It has long been known that bats and birds can host a wide range of viruses which appear for most of the time to do them no harm. Famously and recently, wide spread infection of humans resulting from a viral ‘species jump’ from the above animals has shown how badly humans can respond to the infections viz the current coronavirus pandemic.
It has been shown in bats that despite the presence of the viruses, an inflammatory response is not induced and there is little evidence of significantly elevated antibodies. It is speculated that vigilance by their own killer T cells keeps the virus loaded infected cells low.
The animals usually only shed the viruses ( ie get ill ) during stressful times such as food scarcity, loss of habitat and reproduction.
Below is an hypothesis that I have from the perspective of a biochemist and gerontologist to explain the mystery of the difference between humans and say bats with regard to susceptibility to Coronavirus.
Once a virus enters an organism, by whatever root, its ‘goal’ (in a teleological sense) is to reproduce. To do so it needs access to the full synthetic apparatus present in every cell which it will hijack for viral reproduction.
Entry into a cell is in itself a tricky process and viruses have a range of binding and injecting strategies which are subject to a dynamic and fluid battle between the ‘burglar and the household’ using a dazzling array of molecular ‘picks and locks’.
If successful, the cell will be turned over to virus production and will eventually burst and release its payload. Here a second level of defence comes into action. The increase in viruses will alert the immune system which will cause inflammation ( basically increased transport pathways to infected areas), the production of antibodies which can clump viruses into easy to eat lumps, and the emergence of T-cells and other phagocytes which set about the killing infected cells and removing inactivated viruses.
If the response above is extreme, usually due to sudden high viral loads the response itself can be life threatening as seen so often with critically ill Covid patients. But what about bats?
Are they just being ‘laissez faire’ about virus reproduction? Are the locks on their cells unpickable?
I have another hypothesis.
I think the 'tipping point' between 'latent' and 'lytic', the virologists' terms for the emergence of viruses that can stay present but hidden for long periods ( herpes or HIV would be well nown examples), occurs through events inside the cell.
What happens to the nuclear material ( DNA or RNA) that gets into a cell? It is naked, that is unlike our DNA which has a coating of protein. It is exposed. The Corona Virus RNA is particularly vulnerable being the longest RNA genome discovered. As a result it fragments easily. Indeed sensitive PCR tests pick up RNA fragments from Covid19 long after active ( ie whole genome ) viruses have gone.
When this material enters a cell it is potentially entering a maelstrom of free-radicals released via mitochondria during oxidative metabolism. For a really energetic cell like those in bird or bat muscles it is a veritable firestorm. Damaged DNA is useless to the virus and will float around until it is recycled. It cannot hijack the cells apparatus as the code to do so is damaged.
Using the reasoning above, what would be good ‘victims’ for the virus. The answer is cells that have all the equipment to make viruses but are pretty much powered down.
For example storage cells like adipose tissue (fat), unused muscle cells ‘parked and marked for destruction’ in the old or simply sedentary, would be ideal targets for the virus.
Also hypoxic cells, or cells with poor sugar regulation would be vulnerable as here mitochondria can be turned down and anaerobic respiration is significant. All of the above are ideal for a virus ‘free for all’ and all are very characteristic of the modern human.
In summary I am hypothesising (guessing) the focus on the immune system’s response to infection is partly a red herring. The immune system’s panic comes from an understandable reaction to full scale viral production.
Part of the answer to our vulnerability and the bats’ indifference may then actually be due to processes inside the cell rather than the response of the immune system.
60 years ago, globally, there were vastly fewer obese, inactive humans many kept alive into morbid old age by drugs. Would they have suffered quite so badly from the pandemic?
Our Prime MInister will do well to keep active and lose that flab.
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