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.

Living with plague: Lessons from the Soviet Union’s antiplague system

“…The shift from eradication to control was possible because of enhanced ecological understanding of the complete plague system and acceptance of plague as a disease of endemic wildlife. Spillover to humans is more likely with epizootics, which occur only at particular rodent and flea species’ density thresholds (77, 78). Humans are exposed through flea bites and direct contact (eating, skinning, or butchering infected animals).…”

Disease reservoirs: from conceptual frameworks to applicable criteria

“…For multi-species reservoir systems, the transmission route between the target and non-target populations may be indirect ( Figure 2 , connection between b and target), possibly incorporating different hierarchical levels of a non-target community. 4 , 61 The type of transmission route dictates the form of evidence needed to prove that a feasible transmission route exists between the reservoir and target. For simplicity, we define vectors as part of the transmission route, although under certain circumstances (for example, permanency or substantial amplification in the vector), they may act as part of the non-target community.…”

“…For simplicity, we define vectors as part of the transmission route, although under certain circumstances (for example, permanency or substantial amplification in the vector), they may act as part of the non-target community. 61 Four basic requirements need to be met to make a compelling argument for the existence of a feasible transmission route: (i) spatial (direct or indirect) and temporal connectivity between the reservoir system and the target population, (ii) pathogen involvement in this feasible transmission route, (iii) proof of viability of the pathogen during the proposed transmission route and (iv) a longitudinal approach that requires the isolation of a pathogen multiple times in a given transmission route ( Table 1 ).…”

“…Furthermore, to include all parts of a reservoir population, long-term investigations must focus on the transmission between the non-target and target groups as well as feasible transmission within the non-target community. 61 Unconnected maintenance host populations may become a future reservoir through temporal shifts of the ecosystem.…”

Plague in Zimbabwe from 1974 to 2018: A review article

“…Anthropogenic activities are also involved in altering plague dynamics due to modification of landscape and faunal composition of the foci and adjacent areas, thereby increasing human cases. In the current transitional state as a whole, plague is at risk of becoming a public health problem in poor countries in which ecosystem erosion, anthropic invasion of new areas, and climate change increase human contact with rodent reservoir systems [49].…”