Meeting Abstract
Epidemiological theory often treats host susceptibility as a fixed constant. Is this assumption accurate and how do deviations from it affect our understanding of infectious disease? I explore the causes and consequences of variable susceptibility in a one-host one-parasite system: the zooplanktonic host, Daphnia dentifera, and its fungal parasite, Metschnikowia bicuspidata. From June to December 2017, I tracked Metschnikowia epidemics in six naturally occurring Daphnia populations and measured host susceptibility and its underlying immunological traits to identify its relative role in driving epidemic emergence. I found that epidemics could not be predicted from Metschnikowia exposure alone but depended critically on the interaction between parasite exposure and host susceptibility. Daphnia hosts were exposed to low levels of Metschnikowia for months preceding epidemics, and epidemics only emerged when Daphnia immune defenses declined to a critical minimum. Host susceptibility showed strong variation across scales, from the individual-level to the within- and among-population levels, and I discuss the extent to which ecological factors may be driving this variation.
