Why France is now threatened by dengue fever, Crimean-Congo fever and chikungunya

What new threats threaten us in the short term and how can we prepare for them? This is the meaning of the conclusion of the Committee for Monitoring and Forecasting Health Risks (Covars), published on April 9, entitled “Assessment of the risks of major exceptional situations for human health in France in 2025-2030.”

The vigilance is focused “primarily” on “zoonoses,” transmitted from animals to humans, and “arboviruses,” transmitted, in particular, by mosquitoes. One thing is certain: in addition to the already observed effects of heat peaks and an increase in the number of “extreme” events (floods, droughts, storms, etc.), climate change will change the situation, as ecologist Patrick Giraudoux, professor emeritus at the University, explains. Franche-Comté and member of Kovar.

You say that global warming will lead to more disease vectors. For what ?

For a simple reason: periods of favorable conditions for certain vectors will lengthen. Take the example of mosquitoes. Until now, they are dormant in winter, and the length of the summer season allows them to complete only a few breeding cycles. If heated earlier and later, these cycles are mechanically multiplied. You do “x2”, then “x2”, then “x2” again, and you have an exponent that sets…

Ultimately, we risk having a much larger mosquito population, meaning more bites and an increased likelihood of disease transmission.

There is also chikungunya, which is transmitted by the tiger mosquito. It is possible that the metropolis will face a major epidemic, similar to the one that has already struck Reunion Island. The same goes for the Zika virus, which has spread widely in South America.

For the same reason, ticks, which are among other carriers of Lyme borreliosis, will also reproduce in significant numbers.

Photo by Nicolas Barrault

Should we be afraid of the emergence of new species that have not yet taken root in our soil?

Indeed, under these new conditions, some species will be able to move further north until they colonize our territory. This is the case, for example, with the Hyalomma tick, a carrier of a virus that is quite dangerous to human health and causes Crimean-Congo fever. Its influence has already been confirmed in Spain. It has now arrived north of the Pyrenees and in all likelihood will move into the Rhone Valley and spread throughout the country. Thus, we have both a new species and a new disease.

To this should be added the fact that some species, despite global warming, manage to adapt to initially unfavorable conditions for them. This is the case with the tiger mosquito, which was able to gradually invade Europe.

On the other hand, the quality of the air we breathe is expected to continue to deteriorate, as will the quality of water…

As for the air, concern is focused, among other things, on ozone, the concentration of which increases with the arrival of sunlight – hence the peaks that we very often observe in the summer, in large cities. This is problematic because it is an oxidizing agent that increases respiratory infections.

As for water, as it becomes rarer, it will also become more concentrated. Consequence: The concentration of chemical contaminants and pathogens will inevitably increase to the point that some sediments will become undrinkable.

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With what consequences?

If you have one bacteria per cubic meter, the risk of infection – such as E. coli – is low. If you have thousands, the risk becomes high. On the other hand, in some very degraded environments, the population will be forced to seek water where it is undrinkable due to the lack of alternatives. Then we could witness cholera epidemics, especially abroad.

Could the destruction of biodiversity create uncertainty or even an increased risk to our health?

This is an essential point that is often underestimated and misunderstood. There are thousands or even hundreds of thousands of species in an ecosystem. If you remove just one, it won’t be obvious. However, this disappearance will leave an empty ecological niche. Thus, species that previously competed with it will expand their perimeter and occupy the remaining vacant space. These are what we call opportunistic species.

Let’s take a simplified example of a seven-species system. You take three. Let’s imagine that one of the remaining species takes advantage of this to expand its range and increase its biomass in terms of population. The consequence of this is the impoverishment of biodiversity: the same species, relatively homogeneous at the genetic level, will take over the rest.

Photo by Renaud Baldassin

In the case of agricultural systems, the same species is often grown, very monoclonal and therefore not very diverse for obvious productivity reasons. This type of species will be very vulnerable to parasites, fungi and other insect pests. To counter this fragility, traditional agriculture uses phytosanitary products, with all the public health problems this creates.

Poor biodiversity is therefore much more difficult to manage than highly diverse environments. The whole becomes extremely variable and sensitive to any external disturbance, be it a change in temperature, drought or disease.

Could the growing human influence on nature make the situation worse?

This is confidence. If we consider the biomass or weight of all mammals on a planetary scale, we humans represent 36% and our pets represent 60%. Thus, 96% of this biomass consists of approximately twenty species. The remaining 4% are wild mammals, numbering at least 6,500 species!

The very structure of this system favors pandemics: from wild animals we easily move to domestic animals, which play the role of incubators, and then to humans, whose biomass is absolutely significant.

Faced with these multiple warning signs, is our warning and prevention system sufficient?

This is the whole challenge posed by Kovars, who recommends being much more proactive than what is being done today. We will have to think about improving what we have in the face of situations that are ultimately quite predictable.

From this point of view, Covid was something of a dress rehearsal. It is certainly a pathogenic virus, but the mortality rate is not that high—about ten times that of seasonal influenza, which is certainly not negligible, but is still much lower than some tropical viruses.

Therefore, we must take this threat very seriously and prepare for it. The question is not whether there will be pandemics in France in the near future – there will be, that’s for sure – but to be prepared when they come.

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