this may happen, and even so, it takes a large number of contacts between species for the virus to jump to another animal.
When a virus transmits from one species to another, it usually doesn’t spread rapidly because it is not well-suited to this species. But the virus can mutate over time in the new hots to generate variants that are better suited to the new host.
Interspecies transmission, spillover, host jump, or cross-species transmission (CST) is the capacity for a foreign virus to infect the new host once it is introduced.
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Viruses that Jumped from Animals to Humans
For many years viruses have hopped from animals to humans. COVID-19 or SARS-CoV-2, the new Coronavirus, is the third pathogenic coronavirus to originate from the animals to humans. The first case occurred in 2003 and was called SARS-CoV. It caused SARS, severe, and unusual pneumonia. The other coronavirus to emerge from animals is MERS-CoV, around 2007 in the Middle East, and also caused similar respiratory diseases known as Middle East Respiratory Syndrome (MERS). Since it was first identified, there have been 2,494 cases of MERS-CoV and nearly 900 deaths. On the other hand, the SARS-CoV outbreak had 8,000 cases and around 800 deaths.
COVID-19 pandemic is proving to be larger, with almost 800,000 infections and nearly 40,000 deaths.
How these viruses accomplish inter-species transmission
All these pathogenic viruses appear to have emerged from animals, including the current SARS-CoV-2 pandemic. This means they are all zoonoses diseases.
MERS-CoV and SARS-CoV are believed to have originated from bats, before jumping to an intermediary animal such as camel and civet cat, before spreading to humans.
Genetic study of novel coronavirus sequences indicates that their closest genetic kin seems to be bat coronaviruses, with pangolin as an intermediary species- an endangered animal species trafficked in China for meat and scales.
Currently, four coronaviruses cause flu-like illnesses in humans. They include HCoV-229E, HCoV-OC43, HCoV-NL63, and HCoV-HKU1.
While there are different types of zoonotic viruses that originated from animals to humans, the mode of transmission seems the same – through close contact. The transmission relies on the fundamental principle, namely access, and ability. Can the virus reach the cells of its new host? And if can the proteins in the virus’ cells recognize and bind to the receptors on the host’s cells?
If the foreign virus can meet these two requirements, then that is all it takes for the transmission to take place.
Coronaviruses have become pretty proficient at figuring out how to adapt to the receptors to gain entry to humans’ cells. The viruses have been found to use a surface glycoprotein – a sugar-coated protein – called spike protein to attach to the host cells. The spike protein makes the virus look like a crown, thus the name “corona.”
Most of the viruses that infect humans from animals seem to clasp onto one to one of the three specific receptors on the host cells.
The part of the protein that binds with the viruses – the S1 subunit – can vary significantly, enabling the virus to bind to many different human and other mammals.
The current coronavirus uses a human receptor called angiotensin-converting enzyme 2, while MERS and 229E use dipeptidyl peptidase 4 (DDP-4) and aminopeptidase N (APN) respectively.
All these binding proteins are available in epithelial or surface of the cells of the human respiratory tract, making it soft targets to any airborne virus.
Recent studies show that that the current coronavirus uses ACE2 as a receptor, just like the SARS-CoV.
Besides the coronaviruses, there is also another common type of virus that originates from animals and jumps to humans – influenza. Nearly all types of influenza viruses emerge from waterfowl like geese, terns, ducks, gulls, and other closely related species.
Many viruses also originate from birds and hop to humans.
Also, zoonotic viruses have just a single thread of genetic material known as RNA, which can be easily copied or mutated compared to the double-stranded human DNA, which is responsible for their virality.