Urbanization, globalization of transport, global warming… All these phenomena favor the spread of tropical diseases. Dengue, Zika, chikungunya… are arriving in Europe. Is this phenomenon inexorable? Are we ready to face it? Professor Olivier Bouchaud, infectious disease specialist, and Dr Louis Lambrechts, research director at the Institut Pasteur, answer us.
“Health transition” or the comeback of infectious diseases
To better understand the impact of infectious diseases on public health, it is first necessary to address the little-known notion of “health transition”. Research on the subject has shown that until the Neolithic period, infectious diseases dominated mortality worldwide. Over the past few decades, however, a phenomenon of “health transition” has emerged – a little more advanced in the Western world -, leading to a decline in infectious diseases in favor of non-communicable diseases (cardiovascular diseases, metabolic diseases such as diabetes, obesity, cancers and chronic respiratory diseases). Today, these weigh heavily on morbidity and mortality in the world: 2/3 of deaths are due to non-communicable diseases.
However, epidemiological observation shows that a new health transition is underway and is superimposed on this phenomenon. Epidemic infectious diseases are indeed returning to the forefront and this resurgence is linked to several factors.
Uncontrolled urbanization promotes infectious diseases
Before World War II, 70 to 80% of the world’s population lived in rural areas. Today, the trend is reversing: the urban population exceeds 50% and this phenomenon is expected to accelerate further in the years to come. Non-communicable diseases (due to lack of physical exercise, sedentary activities, access to a less healthy diet – rich in fats, sugar and salt -, pollution and exposure to carcinogenic substances, etc.) are widely installed.
But infectious diseases are regaining ground: urbanization, in certain countries, has in fact led to a concentration of populations in poor conditions. Slums, squats, informal housing, poor and precarious communities are crowded together, in difficult, even dramatic, hygienic conditions. “This considerable population density is restarting epidemics that have been relatively controlled until now., explains Professor Olivier Bouchaud, infectious disease doctor, head of the infectious and tropical diseases department at the Avicenne hospital in Bobigny. “Some epidemics are linked to vectors, mainly mosquitoes, but also to other insects: the multiplication of these vectors leads to more epidemics. Finally, others are linked to food infections (Salmonella, Escherichia coli, etc.) and overlap with another massive phenomenon: the spread of multi-resistant bacteria..
Man is increasingly investing in wild environments.
Humans are also investing in natural and wild environments, which has the effect of disrupting the natural balance but also putting them in contact with more and more infectious agents. The re-emergence of the Ebola virus in West Africa from 2013 to 2016 is a good example. “This virus, until then under control, affected isolated local populations, and gave rise to epidemics that could be easily contained due to poor external communication. From 2013, anthropization allowed person-to-person transmission, in a densely populated area: the virus therefore spread quickly.explains Professor Olivier Bouchaud.
The intensification of means of transport
The movement of travelers around the world is increasingly important. This intensification of travel, particularly by air, means that travelers carrying these viruses in their blood (sick or not) may be bitten by a mosquito (Aedes albopictus in particular), which then spreads these diseases on French and European soil. “This explains why viruses like dengue are transmitted to people who have never traveled. These are what we call “indigenous cases”, explains Dr Louis Lambrechts. “The more travelers there are from areas of endemic transmission of these tropical viruses, the greater the risk that Aedes albopictus – in particular – locally transmits these viruses. These indigenous cases are increasing from year to year because the tiger mosquito has progressed both in distribution area and density.
“The more mosquitoes there are, the greater the risk of high local transmission.”, adds the specialist. “Unfortunately, we will have to get used to the fact that you can contract dengue or chikungunya without having traveled.” A shocking fact for the general public, but an unsurprising reality for specialists who have been observing this development for many years already.
The “tiger mosquito” invades Europe
In Europe, the mosquito Aedes albopictus (commonly called “tiger mosquito”), native to Asia, is the main mosquito having an impact on public health. Exotic, invasive, detected in Europe in Menton for the first time in 2004, this mosquito has since colonized most of the metropolitan territory (only Brittany and part of northern Europe still resist!). It thus transmits viral infections called “arboviruses”, notably dengue (of which there are four different forms of virus), Zika, chikungunya, Rift Valley fever, yellow fever, fever Mayaro fever or West Nile fever… all illnesses we will hear more about in the future. “Currently, dengue is the most important re-emerging disease, in terms of frequency, and the one that should be feared the most.continues the infectious disease specialist.
Europe offers a habitat very conducive to the development of the tiger mosquito which, unlike its peers, tends to bite during the day and not just at night. “This makes mechanical wrestling more difficult, underlines Dr Louis Lambrechts, research director and head of the Virus-Insect Interactions Unit at the Pasteur Institute. “Mosquito nets, for example, are effective in protecting against nighttime bites. But during the day, our means of prevention are more limited.” This mosquito also has a greater capacity for adaptation than that of traditional mosquitoes: it is in fact capable of colonizing very varied environments (rural as well as urban, tropical as well as temperate) or of recolonizing forest environments for example (this is the case in Gabon where it was introduced less than twenty years ago).
Mosquitoes love global warming
The increase in temperatures on our planet does not explain, in itself, the resurgence of infectious diseases as we can hear here and there, but it is undeniably an aggravating factor because it promotes the activity of mosquitoes through the world and the multiplication of these. “The warmer it is, the more mosquitoes have a prolonged period of activity, especially in the fall., notes Dr. Louis Lambrechts. “The warmer it is, the more mosquitoes multiply and the faster the virus, inside the mosquitoes, also develops and will therefore be transmitted quickly. These mosquitoes are also perfectly capable of overwintering and surviving insecticides.”
Insecticides becoming less and less effective
We are indeed observing greater resistance among mosquitoes to insecticides (notably pyrethroids), coupled with less use of the latter since we now know that they are harmful to health and the environment. “European standards for insecticides are fully justified from the point of view of health and environmental toxicity. But as the list of authorized insecticides has been reduced and these products have lost some of their effectiveness, mosquitoes are becoming de facto more difficult to control.adds Professor Olivier Bouchaud.
Europe less sheltered from epidemics than before
For several decades, Europe had managed to get rid of a certain number of tropical diseases and logically believed itself to be safe from all these infectious phenomena. Unfortunately, recent news since Covid-19 clearly shows that this is no longer the case. “The Aedes albopictus mosquito exposes us, especially in summer, to epidemics which can be significant., reveals Professor Olivier Bouchaud. “In Europe but also in Italy, every year, there are a few micro-epidemics of dengue or chikungunya. In the medium term, we cannot imagine that these epidemics will prove massive and dramatic from a health point of view, but it is true that Europe is once again becoming exposed to tropical diseases which had disappeared for a certain number of years. years”.
Fewer and fewer weapons against infectious diseases
On the question of the resurgence of infectious diseases, Professor Olivier Bouchaud wants to be reassuring. “As long as Europe remains a haven of peace and economic prosperity, the weight of these tropical epidemics will still be limited.he believes.
On the other hand, he says he is more worried about the question of infectious diseases that are not specifically tropical (i.e….