Infectious diseases are of important concern to human health and species conservation, as exemplified by the rise of emerging zoonotic diseases globally over the past decades, and most starkly by the COVID-19 pandemic. The current climate and prediction of more emerging pandemics in the future, demonstrate that now, more than ever it is important to further our understanding of infectious disease epidemiology and transmission dynamics. The overall objective of this project was to disentangle the multifarious drivers of seasonal epidemics including temperature, host demography, transmission route and the broader ecological community through a combination of observational, experimental, and theoretical work using the Daphnia magna-Pasteuria ramosa model host-parasite system.
Daphnia magna, are small aquatic crustaceans that live throughout the northern hemisphere. They mainly reproduce clonaly, but produce sexual resting eggs when conditions are poor, which can hatch when conditions improve. Pasteuria is a common bacterial parasite of Daphnia, which causes 100 % mortality in in infected hosts, and removes their ability to reproduce. It also causes the host to turn bright red in colour, making it easy to differentiate infected from uninfected individuals. It is transmitted from environmental reservoirs in the sediment, when Daphnia pick up spores while filter feeding.
In the wild, seasonal epidemics of the parasite are observed in freshwater ponds. As temperature increases in the spring, Daphnia start to hatch from their resting eggs and their population increases in size. Other aquatic insects begin to hatch, including those that may prey on Daphnia. It has been hypothesized that these small predators may release parasite spores into the water when feeding on infected Daphnia (sloppy feeding) and contribute to transmission. A few weeks later, infections begin to be observed, and prevalence peaks around midsummer, usually reaching nearly 100 %. At the same time, the proportion of hosts in the population that are susceptible to many strains of the parasite declines, reaching nearly 0. The epidemic then declines and ends in the late summer.
Because all of these things are happening simultaneously, it is difficult to attribute these dynamics to any particular driver. The objective of this project was to investigate how these various factors (temperature, transmission from the spore bank, sloppy feeding in the free water and selection for host resistance) interact to influence the repeatable seasonal epidemic dynamics observed in nature in order to further understand the drivers of epidemics, by pairing observational and experimental work.
