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Hotter, more polluted, more inhospitable… The planet has changed under the effects of human activity and, in this new environment, infectious diseases are progressing.
Recent studies show the complex consequences of climate change caused by humans, between the spread of certain diseases and new modes of transmission for others.
In particular, vector species such as the mosquito which thrive in a more humid climate with high temperatures, and animals carrying diseases which come closer to humans as their habitat disappears.
Loss of biodiversity appears to play a major role in the proliferation of diseases, according to a study published this week in the scientific journal Nature.
Nearly 3,000 databases from previous work were analyzed to discover how loss of biodiversity, climate change, pollution, destruction or modification of habitats, as well as the introduction of new species affect the development of diseases. infectious in humans, animals and plants.
The erosion of biodiversity emerged as the first factor, followed by climate change and the introduction of new species.
The parasites target species that are present in mass and therefore offer more potential hosts, underlines Jason Rohr, one of the authors, professor of biology at the University of Notre Dame.
And species with larger populations are more likely to “devote themselves to their growth, reproduction and propagation to the detriment of defense against parasites“, he explained to AFP.
On the other hand, rarer and more resistant species are more vulnerable to the decline of biodiversity, which results in a greater proportion of species that are abundant and sensitive to parasites, detailed the scientist.
“If there is more generation of parasites or vectors, then there may be more diseases“, summed up Mr. Rohr.
Unequal consequences
However, the human footprint on the planet has not only increased the risks. Loss or change of habitat has been associated in some cases with a decline in disease, especially thanks to advances that have come with urbanization, such as running water and sewers.
And the consequences of climate change are not the same everywhere.
In tropical regions, the hotter and more humid climate is leading to an explosion in dengue cases. But in Africa, drier conditions could help stem the spread of malaria.
A study published this week in the journal Science modeled the interactions between climate change, precipitation and hydrological processes such as evaporation and the rate at which water penetrates soils.
This research predicts a greater reduction in areas suitable for transmission than predicted by analyzes based on precipitation alone.
The study suggests that the malaria transmission season could be four months shorter in parts of Africa than previously estimated.
However, these observations are not necessarily good news, said Mark Smith, research associate professor of hydrology at the University of Leeds and lead author of this study.
“The areas conducive to (the spread of) malaria will change“, he told AFP.
And the population is expected to grow rapidly in areas where malaria will still be prevalent or become transmissible, increasing the incidence of the disease.
Smith warns that conditions that are too harsh for malaria can also be harsh for humans, using water availability as an example.
Diseases, climate, same fight
The links between climate and infectious diseases nevertheless suggest that climate modeling can help anticipate epidemics.
Local temperatures and precipitation forecasts are already used to predict increases in dengue cases, but they only provide short-term information and are not always reliable.
The Indian Ocean Basin Index (IOBW), which measures average water surface temperature anomalies, could be an alternative.
Another study published in the journal Science this week indeed notes the existence of a close correlation between IOBW fluctuations and dengue epidemics in the Southern and Northern hemispheres.
As the study is retrospective, the predictive potential of IOBW has not been proven, but monitoring this index could help authorities be better prepared.
In any case, tackling infectious diseases means tackling climate change, summarizes Mr. Rohr.