The Three Ts of Pathogen Evolution During Zoonotic Emergence

Abstract

When novel zoonotic diseases like Sars-CoV-2 emerge, they are likely to be poorly adapted to humans. Effective control measures will suppress transmission before significant evolution can occur, but extended transmission in human populations allows time for selection pressures to act. In this review, we discuss these selection pressures with the aim of better understanding the factors shaping both transmission and virulence in zoonotic pathogens as they become established. We discuss how selection pressures during epidemics of emerging zoonotic disease are determined by the three Ts: trade-offs, transmission, and time scales. In short, virulence and transmission may trade-off, but transmission is likely to be favored by selection early in emergence. However, the relative selection pressures on transmission and virulence shift depending on the time scale of the epidemic. Predicting pathogen evolution in zoonoses therefore depends critically on understanding both the trade-offs of transmission-improving mutations and the time scales of selection.

2020 Hindsight: Should evolutionary virologists have expected the unexpected during a pandemic?

“…When a virus switches host, there is no clear way to predict how it will evolve (Geoghegan and Holmes 2018; Visher et al. 2021); the virus can become more or less pathogenic provided it maximizes transmission. As viruses are error‐prone and have such high population sizes, it has been suggested that every possible single mutation will be generated in an individual (Sanjuán and Domingo‐Calap 2016).…”