Plague ecology is importantly characterized by sporadic epizootics, followed by periods of cryptic dormancy prior to subsequent outbreaks. Disease emergence is difficult to forecast due to complex interactions among pathogens, hosts, reservoirs, and their environment. This complexity has obscured our understanding of inter-epizootic plague dynamics enabling local persistence and re-emergence despite apparent absence. We present a multi-compartment ordinary differential equation model, with deterministic and stochastic implementations, to describe how soil-borne amoebae can act as reservoirs for plague bacteria and facilitate re-emergence of these pathogens under natural conditions. This model considers both top-down and bottom-up interactions that occur in the natural disease system of Yersinia pestis and is characterized by host, vector, bacteria, amoeba, and environmental parameters. We compared the ability of five putative reservoir mechanisms (host mammals, fleas, host carcasses, soil, or amoebae) to facilitate the survival of plague bacteria between outbreaks. Finally, we analyzed the likelihood for stochastic dynamics arising from each mechanism to explain bacterial persistence via stuttering infection chains or long-term dormancy.
Results/Conclusions:
This model indicates that populations of soil-borne amoebae can remain sufficiently abundant and infectious over biologically relevant time scales to facilitate inter-epizootic persistence and re-emergence of Y. pestis. We performed sensitivity and threshold analyses on various model parameters to determine that host density and the connectedness of their burrows strongly contribute to plague reemergence and epizootic dynamics, whereas rates of host resistance to plague contributed to the extinction of the pathogen. This model, in conjunction with recently identified amoeba-resistance phenotypes in Y. pestis, provides additional evidence supporting amoebae as plague reservoirs. These findings stress the importance of recognizing pathogen-harboring amoebae as potential threats to global health, agriculture, conservation, and biodefense.