The structure of infectious disease outbreaks across the animal-human interface

Despite the enormous relevance of zoonotic infections to worldwide public health, and despite much effort in modeling individual zoonoses, a fundamental understanding of the disease dynamics and the nature of outbreaks arising in such systems is still lacking. We introduce a simple stochastic model of susceptible-infected-recovered dynamics in a coupled animal-human metapopulation, and solve analytically for several important properties of the coupled outbreaks. At early timescales, we solve for the probability and time of spillover, and the disease prevalence in the animal population at spillover as a function of model parameters. At long times, we characterize the distribution of outbreak sizes and the critical threshold for a large human outbreak, both of which show a strong dependence on the basic reproduction number in the animal population. The coupling of animal and human infection dynamics has several crucial implications, most importantly allowing for the possibility of large human outbreaks even when human-to-human transmission is subcritical.