CLIMATE, EMERGING DISEASES , DANGEROUS CONNECTIONS FOUND
Climate change may
affect human health directly or indirectly. In addition to increased threats of
storms, flooding, droughts, and heat waves, other health risks are being
identified. In particular, new diseases are appearing, caused by infectious
agents (viruses, bacteria, parasites) heretofore unknown or that are changing,
especially under the effect of changes in the climate (change of host, vector,
pathogenicity, or strain). These are so-called "emerging" or
"re-emerging" infectious diseases, such as leishmaniasis, West Nile
fever, etc. According to the WHO, these diseases are causing one third of
deaths around the world, and developing countries are on the front line.
A
difficult relationship to establish
Several parameters may
be behind this increased spread of pathogens and their hosts (vectors,
reservoirs, etc.). Climate change modifies temperature and humidity conditions
in natural environments, and therefore alters the transmission dynamics for the
infectious agents. It also affects the range, abundance, behaviour, biological
cycles, and life history traits of the microbes or related host species,
changing balances between pathogens, vectors, and reservoirs. However, these
effects remain poorly explained, in particular because they require an
understanding of the long-term spatial or temporal changes to the phenomena.
Therefore, it is difficult to establish a direct link between climate change
and the overall evolution of infectious pathologies.
Decreased
rainfall rhymes with epidemic
Providing some
clarification on this question for the first time, a study by IRD researchers
and their partners has shown the relationship over a 40-year period between
climate change and epidemics of a disease emerging in Latin America: Buruli
ulcer. Rising surface temperatures in the Pacific Ocean tend to increase the
frequency of El Niño events, which especially affect Central and South America
approximately every five to seven years, causing waves of droughts. The research
team compared changes in rainfall in the region with changes in the number of
cases of Buruli ulcer recorded in French Guyana since 1969 and observed the
statistical correlations.
In fact, the decrease
in rainfall and runoff led to an increase in areas of residual stagnant water,
where the bacteria responsible, Mycobacterium ulcerans, proliferates. The greater access to swampy
habitats that results from this facilitates frequentation by humans (fishing,
hunting, etc.) and thus intensifies human exposure to the microorganism living
in this type of aquatic environment. This result, published inEmerging Microbes and
Infections -- Nature, was made possible
through long-term time series data.
In light of the
rainfall conditions in recent years, the researchers fear a potential new
outbreak of Buruli ulcer in the region. Beyond an improvement in forecasting
the risk of an epidemic, this study highlights the need to consider a set of
parameters and their interactions. Contrary to the accepted idea, less rainfall
does not mean a certain decrease in the prevalence of infectious diseases, as
shown by this example. Similarly, the expected warming of the atmosphere could
provide temperature conditions unsuitable to the development cycle of some
pathogenic agents, such as for malaria in Africa.
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