Modeling Susceptible Infective Recovered Dynamics and Plague Persistence in California Rodent–Flea Communities

Abstract

Plague persists as an enzootic in several very different rodent-flea communities around the world. In California, a diversity of rodent-flea communities maintains the disease, and a single-host reservoir seems unlikely. Logistic regression of plague presence on climate and topographic variables predicts plague in many localities where it is absent. Thus, a dynamic community-based analysis was needed. Deterministic Susceptible Infective Recovered (SIR) models were adapted for plague and analyzed with an eye for insights concerning disease persistence. An R simulation program, Plaguesirs, was developed incorporating multihost and multivector SIR dynamics, demographic and environmental stochasticity, density dependence, and seasonal variation in birth and death. Flea-rodent utilization matrices allowed us to get transmission rates as well as flea carrying capacities. Rodent densities allowed us to estimate host carrying capacities, while maximum birth rates were mainly approximated through an examination of litter phenology and demography. We ran a set of simulations to assess the role of community structure in maintaining plague in a simulated version of Chuchupate campground in Ventura County. Although the actual campground comprises 10 rodent and 19 flea species, we focused on a subset suspected to act as a reservoir community. This included the vole Microtus californicus, the deer mouse Peromyscus maniculatus, the Ceratophyllid fleas Aetheca wagneri and Malareus telchinum, and the Leptopsyllid flea Peromyscopsylla hesperomys. The dynamics of 21 subsets of this community were simulated for 20 years. Single-rodent communities showed much lower disease persistence than two-rodent communities. However, so long as Malareus was present, endemicity was enhanced; removal of the other two fleas slightly increased disease persistence. Two critical features improved disease persistence: (1) host breeding season heterogeneity and (2) host population augmentation (due to two similar host species instead of one). Voles are winter-spring breeders compared to the spring-summer deer mice. While host synchronicity may enhance epidemics, host asynchronicity favors endemics.

Host Resistance, Population Structure and the Long-Term Persistence of Bubonic Plague: Contributions of a Modelling Approach in the Malagasy Focus

“…Some of these could play an additional role in Madagascar, others should not have any impact, and all of them could be involved in plague persistence in other foci. Indeed, the existence of multi-plague reservoirs [5], [15], [55] seems unlikely in Madagascar, as R. rattus is margely dominant in rural communities, representing at least 95% of the captures [3], [65]. Also, although plague persistence in soils may exist in very peculiar situations (e.g., [66]) or in steppic environments [16], [17], it has never been demonstrated in Madagascar [17].…”

“…Also, although plague persistence in soils may exist in very peculiar situations (e.g., [66]) or in steppic environments [16], [17], it has never been demonstrated in Madagascar [17]. Alternatively, the heterogeneity in the phenology of the host reproduction [15], the direct transmission of Y. pestis inside burrows (through for example the release of the bacillus as aerosols [18], [19]), or the fact that resistant rats might be infectious for a short period of time before recovering (not shown for rats but observed for mice, [67]) or might release infectious fleas at their death could play additional roles in Madagascar and remain to be tested.…”

“…measles [8], [13], [14]. A few other mechanisms are thought to favour the persistence of plague, such as the presence of multiple hosts [5], [15], the possible persistence of Y. pestis in soils [16], [17], the direct transmission between rats inside burrows [18], [19] or the heterogeneity in the phenology of the host reproduction [15]. These alternative explanations will be discussed at the end of the article.…”

“…Explaining the endemism of plague has been the objective of several theoretical studies [15], [20]–[22]. However, the roles of resistance and of metapopulation structure of rodent reservoirs for plague persistence have rarely been explored separately.…”

Critical Contact Rate for Vector–host–pathogen Oscillation Involving Co-Feeding and Diapause

“…Ticks can be found primarily in forests, wooded parks, grasslands and low-growing dense bush, they normally have three post-egg stages: larva, nymph and adult. [1][2][3][4][5] Mathematical models have been developed to estimate tick-host interaction and pathogen transmission dynamics. Rosà and Pugliese 6 assessed how changes in host species densities can influence the persistence of tick-borne pathogen.…”