Cross-scale dynamics and the evolutionary emergence of infectious diseases

Sebastian J. Schreiber; Ruian Ke; Claude Loverdo; Miran Park; Priyanna Ahsan; James O. Lloyd-Smith

Similar mechanisms dictate the influence of mutant viral strains circulating in the reservoir, particularly for long-term infections (Fig 4C,D) . If the cross-scale reproductive number α of a mutant virion is greater than one, so that the mutant frequency rises easily in the infected host population, then even low frequencies of mutants in the reservoir lead to substantial risk of emergence. Indeed, for long-term infections with α > 1, emergence becomes almost certain when there are mutants in the initial spillover inoculum. Conversely, when the cross-scale reproductive number α is less than one, emergence probability scales with the proportion of mutants in the initial dose, and when α 1, the initial dose must consist almost entirely of the mutant strain in order to pose any major risk. These findings yield direct lessons for the growing enterprise of conducting genetic surveillance on zoonotic pathogens in their animal reservoirs [97] [98] [99] [100] . A crucial requirement for effective genetic surveillance is the ability to identify genotypes of concern; the integration of various research approaches to address this question, and estimate key quantities, is an on-going research challenge [101] [102] [103] . Risk to humans increases if there is any non-zero proportion of mutant viruses in the spillover inoculum, so tracking the presence of such mutants is beneficial. Surprisingly, the quantitative frequency of mutants in the initial dose has All rights reserved. No reuse allowed without permission.