Climate change

Climate change and extreme weather will have complex effects on disease transmission

Temperature fluctuations such as heat waves can have very different effects on infection rates and disease outcomes depending on the average background temperature, according to a report published today in eLife.

The study suggests that it will become increasingly difficult to predict the consequences of climate change on host-pathogen interactions as global temperatures rise and extreme weather events become more frequent.

Infectious diseases have profound ecological effects on human, agricultural and wildlife populations. It is well known that interactions between pathogens and their hosts are sensitive to temperature changes. But what is less well understood is how a sudden and extreme change in temperature affects this relationship and how it influences overall infection rates and disease outcomes.

“Climate change is expected to increase not only average temperatures, but also temperature fluctuations and the frequency and intensity of extreme weather events,” says co-first author Pepijn Luijckx, William C. Campbell Lecturer in Parasitic Biology , Trinity College Dublin, Ireland. “Yet, although studies have quantified the effects of rising average temperatures on host and pathogen characteristics, the influence of variable temperature regimes such as heat waves remains largely unknown.”

Luijckx and the team examined the effects of different temperatures on various traits of a host organism — a small crustacean called Daphnia magna — and its known intestinal parasite, Odospora colligata. Parasite transmission is representative of classic environmental transmission, similar to that seen with diseases such as SARS-CoV-2 and cholera.

The team examined how the organisms responded to three distinct temperature regimes: one constant temperature and two variable regimes, with daily fluctuations of +/- 3°C and three-day heat waves of 6°C above the ambient temperature. They then measured the crustacean’s lifespan, fertility, infectious status and the number of parasite spores in its gut. Then they processed the data in a statistical model to compare the impact of the three different temperature regimes.

The team found that daily temperature fluctuations reduced the infectivity and spore load of the parasite compared to those kept at a constant average temperature. However, in contrast, the infectivity of parasites after a heat wave was almost the same as the infectivity of those kept at constant temperature.

Additionally, the number of spores in the crustacean host increased after the three-day “heat wave” when the constant background temperature was 16°C, but this load was reduced at higher temperatures. This suggests that the effects of temperature variation differ depending on the average background temperature and whether this is close to the optimum temperature for the parasite.

Host fitness and reproductive success were generally reduced in crustaceans exposed to parasite spores or when subjected to variable temperatures. The difference between host and pathogen responses suggests that, under certain circumstances, the parasites were able to withstand the sudden change in heat better than their hosts.

“Our results show that temperature variation alters the outcome of host-pathogen interactions in complex ways. Not only does temperature variation affect different host and pathogen traits in distinct ways, but the type of variation and the mean temperature at which it is applied is also important,” Luijckx concludes. “This means that changing patterns of climate variation, superimposed on changes in mean temperatures due to global warming, can have profound and unforeseen effects on the dynamics of illnesses.”

Alongside Pepijn Luijckx, the research team includes co-first author Charlotte Kunze (Carl von Ossietzky University of Oldenburg, Germany, and Trinity College Dublin), Andrew Jackson and Ian Donohue (both Trinity College Dublin).

Their study was funded by Science Foundation Ireland and the Irish Research Council.

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