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ANNEX 4. Antimalarials and malaria transmission

ANNEX 4
antimalarials and malaria transmission

a4.1 principles of malaria transmission

Malaria is spread among people by a mosquito belonging to the genus Anopheles. The
female mosquito is infected by gametocytes, the sexual stages of the parasite when it
takes a blood meal from an infected person. Gametocytes undergo further development
in the insect for a period of 6–12 days, after which transformed parasites as sporozoites
can infect a human through the bite of the infected mosquito.
A4
The intensity of malaria transmission in an area is the rate at which people are inoculated
with malaria parasites by infected mosquitoes. It is expressed as the annual entomological
inoculation rate or EIR, which is the number of infectious mosquito bites received by an
individual in one year. The EIR determines to a large extent the epidemiology of malaria
and the pattern of clinical disease in an area. The high end of the malaria transmission
range is found in a few parts of tropical Africa, where EIRs of 500–1000 can be reached
(1). At the low end of the range are EIRs of 0.01 or below, as found in the temperate
climates of the Caucasus and Central Asia where malaria transmission is only barely
sustained. Between these extremes are situations of unstable seasonal malaria, such as
in much of Asia and Latin America, where EIRs lie below 10, and often around 1–2, and
situations of stable but seasonal malaria, as in much of West Africa, where the EIR is in
the range 10–100.
The proportion of infected mosquitoes in a locality is related to the number of infected
and infectious humans in the area; therefore, lowering the infectivity of infected persons
to mosquito vectors will contribute to reducing malaria transmission and to eventually
reducing the incidence and prevalence of the disease. However, the relationship between
EIR and the prevalence of malaria is complex, and it is affected by the extent of immunity
to malaria, the pattern of its acquisition and loss, and to whether or not there is effective
drug treatment in the area. The hypothetical relationship represented in figure A4.1
assumes no drug treatment. In areas of low transmission where EIRs are below 1 or 2, a
reduction in the inoculation rate will result in an almost proportionate reduction in the
prevalence (and incidence rate) of malaria. In EIRs in excess of 10, where there is great
redundancy in the infectious reservoir, larger reductions in transmission are needed to
make a significant impact on malaria prevalence. The experience with major interventions,
such as the use of insecticide-treated nets and artemisinin-based combination therapies,
suggests, however, that effective transmission-reducing interventions will be beneficial
with respect to mortality and even morbidity in most situations (2,3).
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