Vacated niches, competitive release and the community ecology of pathogen eradication

Abstract

A recurring theme in the epidemiological literature on disease eradication is that each pathogen occupies an ecological niche, and eradication of one pathogen leaves a vacant niche that favours the emergence of new pathogens to replace it. However, eminent figures have rejected this view unequivocally, stating that there is no basis to fear pathogen replacement and even that pathogen niches do not exist. After exploring the roots of this controversy, I propose resolutions to disputed issues by drawing on broader ecological theory, and advance a new consensus based on robust mechanistic principles. I argue that pathogen eradication (and cessation of vaccination) leads to a ‘vacated niche’, which could be re-invaded by the original pathogen if introduced. Consequences for other pathogens will vary, with the crucial mechanisms being competitive release, whereby the decline of one species allows its competitors to perform better, and evolutionary adaptation. Hence, eradication can cause a quantitative rise in the incidence of another infection, but whether this leads to emergence as an endemic pathogen depends on additional factors. I focus on the case study of human monkeypox and its rise following smallpox eradication, but also survey how these ideas apply to other pathogens and discuss implications for eradication policy.

Towards the endgame and beyond: complexities and challenges for the elimination of infectious diseases

“…International organizations (WHO, OIE) may specify periods for maintained control with no detected cases for certification of freedom, but there has been little research on optimizing these targets. Furthermore, should eradication succeed, post-elimination measures ought to be in place to minimize the risk of re-emergence of the same or a related pathogen that could invade a newly vacated niche [91], but this is an even more uncertain area.…”

“…Where cross-immunity is repressing a co-infecting pathogen, elimination of one pathogen may release the other, with potentially negative consequences if the second pathogen is more problematic (note that this may also occur through relaxation of exclusion via resource-mediated competition) [91]. …”

Pathogen evolution and the immunological niche

“…A). These interactions describe the pathogen's role in the community of immune factors and other pathogens, and hence the immunophenotype encompasses several of the definitions of the concept of the ecological niche . Following mathematical niche theory, the immunophenotype P of a pathogen p in host H in a pathogen community C can be formalized as: with the arguments of F corresponding to the strength, duration, and breadth of interactions, respectively.…”

Enhanced immunosurveillance for animal morbilliviruses using vesicular stomatitis virus (VSV) pseudotypes

“…Continued immunosurveillance will ascertain whether PPRV is spreading more widely; broadening its host species in a post-RPV world. Finally, we should consider whether RPV eradication has created a vacated niche [57] for PPRV other morbilliviruses such as CDV, or a novel RPV-related bovine morbillivirus.…”