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Simple to understand: climate change and infectious diseases


The consequences on the evolution of infectious diseases, both animal and human, remain difficult to assess.

It is becoming increasingly clear the disaster scenarios that are periodically announced will not be limited to distant., "exotic" infections, affecting only tropical countries.

We will be more and more exposed to them because of the "globalization" of infectious agents which, by air, are on our doorstep.

Moreover, there is a risk that undesired viruses, bacteria, or parasites will soon be introduced into our country.

Simple to understand: climate change and infectious diseases

What do we mean by climate change?

In fact, several climate events can combine their effects and lead to various consequences.:

1. Global warming is a slowly progressive phenomenon.

Thus, during the 20th century, it resulted in a rise of 0.5 to +0.6°C rise in ambient temperature on a global scale.

By 2100, an additional increase of + 1.4-1.5 to + 5.8-6°C is predicted.

At the same time, ocean water levels would rise by 40-50 cm by 2080 due to 2080, due to the melting of glaciers, polar ice caps, and sea ice.

Low-lying coastal areas and deltas (such as the Ganges and Brahmaputra rivers in Bangladesh) would be flooded.

Also profoundly disrupting ecosystems where mosquito vectors and wild birds are abundant reservoirs of viruses.

These floods would also cause population displacements of populations and famines.

2. The ENSO (El Niño/Southern Oscillation) phenomenon is the best-known example of natural, quasi-periodic climate variability.

It causes an increase in ocean temperature (El Niño, the baby Jesus, because it occurs at Christmas time in Latin America).

Also in atmospheric pressure over the entire Pacific basin (Southern Oscillation), this affects the climate of the entire planet.

The interval between two El Niño episodes are 2 to 7 years, and they can be followed by colder episodes called La Niña.

Each new El Niño event can exacerbate the effects of global warming.

By causing long periods of drought and famine in one part of the world, floods, and mudslides elsewhere, and sweeping everything in their path.

However, the latter disasters are rarely followed by epidemics, contrary to what is regularly announced.

3. Tropical cyclones (hurricanes, typhoons), which are also devastating, have tended in recent years to be more frequent and more violent.

In France, the increase in ambient temperature could be, in the future, of 2 to 3 °C, in winter, and 3 to 4 °C, in summer.

Heat waves, such as the 2003 summer heat wave, could become more frequent and more prolonged.

The Biology of Infectious agents

Regardless of their nature, microscopic infectious agents have multiplication cycles that are sensitive to temperature.

- Bacteria, protozoa, and microscopic fungi depend for their reproduction on an optimal temperature for growth.

Their multiplication is slowed at both suboptimal and super optimal temperatures.

In the latter case, bacteria develop stress proteins (heat-shock proteins) to protect them from the harmful effects of high temperatures.

But this process has its limits.

For eubacteria, bacterial multiplication ceases at certain temperatures.

- Viruses also have a thermal optimum for replication, above and below which the virions produced can manifest new properties.

In the case of polioviruses and ortho-poxviruses (vaccinia, monkeypox), suboptimal temperatures tend to select "cold" mutants.

These mutants are of attenuated virulence and supra optimal temperatures "hot" variants of increased virulence.

- Vector-borne diseases are especially sensitive to environmental variations.

Ambient temperature, as well as relative humidity, has direct effects on the biology of hemato-phagous vectors involved in the transmission of certain infectious diseases.

In the case of mosquitoes, high ambient temperature, at least within certain limits, accelerates their development from egg to adult.

At the same time as it can change their geographic distribution.

- The shortening of the extrinsic incubation time (EI) of the pathogen in the vector is also a consequence of the rise in temperature.

The EI measures the time between when the hematophagous vector becomes infected, by drawing blood from an infected animal or patient.

It becomes capable of transmitting the infection to a susceptible individual.

Its shortening increases the chances of transmission of the pathogen.

- The effects of climatic factors on wild reservoirs are also important.

Thus rodents that are reservoirs of Hantavirus, arena-virus, flavi-virus, Borrelia, or Yersinia pestis perish in large numbers during periods of extreme drought.

Only to swarm when abundant rainfall provides them with food again; grain, insects, etc…

They may also disappear en masse due to flooding.

In addition, the increase in ambient temperature can modify the hibernation rhythms of small mammals, particularly bats, which ensure the conservation of many viruses.

Similarly, in birds, it has been shown that global warming has led some of them to migrate earlier (by 8 to 10 days).

And then to lay eggs earlier, both in Europe and in the United States.

This may have an effect on the enzootic cycles of arbo-viruses in which wild waterfowl are involved, and even on avian influenza.

Thus, climate change is having or will have an impact on the epidemiology and evolution of infectious diseases.

 The precise impact is difficult to assess.

Other more immediate perils

They are represented by the extension or importation of dangerous viruses into our climates, as climate change may favor the development.

Or even the proliferation of their vectors.

Viruses move around a lot and, recently, they have been noticed by intercontinental movements.

In 1995, Japanese encephalitis, an avi-virus transmitted by mosquitoes, succeeded in crossing the Torres Strait, between Papua New Guinea and northern Australia.

One of its vectors from Asia, Culex gelidus, was found only a few kilometers from Brisbane airport.

In 1999, the West Nile virus, another flavi-virus, crossed the Atlantic.

It came from Israel, landed in New York, and then and then spread to almost all of the United States and to southern Canada.

Where, it causes numerous cases of encephalitis in humans each year.

In 2003, Rift Valley fever, a mosquito-borne phlebovirus infection previously confined to tropical Africa.

It spread to Yemen and Saudi Arabia, causing 140 and 87 deaths respectively.

In 2003, the monkey-pox virus, an orthopox-virus related to human smallpox, previously confined to the two forest blocks of West and Central Africa.

Also, it crossed the Atlantic, infecting 82 people in the USA.

The patients were lovers of exotic African rodents, "adorable balls of fur", but, in reality, very dangerous little companions in terms of health.

What are the risks for our country?

Recent events prove to us that the metropolitan territory is not immune to epidemiological threats from its own soil or from overseas territories.

1. The West Nile virus reawakened in 2000 in the Camargue, a natural focus of infection known since the 1960s.

There, it caused an equine epizootic in the departments bordering the Rhone delta: 76 cases and 21 deaths in horses.

Fortunately, the infection did not spread to humans.

However, in September 2003, several cases were reported in Fréjus, in the Var department, at the same time as at the same time.

2. Chikungunya virus, a mosquito-borne alphavirus, never having occurred in the southwestern Indian Ocean, made, from February 2005, a spectacular incursion.

It reached successively the Islamic Republic of the Comoros, Mayotte (French, island), Mauritius, Reunion, Seychelles, and Madagascar.

The most serious impact, both sanitary and economic, occurred on Reunion Island, where some 300,000 cases have been reported.

This is representing more than a third of the population of Reunion Island.

In addition, we have seen the appearance of serious forms of the disease, which had not been described until then.

It is affecting the central nervous system, the heart, the kidneys, the liver, the skin (bullous eruptions)…

And, above all, several cases of transmission from mother to the child. With serious repercussions on the newborn.

In addition, more than 200 deaths, more or less attributable to this virus, have been recorded.

However, as soon as Mayotte was affected, many imported cases appeared in metropolitan France, in the Comorian community.

And, during the second epidemic outbreak in Reunion Island, they multiplied in the Paris region and in the South of France.

The Albopictus mosquito, the designated vector of this epidemic, is already established in the south of France.

It is also possible that dengue fever, which is currently epidemic in French Guiana.

It could also arrive in France, taking advantage of the presence of the same vector on our soil.

Therefore, without losing sight of what is happening in the rest of the world.

We must be particularly vigilant in the face of threats that are no longer only theoretical and distant for our country.

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