Spatiotemporal dynamics of free-living stages of a bacterial parasite of zooplankton

Abstract

Parasites are generally thought of as being intimately tied to their hosts, yet many parasites produce free-living stages. This raises the question: what are parasites doing when they are not being parasitic? We studied the spatiotemporal dynamics of free-living infectious stages and asked whether these dynamics were correlated with infections in 2 focal host species. We used a com- mon and virulent bacterial parasite, Spirobacillus cienkowskii, which infects Daphnia spp. Densities of free-living infective stages were high in a stratified, eutrophic lake (up to ~10 5 to 10 6 cells l -1 ), but also spatiotemporally variable. There was a positive correlation between the density of these free- living stages and the prevalence of infected Daphnia on the subsequent sampling date. This suggests that free-living stages increase in environmental reservoirs prior to the start of epidemics in Daphnia. We also studied the ability of free-living stages to persist outside their hosts for long periods of time. In laboratory microcosms, we found that S. cienkowskii persisted under simulated environmental conditions for more than 2 mo, before declining to below our method's detection limit after approxi- mately 3 mo. Overall, our study of a common parasite of Daphnia reveals the potential importance of free-living stages to epidemic dynamics, and suggests that it is important to consider environmental reservoirs when studying disease dynamics.