The unexpected ways heat is changing how parasites spread disease

When researchers mimicked heat waves in the lab to see how climate change might affect the spread of disease, they found that dialing up the temperature had the potential to lead to two very different outcomes: A spike in the population of disease-spreading parasites or a collapse in their numbers.

While scientists have known for decades that heat waves lead to the proliferation of  disease, from mosquito-borne malaria to respiratory infections like pneumonia, it’s becoming increasingly clear that, contrary to the conventional wisdom, such diseases won’t spread evenly.  The new research, which was published in PLOS Climate on Wednesday, suggests that factors like how long heat waves last and how hot they get can determine whether a community is hit by a disease breakout or spared.

“It’s actually a lot more complicated than we first originally thought,” says Niamh Mc Cartan, lead author of the study and a doctoral candidate at Trinity College Dublin. “Everything is quite context specific. A heatwave happening in Ireland might have different implications to a heat wave happening in Spain” or anywhere else in the world.

The study is based on water fleas, tiny crustaceans commonly used in research studies, but the findings hold relevant lessons for the spread of human disease.  They also show the need for climate models that go beyond average temperatures to consider other heat nuances, she says. As climate change leads to more frequent and intense heat waves, the additional information could help public health officials better understand the spread of disease and help them protect people.

How does a heatwave affect parasites?

The experiment exposed water fleas to parasites, putting them through more than 60 different heat wave permutations.  Researchers mimicked extreme heat by placing jam jar-like vessels with the water flea hosts into baths of varying temperatures and manipulated factors like how long the fleas endured hotter temperatures and when they were exposed to parasites.

For example, the number of parasites in water fleas that had been infected at the beginning of a hot spell increased by almost two-and-a-half times. But in water fleas that had been exposed to parasites long before the temperatures climbed — 20 days before — the number of parasites dropped by more than 10 times. The parasite that was studied has a limited heat tolerance, and at times it simply got too hot for it to survive, Mc Cartan says.

Although the researchers used water fleas, which don’t spread human diseases, the findings shed light on how parasites in other hosts that can infect people might react to temperature fluctuations, including malaria-transmitting mosquitos.

“You cannot do an experiment to put people infected with a disease and expose them to high temperatures,” says Camilo Mora, a professor at the University of Hawaii at Manoa who has studied climate change’s impact on disease. “That experiment is just never going to happen. But that doesn’t mean that we as humans are not going to get affected by this.”

The spread of parasites also has consequences for the environment. The water fleas used in the experiment, known as Daphnia magna, are an important food source for fish and other organisms. If parasitic infections cause them to die off, less food will be available to their predators. “Because Daphnia are central to many freshwater ecosystems, their health can be a good indicator of overall ecosystem functioning,” says Mc Cartan.

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