Important note: before opening this new chapter, we have just learned of a terrible earthquake in Morocco. While the direct consequences are dramatic, with more than 2,000 deaths so far (09/10/23), the health consequences, and in particular the development of outbreaks, are still to come: we invite you to reread the article we devoted to this subject by following this link [1].
A case of dengue fever was identified in the Lyon metropolitan area, in Couzon-au-Mont-d’or in August. The local authority specifies that this is an “imported case” from Martinique, which means that the person contracted the disease by being bitten by a mosquito in an endemic country.
What’s it about [2]?
Since 2010, some 30 “native cases” – people who have never travelled to a dengue-infected country – have already been detected in Alpes-Maritimes, Var, Bouches du Rhône, Hérault and Gard.
In 2022, 66 native cases were reported, affecting new departments such as Haute Garonne, Hautes-Pyrénées and Pyrénées Orientales.
It would appear that the disease is becoming established in France. In the Rhône [3] department alone, 13 cases of dengue fever were detected between May and August.
Dengue is transmitted by the tiger mosquito, as are 2 other arbovirus diseases: Zika and Chikungunya.
In fact, the increase in the number of cases of Dengue fever in France is correlated with the increase in the colonization of areas by the tiger mosquito (Aedes albopictus), which by January 1, 2023 was colonizing 71 of the 96 départements in mainland France. In the Americas, 2.8 million cases of dengue fever have been identified in 2022, more than double the number in 2021, and Peru is the victim of an unprecedented outbreak.
Climate change affects human health by disrupting ecosystems, and direct impacts are more frequent and more intense. Indirect impacts, i.e. effects on natural systems such as air quality, water availability, biodiversity and food production, also play an important role.
Heatwaves and ever-rising temperatures exceed thermoregulatory capacities: mortality rises due to the combined effect of an ageing population and the intensification of heatwave periods.
Extreme weather events such as storms, floods, droughts and fires cause direct and indirect deaths through :
- air pollution (fine particles) ;
- the spread of water-borne diseases (leptospirosis, cholera, etc.);
- the consequences of malnutrition due to famine;
- mental health-related trauma…
The availability of new water resources favors the emergence of water-borne infectious diseases (gastroenteritis, bacillary or parasitic dysentery).
Food insecurity is the consequence of agricultural, fishing and livestock crises (drought, reduced yields, etc.). The migration of fish stocks towards the poles deprives tropical populations of their main source of food.
Pathogens, hosts/vectors and modes of transmission to humans
An increasing number of studies show the link between climate change and the growing threat of zoonoses, i.e. diseases transmitted from animals to humans.
Climate change has an impact on a number of factors, particularly the environment, leading to changes in the spatio-temporal distribution of infectious diseases, and in the frequency and severity of outbreaks. Above all, these changes will affect the survival, reproduction, life cycle and habitat of disease-transmitting hosts.
For example, climatic conditions have been shown to be more favorable to the transmission of the dengue virus and the Plasmodium falciparum parasite responsible for malaria on a global scale since 1950: the vectorial capacity of the tiger mosquito for dengue transmission has increased by 19% worldwide and 40% in Europe. For malaria, the vectorial capacity of Aedes has increased by almost 150% in the Western Pacific region.
Climate change is extending the mosquito’s period of activity (currently May to September), and the high temperatures favor the multiplication of viruses in mosquitoes.
Lyme disease
The incidence of Lyme disease [4] (transmitted by ticks of the genus Ixodes) is on the rise in several regions of northern Europe and at higher altitudes, most certainly favoured by milder winter temperatures.
In North America, warmer temperatures are expanding the range of ticks carrying Lyme disease.
Warming temperatures are helping to extend the geographical range of the disease. Ticks thrive at temperatures above 7.2°C and in wetter climates, so warming North America offers more favorable habitats for these ticks. As with mosquitoes, a warmer climate can accelerate the time it takes for a young tick to become an adult, shortening the overall reproductive cycle. Similarly, milder winters enable some ticks to survive the cold season and remain active for longer each year. Climate change may also affect other environmental factors, such as deer population levels and other hosts.
As average global temperatures rise, the number of new cases of Lyme disease reported to the Centers for Disease Control and Prevention (CDC) in the USA has almost doubled since the early 1990s, to around 30,000 cases a year.
In Canada, the number of cases has risen from a few hundred to several thousand in recent years.
Dengue and Chikungunya
As mentioned in the introduction, these arbovirus diseases have proliferated not only in France, but also in southern Germany, Italy and Croatia. Global warming is also creating favorable conditions for the return of A. albopictus and A. aegypti mosquitoes to Europe. Once the vector is established, cases of autochthonous transmission can occur if the pathogen has been imported by people infected in endemic areas (imported cases).
In Southeast Asia, cases of dengue fever have soared, as longer rainy seasons and more frequent and severe flooding allow mosquitoes to thrive.
Aedes mosquitoes thrive in warm, humid environments. As temperatures rise, the insects can survive in areas that were previously too cool for them. Warmer temperatures also reduce the time needed for young mosquitoes to become disease-carrying adults. In addition, mosquitoes generally lay their eggs in stagnant water, so flooding can lead to an increase in dengue cases as the mosquito population grows.
Cases can also increase in times of drought, as people are more inclined to store water in containers, where mosquitoes prefer to lay their eggs.
Hantavirus [5]
This is a viral disease of rodents. In humans, it causes pulmonary infections (coughing, shortness of breath) or hemorrhagic fevers with renal syndrome, which can be severe. Contamination occurs through contact with sick rodents or their droppings, or by inhalation of aerosolized viral particles from rodent excrement. Climatic factors, such as rising summer and autumn temperatures and abundant rainfall, may be responsible for the resurgence of the disease. In several parts of Europe, an association between a warmer climate and Hantavirus infection has already been observed,
In South America, there is concern that increased rainfall variability could lead to a rise in rodent-borne Hantavirus disease.
In recent years, there has been an increase in person-to-person transmission of the Andean Hantavirus, found in Argentina and Chile.
Vibrio [6] spp.
They are responsible for water-borne diseases, which have increased in recent years, particularly in the Baltic Sea, where since 2018, the entire coastline is conducive to the proliferation of the bacteria due to warmer surface water. Skin infections are generally benign, but fatal cases of septicemia have also been observed in immunocompromised patients in recent years.
Ebola [7]
Many of the effects of climate change are likely to provide better conditions for disease-carrying animals. For example, a warmer, wetter climate in the forests of the Democratic Republic of Congo (DRC) could produce more vegetation to feed more host animals. The virus is therefore more likely to spread to humans. Previous Ebola outbreaks have coincided with the transition from dry seasons to periods of heavy rainfall.
At the same time, people living in regions experiencing more frequent droughts could suffer from food insecurity. This could push them further into forested areas in search of bushmeat (raw or minimally processed meat from animals such as bats and monkeys) and other foods, exposing them to an increased risk of coming into contact with the virus.
Conclusion
The infectious diseases that spread under the influence of climate change are mainly those involving intermediate hosts such as mosquitoes, ticks, rodents and even bats. Under the influence of rising temperatures or changing humidity, geographical areas are changing. However, these phenomena are relatively recent, and we must also take into account the evolution of human practices and social and economic developments, which may be the source of their emergence and the driving force behind their possible spread.
References
H. Hachad, A. Depoux, Introduction aux impacts sanitaires du changement climatique [8], LPM formation, 2021.
C. Klobucitsta and L. Maizland, Perilous pathogens : how climate change is increasing the threat of diseases [9], Council on foreign relations, 2022.
JF Guégan, Impacts sur l’épidémiologie des maladies infectieuses et risques épidémiques émergents [10], LPM formation, 2021.
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