Researched and probed: the differential influences of global warming on disease incidence

Changes in temperature, such as heatwaves, can have drastic differential influences on disease incidence and severity of illness based on the average composition of temperature, according to a study published in eLife on February 15, 2022.

According to the study, as global warming continues and intense weather occurrences become more common, it will be highly impossible to forecast the effects of climate change on host-pathogen interactions.

Global warming and climate change.

Infectious diseases have far-reaching ecological consequences for human, agricultural, and wildlife populations. Temperature changes are considered to induce pathogen-host responses. What seems to be less clear is how spontaneous and drastic temperature changes actually impact this relationship, and how this tends to affect overall transmission rates and disease outcomes.

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

Luijckx and colleagues studied the impacts of temperature on specific features in a host organism – a small crustacean called Daphnia magna – and its known gut parasite, Odospora colligata. The parasite is transmitted in the same way that diseases like SARS-CoV-2 and cholera are transmitted in the surrounding.

The researchers investigated how organisms reacted to three different temperature conditions: a steady temperature and two variable regimes with daily fluctuations of +/- 3°C and three-day heatwaves of 6°C above room temperature. They subsequently assessed the crustacean's lifespan, fertility, infection status, and parasite spore count in the gut. The data was then processed in a statistical model to draw comparisons over the effect of the three varying temperature conditions.

The researchers discovered that daily temperature changes reduced the parasite's risk of transmission and spore burden compared to those maintained at constant average temperature. The transmissibility of parasites following a recent heatwave, on the other hand, was nearly the same as that of those kept at a constant temperature.

Global warming and climate change diseases.

Furthermore, the quantity of spores in the crustacean host multiplied after a three-day 'heatwave' with a background constant temperature of 16°C, but this burden was lowered at higher temperatures. It then implies that the implications of temperature gradient vary largely depends on the average background temperature and if it is nearly equal to the parasite's proper temperature.

In general, host suitability and reproduction were lowered in crustaceans susceptible to parasite spores or subjected to variable temperatures. The disparity in host and pathogen reactions suggests that in certain cases, the parasites were capable of withstanding the dramatic fluctuations in temperature better than their hosts.

“Our findings 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 a distinct way, but the type of variation and the average temperature to which it is applied also matter,” concludes Luijckx. “This means that changing patterns of climate variation, superimposed on shifts in mean temperatures due to global warming, may have profound and unanticipated effects on disease dynamics.”