Dynamics of viral index infections in novel hosts


Whether a pathogen entering a new host species results in a single infection or in onward transmission, and potentially an outbreak, depends upon the progression of infection in the index case. Although index infections are rarely observable in nature, experimental inoculations of pathogens into novel host species have a long history in biomedical research. This provides a rich and largely unexploited data source for meta-analyses to identify the host and pathogen determinants of variability in infection outcomes. Here, we analysed the progressions of 514 experimental cross-species inoculations of rabies virus, a widespread zoonotic pathogen which in nature exhibits both dead end infections and varying levels of sustained transmission in novel hosts. Inoculations originating from bats rather than carnivores, and from warmer to cooler-bodied species caused infections with shorter incubation periods that were associated with diminished virus excretion. Inoculations between distantly related hosts tended to result in shorter clinical disease periods, which will also impede transmission. All effects were modulated by infection dose and together suggest that increased virulence as host species become more dissimilar is the limiting factor preventing onward transmission. These results explain observed constraints on rabies virus host shifts, allow us to evaluate the risk of novel reservoirs establishing, and give mechanistic insights into why host shifts are less likely between genetically distant species. More generally, our study highlights meta-analyses of experimental infections as a tractable approach to quantify the complex interactions between virus, reservoir, and novel host that shape the outcome of cross-species transmission.