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.
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.
