I don't know what would differ exactly from humans as to how the virus can propagate, so I'd be interested in more details. Maybe it just doesn't infect/ and thus get through their respiratory system at all?
Firstly there's a assumption here that the animal had exactly the same strain that is infecting humans as opposed to a closely related but 'dog targetting' variant of the species.
Viruses are incredibly specific to their host species. If a host cell doesn't express some cell surface antigen that the virus needs to get into the cell, it won't. If a host cell doesn't have the right variant of some enzyme involved in manufacturing or assembling new virions then the virus won't reproduce.
The response to viruses is also highly variable within species. For most people Epstein-Barr virus causes the disease Mononucleosis/Glandular Fever, but in some people (particularly in Sino-Asian populations) it causes a type of leukaemia.
All it takes for a virus to cause/not cause disease, or be infectious/no infectious can be as little as a single gene variation in the host species.
I don't know why you would say that when we have so much well-documented viral zoonoses. Maybe I am just not understanding.
For a virus to pass from say, birds, to people it needs to be able to infect both. Sometimes this is the case, most often it is not. Sometimes it happens because a strain randomly mutates so that it would infect another species (mutation in viruses happens
all the time) and encounters a new species of host at the same time. Most of the time it doesn't encounter the new species and dies out. Crossing species is the exception not the rule. Vets don't catch Parvo from dogs, vets don't get immune deficiency by catching FIV from cats.
Viral zoonoses are comparatively rare. Bacteria zoonoses are not (e.g. Psittacosis). Bacteria and viruses are worlds apart, viruses need a fully working cell that is compatible with their biochemistry to reproduce, bacteria just generally need warmth, wet and food. You can grow a bacteria on an agar plate, viruses require tissue culture in a compatible species cells. Look at all the diseases that are commonly transmitted between humans and other animals - the vast majority are bacterial or parasitic. The rare viral ones typically require very specific hosts on both sides.
Some viruses only require very evolutionarily primitive cell biology, using cell biology that is common to whole sections of the evolutionary tree. Rabies is one - it'll infect most mammals - but it's still limited in what it can infect, you won't find a lizard with rabies.
"Firstly there's a assumption here that the animal had exactly the same strain that is infecting humans as opposed to a closely related but 'dog targetting' variant of the species."
OK, take herpes B for example. There are clearly documented cases of infection to humans after exposure (scratch or bite) to macaques. Indeed, the macaque is barely symptomatic other than shedding...the human, unfortunately can develop encephalitis in a couple of days. The infected human can also infect other humans.
Same virus. same infection albeit dramatically different effects. What am I missing?
A macaque is, like you and me, a primate. There's a much smaller difference between species of primate than there is between primates and dogs.
Look at it another way. The SARS-Covid-2 virus has a genome with about 30,000 base pairs, that means (crudely) it can only code 10,000 amino acid sequences, quite a few of which are overhead. Compare that to a computer virus with 30,000 bytes assembler instructions and 10,000 actual instructions. That little genetic material has to code for the structure of the virus, how it gets into a host cell, how it gets that host cell to manufacture more virions and so on. The part that says "how to get into a cell to reproduce" has to have a mechanism for doing so, usually this is by 'recognising' a surface protein on the cell and using that to leverage the cell's active transport mechanisms to get into the interior of the cell. If that protein isn't there, or it's a slightly different shape because it's the dog variant rather than the human variant, then the virion never gets to deliver its genetic payload to the cell, so no infection happens.
Lets go back to the computer analogy. If part of that 30,000 instructions is "how to infect a computer" and the virus was targetting Windows then it would fail to infect MacOS because MacOS doesn't expose the same interfaces on the outside that Windows does. They're both x86 operating systems so they have similar mechanisms for, say, networking but the exact details of the networking interfaces of Windows that might allow a virus to get its payload through are different on MacOS and so the virus fails to 'infect' its host.
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