The disease pyramid
Nature is complex. Nature is drive by interactions between different species, and those interactions are driven by environmental impacts. Everything is to some degree connected.
Until now, disease expression and severity was explained by the three way interactions between the host, the pathogen and the environment - the disease triangle. Our review shows how the microbiome of the host influences the interactions between living organisms, the environment and pathogens. Hence, disease expression and severity is the result of four-way interactions, hence forming a pyramid and not a triangle.
Biotic and abiotic environmental factors have a strong influence on the dynamics of diseases in humans and animals. In our study, we focus on an important component: the microbiome. The individual microbiome of a living being is a vital component of immunity. Especially on the skin and in the bowel, i.e. directly at the interface between the individual and pathogens, endogenous bacteria and viruses are highly active.
The international team presents the concept of a disease pyramid with the four cornerstones of environment, pathogen, host and host microbiome. For the first time, the different functions of the microbiome are taken into account. We illustrate this by using the amphibian disease chytridiomycosis caused by the fungal pathogen Batrachochochytrium dendrobatidis.
"The microbiome of living organisms is highly variable. It is only in recent years that researchers have succeeded in using genetic methods to determine the totality of microorganisms. We are now only gradually beginning to understand their role in health prophylaxis and how they interact, for example, with the environmental microbiome, pathogens and the host," explains Adeline.
Diversity of microbiome and habitats strengthens resistance
Our research emphasise that more diverse microbiomes can make the host more resistant to pathogens because they are better able to keep potential pathogens at bay. The study also shows that individuals who inhabit complex and therefore species-rich habitats have a lower mortality rate. We show that the microbiome can act very specifically against pathogens: The symbiotic skin bacterium Janthinobacterium spp. forms an anti-fungal agent as a metabolic product and thus prevents infection with Batrachochytrium dendrobatidis.
Climate change modifies the microbiome
It is assumed that the adaptability of the microbiome can in turn increase the organism's adaptability to environmental influences. There are several examples of this in the animal kingdom. However, environmental changes such as climate change can also throw the microbiome out of balance: "A microbiome in equilibrium can protect against infection in changing environmental conditions," explains Adriana.
"However, it is also shown that environmental changes—especially temperature—have a significant impact on the composition of the microbiome, and thus on the resistance of amphibians to Batrachochytrium dendrobatidis. Climate change will significantly change the distribution of this fungal disease in amphibians," the ecologist predicts.
Dirk explains: "We have to be aware that climate change and biodiversity loss are stress factors for ecosystems, for humans, for animals and for the microbiome. Our research shows that if the different axes of the disease pyramid are destabilised, new infectious diseases can be expected, including for humans."
We have cleared forests, destroyed many landscapes. And there are many pathogens in those landscapes. Pathogens are very natural components; they are there to select against weak individuals. The moment we destroy biodiversity, nature, the environment, we come into contact with these pathogens or animals that carry these pathogens. We must be aware that the current Covid-19 crisis is relatively small compared to the crisis of climate change and biodiversity loss. Why? Climate change and biodiversity loss are stress factors for ecosystems, for humans, for animals and for the microbiome... Our research shows that if the different axes of the disease pyramid are destabilized, new infectious diseases can be expected, including zoonoses.
The presented concept of the disease pyramid is trend-setting for research on human-animal-plant-environment interactions and the resulting risks for biodiversity and humans.
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