Zoonoses As Ecological Entities: A Case Review of Plague

Abstract

As a zoonosis, Plague is also an ecological entity, a complex system of ecological interactions between the pathogen, the hosts, and the spatiotemporal variations of its ecosystems. Five reservoir system models have been proposed: (i) assemblages of small mammals with different levels of susceptibility and roles in the maintenance and amplification of the cycle; (ii) species-specific chronic infection models; (ii) flea vectors as the true reservoirs; (iii) Telluric Plague, and (iv) a metapopulation arrangement for species with a discrete spatial organization, following a source-sink dynamic of extinction and recolonization with naïve potential hosts. The diversity of the community that harbors the reservoir system affects the transmission cycle by predation, competition, and dilution effect. Plague has notable environmental constraints, depending on altitude (500+ meters), warm and dry climates, and conditions for high productivity events for expansion of the transmission cycle. Human impacts are altering Plague dynamics by altering landscape and the faunal composition of the foci and adjacent areas, usually increasing the presence and number of human cases and outbreaks. Climatic change is also affecting the range of its occurrence. In the current transitional state of zoonosis as a whole, Plague is at risk of becoming a public health problem in poor countries where ecosystem erosion, anthropic invasion of new areas, and climate change increase the contact of the population with reservoir systems, giving new urgency for ecologic research that further details its maintenance in the wild, the spillover events, and how it links to human cases.

Fig 1. The four reservoir models proposed by Poland and Barnes [12], revisited by Gage and Kosoy [13].

From the top down: 1. classical model, 2. chronic infection model, 3. fleas as reservoir, 4. Telluric Plague. Horizontal axis informs the spatial occurence of the phenomena, starting from a sylvatic landscape to a more urban/periurban situation, indicating public health risk. The gradient bar indicates the possibility of transition from an enzootic to an epizootic cycle. Rodent species differentiated by color and pattern, flea species by color alone. Arrows indicate the interaction between the components of the cycle in given time T (double arrows), or the progression of each stage of the model. Arrows pointing towards humans indicate human infection in epizootics.

Fig 2. The metapopulation model for Plague reservoir systems.

Discrete subpopulations are disjoint in space but maintain contact through movement of individuals. Population size fluctuations are indicated by the white circle (T1, first assessment) and the gray circle (T2, second assessment), where populations can retract, expand, or extinguish. Arrows indicate movements of individuals (rodents or fleas) that maintain the subpopulations interconnected and working as a functional unit. For detailed information on the effects of population fluctuations on Plague cycles, see Reijniers et al. [15, 16].