Introduction
Seasonality is a major driver of ecological dynamics, and its significance only becomes more important in the context of global climate change, as seasonal conditions are predicted to change (for example, warmer/earlier Springs in temperate regions). Many infectious diseases display strong seasonal dynamics which may in part be due to phenology of the host, parasite or vector, as well as changes in abiotic environmental conditions or a combination of these factors (Altizer et al. 2006, Martinez 2018). Many factors—such as light and temperature, host and parasite phenology, and the phenology of other species in the community—fluctuate approximately simultaneously with seasons, making it difficult to attribute epidemic dynamics to specific factors and furthermore, to understand if these factors affect hosts and parasites in the same way. For parasites with a free-living stage, it is often unclear whether their dynamics are driven by the seasonality of the host population, or the response of the parasite itself to environmental changes. To understand how epidemics may be influenced by changes in seasonal conditions, it is therefore necessary to disentangle how hosts and parasites are affected by these changes.
In temperate regions, temperature is a key factor of seasonality, particularly for ectothermal organisms. The seasonality observed in epidemics of parasites with free-living stages, especially those that overwinter independent of their hosts, suggests that both may respond to temperature differently (Gehman et al. 2018, McDevitt-Galles et al. 2020), but observational work does not readily allow us to disentangle these differing relationships to temperature or to predict how changes to temperature may influence epidemic dynamics. Understanding host and parasite response to temperature has therefore become more vital in the context of ongoing climate change, which is expected to cause changes in infectious disease dynamics (Cook 1992, Haines et al. 2006, Patz et al. 2008, Lafferty 2009, Lafferty and Mordecai 2016, Kirk et al. 2020). The objective of this study is to distinguish between host and parasite seasonality by determining to what degree a parasite is limited by temperature versus the availability, development, and susceptibility of its seasonal host. To understand the drivers of seasonal epidemics, and in particular the compare the effect of temperature on host development to the effect on parasite development, experiments which disentangle these factors through controlled experiments are necessary. Controlling environmental factors such as light and humidity, as well as daily or seasonal fluctuations in temperature, can help to not only disentangle the effects of these factors from each other, but also isolate the response to specific constant temperatures in a way that seasonal observations do not allow. For such work, the use of a biological model system which is well understood and easily maintained in the lab can offer insight into complex natural systems.
The Daphnia magnaPasteuria ramosa system offers the opportunity to further our understanding of infectious disease epidemiology through a combination of well-known epidemic patterns in nature and the ability to manipulate various ecological parameters with controlled experiments. Their trophic position, sensitivity to environmental conditions and the fact that host and parasite are well-understood make this system suitable for disentangling the relative importance of various factors influencing disease dynamics. P. ramosa is a common bacterial parasite of the aquatic microcrustaceanD. magna , causing 100 % mortality of those infected and strong reduction in fecundity, therefore having significant impacts on the host’s population dynamics (Ebert et al. 2004, Duncan and Little 2007, Ebert et al. 2016). The parasite transmits horizontally when transmission stages (spores) are picked up from the water column or environmental reservoirs in the pond sediments (Decaestecker et al. 2004), hereafter referred to as the “spore bank,” where spores can survive decades (Decaestecker et al. 2007). Previous work has suggested that the rate of infection with spores from the water column increases with temperature from around 15 oC (Vale et al. 2008), indicating that infection may in some way be limited by temperature. However, different steps of the natural infection process may differ in their sensitivity to temperature (Hall et al. 2019, Izhar et al. 2020), making it essential to include all steps of the infection process when assessing temperature effects on the seasonal disease outbreaks in natural populations. For example, activation of the resting spores and attachment to the oesophagus of susceptible Daphnia was not different across a range of temperatures from 10 to 25oC (Duneau et al. 2011). However, exposure rate may be reduced at lower temperatures due to generally lowered activity and filtering rates of the Daphnia host (Burns 1969). Furthermore, Spring hatchlings of Daphnia from sexual resting stages may be different in various aspects from the later born asexual offspring and this may influence their likelihood of getting infected. Here we aim to understand the role of temperature in determining the onset of an epidemic.