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Biologic
characteristics and behavioral traits can influence an individual's risk
of developing malaria and, on a larger scale, the intensity of transmission
in a population.
Genetic
Factors
Biologic
characteristics present from birth can protect against certain types
of malaria. Two genetic factors, both associated with human red blood
cells, have been shown to be epidemiologically important. Persons who
have the sickle cell trait (heterozygotes for the abnormal hemoglobin
gene HbS) are relatively protected against P. falciparum malaria
and thus enjoy a biologic advantage. Because P. falciparum malaria
has been a leading cause of death in Africa since remote times, the sickle
cell trait is now more frequently found in Africa and in persons of African
ancestry than in other
population groups.
Persons
who are negative for the Duffy blood group have red blood cells that are
resistant to infection by P. vivax. Since the majority of Africans
are Duffy negative, P. vivax is rare in Africa south of the Sahara,
especially West Africa. In that area, the niche of P. vivax has
been taken over by P. ovale, a very similar parasite that does
infect Duffy-negative persons.
Other
genetic factors related to red blood cells also influence malaria, but
to a lesser extent. Various genetic determinants (such as the "HLA
complex", which plays a role in control of immune responses) may
equally influence an individual's risk of developing severe malaria.
Acquired
Immunity
Acquired
immunity greatly influences how malaria affects an individual and a community.
After repeated attacks of malaria a person develops a partially protective
immunity. Such "semi-immune" persons often can still be infected
by malaria parasites but do not develop severe disease, and, in fact,
frequently lack any typical malaria symptoms.
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Anemia in young children in Asembo Bay, a highly endemic
area in western Kenya. Anemia occurs most between the ages
of 6 and 24 months. After 24 months, it decreases because the
children have built up their acquired immunity against malaria
(and its consequence, anemia). |
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In
areas with high P. falciparum transmission (most of Africa south
of the Sahara), newborns will be protected during the first few months
of life presumably by maternal antibodies transferred to them through
the placenta. As these antibodies decrease with time, these young children
become vulnerable to disease and death by malaria. If they survive to
an older age (2-5 years) they will have reached a protective semi-immune
status. Thus in high transmission areas, young children are a major risk
group and are targeted preferentially by malaria control interventions.
In
areas with lower transmission (such as Asia and Latin America), infections
are less frequent and a larger proportion of the older children and adults
have no protective immunity. In such areas, malaria disease can be found
in all age groups, and epidemics can occur.
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Mother
and her newborn in Jabalpur Hospital, State of Madhya Pradesh, India.
The mother had malaria, with infection of the placenta.
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Pregnancy
and Malaria
Pregnancy
decreases immunity against many infectious diseases. Women who have developed
protective immunity against P. falciparum tend to lose this protection
when they become pregnant (especially during the first and second pregnancies).
Malaria during pregnancy is harmful not only to the mothers but also to
the unborn children. The latter are at greater risk of being delivered
prematurely or with low birth weight, with consequently decreased chances
of survival during the early months of life. For this reason pregnant
women are also targeted (in addition to young children) for protection
by malaria control programs in endemic countries.
See
also: Malaria During Pregnancy
Behavioral
Factors
Human
behavior, often dictated by social and economic reasons, can influence
the risk of malaria for individuals and communities. For example:
- Poor
rural populations often cannot afford the housing and bed nets that
would protect them from exposure to mosquitoes
- These
persons often lack the knowledge to recognize malaria and to treat it
promptly and correctly. Often, cultural beliefs result in use of traditional,
ineffective methods of treatment
- Even
when people know what to do, they often cannot do it because of financial
reasons (no money to buy drugs) or physical impossibility (the nearest
health post is a 3-day walk)
- Human
activities can create breeding sites for larvae (standing water in irrigation
ditches, burrow pits)
- Agricultural
work such as harvesting (also influenced by climate) may force increased
nighttime exposure to mosquito bites
- Raising
domestic animals near the household may provide alternate sources of
blood meals for Anopheles mosquitoes and thus decrease human
exposure
- War,
migrations (voluntary or forced) and tourism may expose non-immune individuals
to an environment with high malaria transmission.
Human
behavior in endemic countries also determines in part how successful
malaria control activities will be in their efforts to decrease transmission.
The governments of malaria-endemic countries often lack financial resources.
As a consequence, health workers in the public sector are often underpaid
and overworked. They lack equipment, drugs, training, and supervision.
The local populations are aware of such situations when they occur, and
cease relying on the public sector health facilities. Conversely, the
private sector suffers from its own problems. Regulatory measures often
do not exist or are not enforced. This encourages private consultations
by unlicensed, costly health providers, and the anarchic prescription
and sale of drugs (some of which are counterfeit products). Correcting
this situation is a tremendous challenge, that must be addressed if malaria
control is to be successful.
Page last modified : April 23, 2004
Content source: Division of Parasitic Diseases
National Center for Zoonotic, Vector-Borne, and Enteric Diseases (ZVED)
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