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 magna —Pasteuria 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.