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[08/25/2004]
 Epidemiological Facies |
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 On the eve of the year 2000, malaria remained
the leading world endemic disease. In Sub-Saharan Africa, it is the
most serious public health problem: almost the whole population,
around 550 million people, is to be found in the malaria-infested
area; nearly 75 % of the population lives in strong endemic areas
and around 18 % is under threat of a malaria epidemic. The WHO
estimates that there are between 270 and 480 million new clinical
cases each year and between 1.5 and 2.7 million deaths. But malaria
is not a homogeneous entity: there exist various epidemiological
facies determined by the interrelations between the vectors, the
parasites, the human hosts and the biotopes (1, 2).
Pr Dominique Baudon, professor in public health, I.M.T.S.S.A.,
BP 46, Marseille, France.
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Diversity of the vectors
In sub-Saharan Africa the transmission is essentially due
to Anopheles gambiae, Anopheles
funestus and Anopheles arabiensis.
These are excellent vectors with a large life-span and a clear
trophic preference for humans: this explains the quasi-continuity
of malaria transmission in this part of the African continent.
Depending on the level of transmission, each individual will
receive, during his lifetime, from 1 to 1,000 infected bites per
annum (2). | Diversity of parasites
Plasmodium falciparum, the species
which kills and which can resist the antimalarial drugs is
predominant. It affects in 90 % to 95 % of the malaria-infected
subjects. Plasmodium
malariae, Plasmodium vivax
and Plasmodium ovale are much more
rarely present and do not seem to be posing a major public health
problem. | The human host
One of the consequences of the man-parasite-vector relationship
is the acquisition by the native populations of a special immunity,
known as "premunition". It requires several years for it to become
effective and is maintained by repeated anopheline infections. It
is acquired at the price of a high mortality rate and appears as
rapidly as the transmission is large and permanent. It avoids the
patient succumbing to severe forms of malaria and therefore to
malarial mortality.
With man, it is necessary to differentiate between "malarial
infection" and "malarial disease". The "malarial infection" is
translated by the asymptomatic transport of parasites: thus, in an
area of intense and permanent transmission, almost all individuals
are carriers of Plasmodia. The very
fact that they carry parasites does not automatically mean that
they are sick. In "malarial" disease, there is a clinical
expression of transport: the most classic form is malaria fever,
but there is a great diversity of clinical pictures from the simple
malarial fever to the fatal pernicious malaria (3, 4). | The epidemiological facies of malaria
1. Degrees of malarial stability The intensity of malarial transmission in Man (number of
infective bites received) and the life cycle of the anopheline
population condition the degree of stability (fixing and rooting)
of malaria in Man, with its consequences. Three areas of stability
can be distinguished: Area of stable malaria: the transmission is intense and
permanent. It corresponds to almost the whole of the equatorial
zones where the rainfall is high and is almost
permanent.
Area of unstable malaria: the transmission is weak and episodic.
Area of intermediate stability: the transmission has experienced a
seasonal increase (2). 2. Diversity of the epidemiological
facies The concept of epidemiological facies was established for West
Africa by P. Carnevale et al. (5), then generalized to the whole Afro-tropical
region by J. Mouchet et al. (2).
Into this notion of facies are integrated the climatic and
phyto-geographical features of the large Sub-Saharan regions of
Africa (forest, savanna, sahel, steppes, plateaux and mountainous
areas). The zones of stability defined above include various
facies. | In the stable malaria areas :
equatorial facies: forests and post-forestal savannas in central
Africa. The anopheline transmission is intense and permanent,
reaching as many as 1,000 infective bites per capita (female
anopheles carriers of sporozoites) and per annum. This is what
provides precocious acquisition of premunition, towards the age of
5 years old. In children, 30 % to 50 % of fevers are attributed to
malaria. The morbidity is spread throughout the year. The serious
forms of malaria, in particular pernicious malaria, are frequent
among young children, but rare among adults, the latter usually
having premunition.
Tropical facies: damp savannas of West and East Africa. The
transmission is subject to long seasonal outbreaks (6 to 8 months),
with 100 to 400 infective bites per capita and per year annum.
Premunition appears later, towards the age of 10. The death rate is
higher in the rainy season (around 80 % of the cases of fever among
children). The serious forms of malaria are to be found until a
more advanced age. | | In the intermediate malaria areas :
sahelien facies: dry savannas. Transmission is by short seasonal
outbreaks (less than 6 months), with 2 to 20 infective bites per
capita and per annum. In the transmission season, nearly 70 % of
the fevers are of malarial origin. Premunition appears much later,
explaining the numerous cases of pernicious malaria in
adults. | | In the unstable malaria areas :
desert facies: steppes
austral facies: plateaux of southern Africa
mountain facies: areas situated above 1000 m in altitude. In these
3 facies, the transmission period is very short and there may be
years without transmission. There is no premunition acquisition.
Malaria expresses itself in the form of epidemics which appear in
the transmission period and may affect almost the whole
population.
In each of the facies described, transmission particularities may
be observed, variations which create real epidemiological hotspots.
For example: rice-growing areas, irrigation dams, coastal lake
areas with salt water, destruction of the "primary" forest creating
a savanna zone, ... |
3. "Urban malaria", a special case In Africa, malaria is an essentially rural endemic disease. No
specifically urban vector exists. In urban environments, the
transmission is overall much weaker than in rural areas: this
explains the lower level of immunity of urban populations. For a
number of years, urbanization has been accelerating: more and more
subjects will be born and will live permanently in towns where the
anopheline transmission is low even non-existent and thus will not
acquire immunity. They will essentially become infected while on
short stays in rural areas (marriages, funerals, etc.) and may
develop severe forms of malaria at any age. Thus, this acceleration
of urbanization in Africa will have two fundamentally antagonistic
effects: one favorable effect, for the years to come it is possible
to foresee a reduction in the malaria incidence rate, the
individuals who have a lower probability than today to become
infected; and a harmful effect, with an increase in the proportion
of severe forms of malaria linked to the absence of immunity. Which
confirms the remark that for Africa, "malaria in urban environments
is the malaria of tomorrow " (6). A catastrophic
hypothesis would be the selection of Anopheles mosquitoes able to
develop in places like those of the Culex (polluted water) with
intense transmission affecting non-immune populations.
| Adaptation of the antimalarial effort to the epidemiological facies
The main objective of the fight against malaria in Africa is to
reduce the morbidity and mortality rate of malaria in Sub-Saharan
Africa. The strategy is based on 3 major elements:
Treatment of the malaria disease cases: the precocious and
effective treatment of the cases represents the best prophylaxis of
malarial mortality.
Personal and collective protection: protection of pregnant women by
chemoprophylaxis; anti-vectoral efforts using materials impregnated
with residual insecticide (mosquito bed nets, screens, curtains,
...) ; domestic insecticide sprays.
Prevention and action against epidemics. The malaria situation is neither homogeneous nor uniform. The
strategies of the fight need to be adapted to each situation,
according to the diversity of the epidemiological facies, but also
taking into account cultural, socio-economic and operational
factors (7, 8, 9, 10, 11).
In the countries or regions with stable malaria, the strategy is
based essentially on the correct care facilities for the malaria
disease (confirmed malaria, unexplained fevers) and personal
protection measures.
In the countries or regions with unstable malaria, the epidemic
risk is great. Knowing the impact of epidemics on malarial
mortality, priority should be given to the anti-vectoral effort
through the use of domestic sprays. In the event of an epidemic,
other than anti-vectoral actions, care activities will be
reinforced. This adaptation of strategies is an indispensable condition for
the success of the actions against malaria. | Bibliographical references
1. BAUDON D. - Aspects épidémiologiques des paludismes en
Afrique sub-saharienne. Bull. Mem. Soc. Med. Paris 1987 ; IV :
3-5.
2. MOUCHET J., CARNEVALE P., COOSEMANS M. et
Coll. - Typologie du paludisme en Afrique. Cahiers Santé 1993 ; 3 :
220-238.
3. PALUDISME : Universités francophones,
UREF. ELLIPSES Ed, 1991, 240 p.
4. CHARMOT G., MOUCHET J.et Coll. -
Paludisme. Cahiers Santé 1999 ; 3 : 211-338.
5. CARNEVALE P., ROBERT V., MOLEZ J-F.,
BAUDON D. - Faciès épidémiologique des paludismes en Afrique
sub-saharienne. Études médicales 1984 ; 3 : 123-133.
6. BAUDON D., LOUIS F.J., MARTET G. - En
Afrique, le paludisme urbain est le paludisme de demain. Med. Trop.
1996 ; 56 : 323-325.
7. BAUDON D., CARNEVALE P., AMBROISE-THOMAS
P., ROUX J. - La lutte antipaludique en Afrique : de l'éradication
du paludisme au contrôle des paludismes. Rev. Epidemiol. Sante
Publ. 1987 ; 35 : 401-415.
8. CARNEVALE P., MOUCHET J. - Lutte
antivectorielle et lutte antipaludique. Med. Trop. 1990 ; 50 :
391-398.
9. OMS - Mise en œuvre de la stratégie
mondiale de lutte antipaludique. Rapport d'un groupe d'étude de
l'OMS sur la mise en œuvre du plan mondial d'action pour la lutte
contre le paludisme, 1993-2000. Série de rapports techniques 1993,
n°839, 67 p.
10. OMS - African initiative for malaria
control in the 21st century. WHO 1998, 18 p.
11. OMS - Rollback Malaria Project :
resources support network for prevention and controlof malaria
epidemics.Doc. OMS 1998, n°CDS/RBM/RSN/EPI/98 |
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