Dengue fever, new evidence on links between health and the environment

What is dengue fever?

Dengue fever is a viral disease that causes a high fever and, in rare cases, progresses to a more severe form, particularly bleeding. Fatalities are rare – about 0.01% of all cases – but not negligible given that hundreds of millions of people around the world become infected every year. The public health issue is therefore important as dengue fever spreads to countries where it has not existed.

Where does the disease strike?

Dengue fever is currently a tropical disease. Overseas, certain French regions – Guadeloupe, Martinique, Réunion… – are regularly hit by major epidemics. The novelty of recent years is the arrival of dengue fever in France. Since the 2010s, a few “native” cases not resulting from contamination outside the territory have been reported each year.
This trend has taken a new step this summer. In mid-September, health officials identified around forty cases, concentrated in the South, a number that is likely to increase further this year. After that, “we are moving towards a prolongation and multiplication of these episodes,” warned epidemiologist Marie-Claire Paty during a Public Health France conference on Friday. “We are not immune to future epidemics” in mainland France, she stressed, admitting that their magnitude would certainly be less than overseas. This progression of the disease results from its mode of transmission.

How does dengue fever spread?

Dengue needs a mediator: mosquitoes. After biting a patient, they may find that they carry the virus and transmit it to their future victims. So does malaria, which is transmitted by Anopheles mosquitoes, while dengue spreads through several species of the genus Aedes.
First of all, there is the tiger mosquito, which was limited to Southeast Asia a few decades ago but is now establishing itself in Europe and America.
In mainland France, this species, absent before the 2000s, has conquered two-thirds of the territory. That doesn’t mean dengue fever cases are that common. However, their spread usually follows the establishment of the tiger mosquito with a delay of several years.
A phenomenon observed in other countries with similar diseases. Italy, one of the first European countries to see the arrival of this insect, has therefore recorded hundreds of cases of chikungunya, another virus transmitted by this mosquito, in recent years.

Why this progress?

The spread of the tiger mosquito is “encouraged by the globalization of trade and travel, progressive urbanization and climate change”, summarized the World Health Organization (WHO) in 2014.
The insect actually only lives in the city, prefers humidity and heat, and can slip into luggage or imported products.
The fight against this mosquito, and moreover against dengue fever, particularly resonates with a concept in full swing in public health: “One Health (one health)”, which insists on the interdependence between human health, animal health and environmental protection.

Also read: Reunion: Reinforced sterile insect technology, very encouraging trials against historical vector mosquito Dengu

How can dengue be fought?

There is a vaccine made by the Sanofi laboratory, but its effectiveness is too insufficient to recommend it to people who have never had dengue fever. The fight against the disease begins with the environment, starting with mosquitoes that spread the virus. So, the “mosquito control” operations are aimed at identifying and destroying the foci of larvae. However, other measures against the effects of climate change can counteract this, such as planting plants to reduce heat build-up in the city. “There may be water collection sites and breeding sites,” Ms Paty warned, but believed it was possible to reconcile the two issues.

However, once installed somewhere, the tiger mosquito generally cannot be completely eradicated. In the long run, the solution would be to exploit them. There are several approaches. One of the most promising is to introduce mosquitoes infected with a bacterium that blocks the circulation of the virus into the wild. Several studies, the most important of which was published last year in the New England Journal of Medicine, suggest that this approach, practiced particularly in New Caledonia, significantly reduces cases of dengue fever.

With AFP

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