- Researchers investigated the effects of different temperature regimes on a crustacean model of disease transmission.
- They found that different temperature regimes affect disease transmission in complex ways.
- They concluded that further research into how temperature regimes affect host-pathogen dynamics is crucial for predicting the effects of climate change on human and environmental health.
Researchers predict that climate change will increase the Earth’s mean temperature, cause temperature fluctuations, and increase the frequency and intensity of extreme weather events. How these changes will affect infectious disease and impact human health, agriculture, and wildlife remains to be seen.
Research shows that temperature variance can modify host-pathogen dynamics. One study found that daily temperature fluctuations increase malaria transmission, and another suggested that short-term temperature fluctuations led to reduced transmission.
Further research shows that how extreme heat events affect host-pathogen dynamics may also depend on the magnitude, duration, and intensity of the heatwave.
For example, one study investigating a parasitoid-insect interaction found that a heatwave increase of 5°C increased parasitoid size while a 10°C increase reduced parasitoid growth.
Knowing how host-pathogen dynamics respond to temperature variation could help researchers and policymakers alike prepare for the effects of climate change.
In a recent study, researchers led by Dr. Pepijn Luijckx, assistant professor in parasite biology at Trinity College Dublin, Ireland, investigated the effects of different temperatures on the host-pathogen dynamics.
They found that temperature variation alters pathogen-host interactions in complex ways that may affect disease dynamics in an unanticipated manner.
“Here, we show that temperature variation with each of the traits we measured […] responds in a unique way to different types of temperature variation,” Dr. Luijckx told Medical News Today. “Given that our mathematical models for disease spread rely on numerous variables, and our results show that each of these may respond in a unique way to both changes in temperature mean and variance, predicting how global warming may alter diseases may be incredibly complex.”
The study appears in eLife.
Daphnia magna
For the study, the researchers examined the effects of different temperatures on small crustaceans called Daphnia magna alongside its gut parasite, Ordospora colligata.
Researchers frequently use Daphnia in ecological model system research, and Ordospora transmission is representative of classical environmental transmission, similar to viral infections such as SARS-CoV-2.
They then subjected the crustaceans alongside their parasites and a placebo infection as a control to three temperature regimes for 27 days:
- constant temperature regimes ranging from 50°F (10°C) to 82.4°F (28°C)
- daily temperature fluctuations ±3°C
- constant temperature regime with a 3-day heatwave rising temperatures by 6°C
The team selected the temperature regimes to mimic temperature events that occur in the study subjects’ natural environments, such as rock pools and small ponds.
Altogether, the researchers observed temperature effects in 492 individuals. During the experiment, they assessed host longevity, fecundity — the ability to produce offspring, infection status, and the number of Ordospora spores within the host gut.
The researchers found that, regardless of temperature regime, Ordospora reached optimal performance at roughly 66.2°F (19°C).
While there was a reduction in infectivity and spore burden among Ordospora-exposed Daphnia in fluctuating temperatures, the infectivity of parasites following a heatwave was almost the same as those maintained at a constant temperature.
The researchers noted that the effects of temperature variation differ depending on the average background temperature and how close this is to the optimum temperature.
For example, spore burden at 60.8°F (16°C) differed by almost an order of magnitude between fluctuating temperature regimes — at 86 spore clusters — and heatwaves: 737 spore clusters.
“That a heatwave of 6°C above ambient can lead to an almost 10-fold higher level of disease burden at 16℃ when compared to fluctuating temperatures was remarkable,” said Dr. Lujickx. “Moreover, that this same heatwave, when applied to different mean temperatures, led to either no difference with fluctuating temperatures or even an opposite outcome was unexpected.”
Host fitness was generally reduced by exposure to Ordospora spores or experience of a variable temperature regime. The researchers found that Ordospora-exposed Daphnia experienced a reduction in reproductive success of 8% in constant temperatures and 24% in daily fluctuation when compared with controls without Ordospora.
This, they say, means that under some circumstances, the parasites may be able to acclimatize to new temperatures faster than their hosts.
Temperature variability hypothesis
To explain their results, the researchers noted that, according to the temperature variability hypothesis, as parasites are smaller than their hosts, they, in turn, have a faster metabolic rate. In unpredictable environments, such as a heatwave, parasites would therefore have an advantage over their hosts.
They noted that host resistance might also decrease due to a trade-off between energy demand for acclimatization and immunity from thriving pathogens.
Dr. Luijckx said that while this is the most promising explanation, some aspects of their findings remain unclear: “While this theory can explain the observed increase in the number of spores of the pathogen at 16℃, it cannot, however, explain why we see that the outcome depends on the mean temperature. Other theories, however, have suggested that when the disease has a smaller temperature tolerance than its host, this may lead to a reduction in disease performance at temperatures that exceed its tolerance.”
The researchers concluded that improving their understanding of temperature variation on host-parasite dynamics is critical for predicting disease dynamics as the climate changes.
Study limitations
The team noted several limitations to their findings. Dr. Luijckx explained that as they only conducted their experiments on one species of water flea and a single disease, they are unsure if their findings may apply to other organisms and lead to higher levels of disease in livestock, agriculture, or disease vectors.
He added: “Moreover, our study was conducted at an individual level. To fully understand disease outbreaks and dynamics, we would need to test how temperature variation and extreme weather affect diseases at a population level. We are planning to do such experiments soon.”
“We have to be careful in extrapolating these results to homeotherms like people, whales, and larger charismatic endangered species,” Dr. Joseph K. Gaydos, VMD, Ph.D., chief scientist at the University of California, Davis School of Veterinary Medicine, told MNT. Dr. Gaydos was not involved in the study.
“Still, that said, what happens to very small, even microscopic creatures has a huge effect on much larger species. Imagine what parasitic changes in krill — a small oceanic poikilothermic plankton — could do to krill populations and how that could affect whales that eat krill,” he continued.
Implications
When asked how these findings may affect public health policy and research, Dr. Luijckx said:
“Our findings, if they apply to other diseases, suggest that ongoing climate change could alter where and when disease outbreaks occur.”
“However, to accurately predict disease spread with ongoing global warming and inform health policy, we will need to first explore the generality of our findings, identify the mechanism responsible for our observations, and test if our findings still hold when we do similar experiments with whole populations.”
Dr. Gaydos added: “We think a lot about how climate change will affect [the] distribution of human, domestic animal, and wildlife parasites like mosquitoes or ticks, but often we forget that even small organisms like freshwater plankton (Daphnia) have parasites and that climate-related changes in host-parasite interaction [are] going to have major implications on up the food chain.”
“That means we, as scientists, need to think a little broader than we currently are thinking. Human health is tied to animal health and environmental health, and the complexities are huge.”
“For me, the biggest take home is this: Kunze, Luijckx, and team have reminded us that despite all of our efforts to predict how our changing climate will affect disease, parasitism, humans, domestic animals, and wild populations, it’s still a crapshoot as to what is going to happen out there.”
“Not only do we need to be paying better attention, we need to get our A-game on to better plan for climate resiliency. Yes, the world is going to change in some of the ways we expect, but there are going to be a lot of curveballs coming our way.”
– Dr. Gaydos
“This paper reminds us that it won’t just be linear — constant higher temperatures will not produce the same things, as more extreme fluctuations and heatwaves can also have differing effects. The only way we can prepare for uncertainty is to do a better job of taking care of the limited natural resources we currently have and ensuring we have as much resiliency in natural systems as we can, so the system will be able to compensate. The global 30×30 initiative is a promising opportunity,” he concluded.
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