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this post was submitted on 07 Sep 2024
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Well shit. Here we go again.
We've known since at least March that about 10 human cases of this new bird flu would allow the virus to mutate and adapt to humans. This is the 14th reported case in the states this year, and the first which could have been transmitted from human to human. We understood what was happening, yet have really done nothing to try to prevent it's spread.
The mutation rate baked into Influenza's reproduction cycle is much more elaborate than coronaviruses, and this isn't exactly a bad thing. When a human catches bird flu from a bird, the mortality rates are pretty burly as this version of the virus attaches to the α2:3 receptor. While this receptor is found throughout the avian digestive and respiratory track, it's only found in the lower lungs of humans. A lower lung infection will always be gnarlier than an upper respiratory infection. Human influenza viruses have a preference for the α2:6 receptor, which is found throughout our airway. This is the primary adaptation which occurs when influenza mutates to infect humans. But a virus is a parasite, so in their ideal world, they wouldn't kill their host. Viruses often do the most damage when adapting to or having recently adapted to a new host. Hopefully, the mutation rate of influenza will result in a shorter pandemic compared to COVID if it ends up taking place.
For the record, the law of declining virulence, while still referenced a lot, has been disproven and was never a real scientific theory, just one scientists educated guess that took off.
Then why does bird flu have more invasive symptoms and a higher mortality rate compared to human influenza's? I was taught by a virologist who's been around the block many times and got her PhD in the USSR. She was adamant that a parasite never wants to kill it's host, as this results in no longer being able to reproduce in the host and shortens it's reproduction time in future host.
Most viral offspring are not capable of infection, as without mutations, viruses would not be able to reproduce effectively and could not adapt to changing environments. To disprove a hypothesis simply means one aspect of the statement is incorrect. So while the cause and effect occurs, the explanation for why wasn't dialed. Or at least, this would be my guess for how it could have been disproven.
The same relationship of the cost of high virulence hurting the fitness of a pathogen may not apply if hosts are kept in proximity that a fast death doesn't necessarily mean the pathogen cannot find a new host.
It may have been how the 1918 flu pandemic was so deadly yet still successful at spreading to new hosts; being spread amongst soldiers in trenches and infirmaries and troop ships compared.
Plus, there are evolutionary pressures on pathogens to become more virulent; using less resources by the host only leads to being outcompeted by more virulent pathogens. While mortality is a limiting factor, there is a temporary benefit to strategies that select for virulence until that limit is reached.
Virulence is defined as causing damage to the host. If the virus kills everyone in that small proximity, the virus will no longer be able to reproduce or spread, plain and simple.
Viruses are super crafty in obtaining their limited number of proteins from the host. However, some viruses bring enzymes with them to get the job done without much help from the host. These are typically larger RNA viruses. Influenza is in this group, but it still steals the 5' cap from the host. This happens without miss as it's able to interact with the host RNA polymerase undetected and then this cap makes it so the influenza RNA appears to be host RNA.
The 1918 influenza was so deadly as it had just jumped to a new host, humans. The same exact influenza responsible for the 1918 pandemic has relatives still in circulation. It's not in circulation as the less virulent versions were more successful. This explicitly demonstrates the preference to be less virulent.
If a virus doesn't need many proteins from the host, it's able to reproduce much more quickly than one dependant on more host resources. The more resources it needs, the increased ability of the immune system to prevent it's reproduction. So in many situations, a lower requirement for host resources can make it more successful. Regardless, it can be very dependent on host resources, like many DNA viruses, and still not be very virulent. A great example of this is Hepatitis D. Virology is a fascinating field and it's highly intricate as a virus is more like it's host than any other viruses. There's not a lot of commonality between different viruses and their reproduction cycles. So viruses in the same family are compared, and the 1918 becoming less virulent shows there's a preference for becoming less virulent over time.