How host genetics dictates successful viral zoonosis
Fig 1
This essay focuses on the most extreme bottleneck in viral disease emergence: The replication of animal viruses in their first human host.
On the left is a typical representation of the zoonosis pyramid, modified from [1,3–5]. The concept here is that animal viruses become increasingly human-adapted through a series of evolutionary steps represented from bottom to top. In our view, these steps are best described as 1) random variants arise in the animal reservoir that are capable of replicating themselves in humans, 2) variants are then selected for the ability to transmit between people, and 3) sometimes, as in the case of HIV-1 group M [12], these viruses become stably maintained in humans and are divorced from their former animal reservoir. The pyramid shape properly demonstrates that increasingly fewer viruses progress through the different stages of zoonosis but is misleading in that it doesn’t represent the scale of probabilities in these events. Instead, based on the arguments laid out in the essay, the pyramid should more correctly be depicted as a pinhole. As shown on the right, our current understanding is that <0.1% of animal viruses have any ability to replicate in humans (step 1), and then fewer still are able to meet the relevant criteria as the process continues upwards. The bottlenecks leading to the latter two steps don’t seem to be as extreme. One study of zoonotic pathogens (those already at step 1) found that 33% are transmissible between humans (step 2), and 3% spread so effectively that they become permanently sustained in humans (step 3) [10]. Herein, we discuss how experimental virology and an understanding of host–virus interactions render clear the reasons for the most extreme bottleneck in this process (green arrow).