A similar situation was seen with anemia, an important
manifestation of malaria infection in this area of Kenya. Parasitemia
had the greatest impact on the hematologic status
of children less than six months of age. Between six months
and four years of age, the increased risk of anemia due to
either high or low density infection was uniform. High density
infections still were associated with increased risk of
anemia for children 5–9 years old, but after 10 years, there
was no significant increase in risk given either level of parasitemia.
As with fever, there appears to be an increasing
ability to limit the hematologic effects of malaria infection
that is developed over time and, probably, cumulative exposure.
The youngest children lack the ability to maintain
hematologic status in the face of any density malaria infection.
With age, they appear to develop the ability to limit the
effects of low density infections first, then ultimately to limit
the effects of high density infections as well. It is important
to note, however, that parasitemia continues to have hematologic
consequences long after it loses its association with
fever.
A number of strategies have been investigated or are in
development to reduce the burden of malaria disease, including
improved case management of both malaria illness
and anemia, use of insecticide-impregnated bed nets, and
vaccines.13–15 In this area and probably most of sub-Saharan
Africa, disease control strategies must be designed to be usable
in the groups at highest risk, specifically young children.
In this western Kenyan community, the ages exhibiting
the greatest burden of malaria illness were, in general, children
less than five years of age and specifically those less
than two years of age.
These results may also help to explain the success or failure
of specific interventions. For example, the reduction in
mortality attributed to insecticide-impregnated bed net use
may be directly associated with the reduction of the incidence
of higher density parasitemia.16–18 Even in areas where
drug-resistant malaria occurs, if a drug still has the ability
to reduce parasite loads there may be decreases in illness
and/ or mortality.19
A note of caution, however, is required; in this population,
there was a significant risk of anemia even among children
with low density infections. Because low density infections
were not significantly associated with fever among children
more than two years of age, inadequate treatment (or potentially
any other intervention, such as vaccines), which limits
parasite densities without eliminating them, may actually increase
the proportion of children with clinically silent malaria
infections who may go on to develop severe anemia. It
is clearly important, therefore, to consider the potential for
deleterious effects of interventions as well as the benefits.
It is clear that effective malaria prevention strategies are
desperately needed in Africa. With a better understanding of
the dynamics between parasitemia, age, and clinical expression
of malarial disease, potential pitfalls of new strategies
and technologies can be anticipated and modifications in design
or implementation can be made. In this way, strategies
that make sense given what is known about the clinical epidemiology
of malaria can be identified and moved forward
efficiently and begin to decrease the burden of malaria in
Africa.