It’s all bad –
No groups of species appear to be especially likely to transfer viruses to humans.
John Timmer – Apr , (9:) (UTC UTC)
On their own, the results would seem to point to the special reservoir model, as hoofed ungulates (like our agricultural animals) and rodents collectively accounted for half the viruses that had transitioned to human hosts. But things got more complex when the authors tried to analyze the properties of a virus that made it more likely to make this transition. The best combination of properties, which could explain about half the probability of a zoonotic jump, was dominated by things like transmission through insects and a relatively simple replication cycle inside cells.
And, while the host’s order on the evolutionary tree appeared to matter at first, it mattered much less once the authors adjusted for a critical factor: how many individual species make up that order. For example, rodent and ungulate species may transmit more viruses to us, but there are a
of species in these groups. If you adjust the rate by species number, the effect largely goes away. If you also control for the fact that we’ve identified far more virus species in mammals than in birds, then the effect becomes little more than statistical noise. The probability that a group of species will transmit a virus to humans becomes a function of how many species are in that group. This is inconsistent with the special-reservoir hypothesis. But things don’t look great for the evolutionary explanation, either. While the zoonotic risk dropped as you got further from primates, this accounted for less than 1 percent of the overall risk.
In fact, if you simply estimated the number of zoonotic jumps based on the species number, groups that seemed threatening start to look fairly mundane. Rodents, for example, would be expected to have given 42 viruses to humans; we’re aware of 41 instances where that took place. Bats would be expected to have transferred 41 viruses to us but have only sent of them. The one exception is, again, the ungulates, which seem to send viruses our way at rates above what we’d expect.
Now what?
The hope was that, by identifying the rules of zoonotic transfers, we could identify groups of species that have an elevated risk of causing problems and thus could be subjected to more careful monitoring. This analysis suggests that these groups may not exist. It doesn’t rule out the possibility that there are groups of species below the order level that are hotspots for zoonotic transfers. But at this point, the number of viruses transferred per group is likely to be small and might not stand out from statistical noise.
That said, some species / virus combinations are notable. For example, while bats are notable for having been the source of SARS-CoV-1 and Ebola, they’re actually most likely to transfer a new species of rabies virus to humans. Other primates are a major source of adenovirus and Dengue species, while rodents tend to transfer hantaviruses and arenaviruses.
While this isn’t especially good news for targeted surveillance efforts, that might not be bad news overall. Having obvious targets might mean we over-focus on those, leaving us vulnerable to risks we hadn’t anticipated.
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