Unhealthy herds and the predator-spreader: Understanding when predation increases disease incidence and prevalence

Disease ecologists now recognize the limitation behind examining host-parasite interactions in isolation: community members-especially predators-dramatically affect host-parasite dynamics. Although the initial paradigm was that predation should reduce disease in prey populations (healthy herds hypothesis), researchers have realized that predators sometimes increase disease in their prey. These predator-spreaders are now recognized as critical to disease dynamics, but empirical research on the topic remains fragmented. In a narrow sense, a predator-spreader would be defined as a predator that mechanically spreads parasites via feeding. However, predators affect their prey and, subsequently, disease transmission in many other ways such as altering prey population structure, behavior, and physiology. We review the existing evidence for these mechanisms and provide heuristics that incorporate features of the host, predator, parasite, and environment to understand whether or not a predator is likely to be a predator-spreader. We also provide guidance for targeted study of each mechanism and quantifying the effects of predators on parasitism in a way that yields more general insights into the factors that promote predator spreading. We aim to offer a better understanding of this important and underappreciated interaction and a path toward being able to predict how changes in predation will influence parasite dynamics.

Automated summary

Disease ecologists have recognized that examining host-parasite interactions in isolation is limiting, as community members, particularly predators, can significantly impact host-parasite dynamics. The current understanding is that predators can potentially increase disease in their prey, and this predator-spreaders are now considered crucial in disease dynamics. However, empirical research on this topic remains fragmented. This study reviews existing evidence, provides heuristics to understand whether a predator is likely to be a predator-spreader, and offers guidance for targeted study and quantification of predator effects on parasitism. The aim is to enhance understanding of this important interaction and develop a predictive ability for how changes in predation will influence parasite dynamics.