Apparent failure and cryptic success of disease control via intermediate host population reduction: Density dependence in copepods has implications for Guinea worm disease transmission

Abstract

All populations experience density dependence, lest they grow infinitely. However, elucidating what forms of density dependence act most strongly for a given species remains incredibly challenging. Identifying the mechanisms that regulate population density is particularly relevant to pest control, which often causes high mortality, reducing population density but enabling recovery when survivors are freed from intraspecific competition and density dependent vital rates (e.g., birth, death, and maturation rates). Understanding these demographic responses is critical for effective pest management, especially when specific life stages of pests differ in the harm they cause. Guinea worm disease (GWD) is a neglected tropical disease with an obligate copepod intermediate host, and importantly, only large-bodied copepods can be infected with GW parasites. GW disease has been the target of control for several decades, with main management strategies including the use of a chemical larvicide, Abate, which is applied to water bodies to cull copepod populations. Despite the wide application of Abate in GW endemic countries, little is known about long-term copepod population dynamics in response to Abate. Thus, we evaluated this mortality-based management of intermediate copepod hosts to control GW transmission, combining mathematical models with a population dynamics experiment designed to mimic Abate pesticide control methods across a range of intensities. Despite initial reductions in both total and stage specific population densities, copepod populations recovered so rapidly that control appeared to fail, enabled by extremely fast maturation rates in low-density populations. Much to our surprise, model simulations of GW transmission showed that although total and stage-specific densities rebounded, infectious adult copepods --the proximate cause of infections--were strongly suppressed, indicating cryptic success. This effect was enabled by the 15-day developmental period of GW larvae within copepods, which blocks the accumulation of infectious copepods between interventions. Ultimately, this study highlights the importance of understanding mechanisms of density dependence when designing and optimizing pest control interventions, as well as interpreting counterintuitive consequences of interventions.