Cross-species transmission and evolutionary dynamics of canine distemper virus during a spillover in African lions of Serengeti National Park.

The outcome of pathogen spillover from a reservoir to a novel host population can range from a "dead-end" when there is no onward transmission in the recipient population, to epidemic spread and even establishment in new hosts. Understanding the evolutionary epidemiology of spillover events leading to discrete outcomes in novel hosts is key to predicting risk and can lead to a better understanding of mechanisms of emergence. Here we use a Bayesian phylodynamic approach to examine cross-species transmission and evolutionary dynamics during a canine distemper virus spillover event causing clinical disease and population decline in an African lion population (Panthera leo) in the Serengeti Ecological Region between 1993 and 1994. Using 21 near-complete viral genomes from four species we found that this large-scale outbreak was likely ignited by a single cross-species spillover event from a canid reservoir to non-canid hosts less than one year before disease detection and explosive spread of CDV in lions. Cross-species transmission from other non-canid species likely fueled the high prevalence of CDV across spatially structured lion prides. Multiple lines of evidence suggest that spotted hyenas (Crocuta crocuta) could have acted as the proximate source of CDV exposure in lions. We report thirteen nucleotide substitutions segregating CDV strains found in canids and non-canids. Our results are consistent with the hypothesis that virus evolution played a role in CDV emergence in non-canid hosts following spillover during the outbreak, and suggests that host barriers to clinical infection can limit outcomes of CDV spillover in novel host species.