[firstplacelast]

Might be wrong about this, but being so far removed and seemingly immunologically resistant is exactly what makes it a dangerous combination. Viruses mutate and recombine at an astonishing rate, so 99.9999% (?) of viral entities won't be able to make the jump between these species, but the one that can might have devastating consequences as it will be wildly different from anything that has infected rats before (and from there it's more likely to infect other mammal populations).

The more exposure between these populations the higher the likelihood that a crossover event occurs.

[neom]

high mutation rate != high crossover rate, and immunological resistance != susceptibility to novel evolution. (it's actually the other way around)

Also to be super clear, I'm not saying rats are immune to bat viruses, I'm just saying in reality it's is too divergent for functional crossspecies transmission and both rats and bats already have their own established, evolved coronavirus ecosystems that solve the same problems (like furin cleavage above). It seems to me near impossible it could actually happen.

[mikeyouse]

It’s incredibly common for bats and rats to be coinfected with the same viruses… this is literally why we were looking at pangolins and raccoon dogs and mink for SC2 and at masked palm civets and ferrets for SC1. The term of art here is “intermediate host”.

The virus doesn’t need to make the animal sick to increase its human transmission risk, it just needs to infect its cells and be in the same host as other circulating viruses to get the crossover and recombination events.

From early host analysis,

> For a precursor virus to acquire the genomic features suitable for human ACE2 receptor binding, an animal host would likely have to have a high population density to allow natural selection to proceed efficiently (27). It is interesting to note that rodent betacoronaviruses have the polybasic cleavage site (38). Considering the above, surveillance and whole genomic analysis of CoVs from rodents are important to elucidate whether these species have any role in the transmission cycle of the virus and to detect the emergence of possible recombinants involving CoVs from these species and those from bats. However, there is not yet any evidence on the role of rodents or squirrels as intermediate hosts.

https://pmc.ncbi.nlm.nih.gov/articles/PMC7297130/

Or more direct examination of the FCS issue between rats and bats:

> Here, we examine the spike protein across coronaviruses identified in both bat and rodent species and address the role of furin as an activating protease. Utilizing two publicly available furin prediction algorithms (ProP and PiTou) and based on spike sequences reported in GenBank, we show that the S1/S2 furin cleavage site is typically not present in bat virus spike proteins but is common in rodent-associated sequences, and suggest this may have implications for zoonotic transfer. We provide a phylogenetic history of the Embecoviruses (betacoronavirus lineage 2a), including context for the use of furin as an activating protease for the viral spike protein. From a One Health perspective, continued rodent surveillance should be an important consideration in uncovering novel circulating coronaviruses.

https://www.sciencedirect.com/science/article/pii/S235277142...