Malaria is an infectious disease caused by Plasmodium blood parasites
which live inside the human host and are spread by Anopheles mosquitoes.
Every year an estimated 225 million new cases and near 800,000 malaria
deaths are reported. Control of the disease is a formidable task involving
all three living organisms (parasite, vector and host) and their environment.
Human hosts are mobile and play an important role in the spread of the
disease. Anopheles mosquitoes, also mobile, have a tendency to adapt
to new environments and are able to counter-act control measures, for
instance by developing insecticide resistance. Plasmodium parasites,
highly adaptive, develop drug resistance, thus rendering existing malaria
treatments useless. Finding new methods and strategies to control and
eliminate malaria is a continuous struggle. Nowadays malaria control
strategies contain a complex of measures complementing one another.
New tools are being developed.
In 2005 a new 5-year malaria program was initiated in Suriname which
combined a number of old, improved and new malaria control measures and
strategies. This thesis describes the effect of these measures and strategies
on malaria incidence and the Anopheles darlingi populations in Suriname
and also evaluates some mosquito monitoring tools.
Suriname has been fighting malaria since the early 1900s. As in many
other countries in Latin America, control efforts were successful in the
1950s and 1960s and Suriname succeeded in getting the coastal area free
of malaria. In the interior, transmission continued and malaria incidence
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increased. An important role in malaria transmission is played by the
mosquito Anopheles darlingi, the primary vector in Suriname. This vector
species is important throughout its distribution area in South America. It has
a preference for human blood and is highly effective in transmitting malaria
even when population densities are low. It combines these capacities with
diverse behavioral characteristics and an ability to adapt to new situations,
which make it a difficult vector to control.
The density of host-seeking Anopheles mosquitoes is an important
factor in determining malaria transmission risk. This density is traditionally
assessed by collecting and counting the number of mosquitoes which land
on a human collector in order to bite; the human landing collections. Most
often the landing mosquitoes are collected before they bite, but obviously
this method may hold a risk for the collectors when used in a malaria
endemic area. Besides, it is a very labor-intensive and expensive method.
Alternative tools are needed. The ability of the CDC Miniature light trap,
the BG Sentinel™ trap and the Mosquito Magnet® Liberty Plus, with carbon
dioxide or a protected human as bait, to act as an alternative to human
landing collections in defining mosquito biting pressure was tested both
for An. darlingi and Anopheles aquasalis. Even though in particular the BG
Sentinel and the Mosquito Magnet Liberty Plus showed potential, none of
the trapping methods proved as effective as the human landing collections.
As it turns out carbon dioxide may not be a sufficient stand-alone bait
for these Anopheles species. Alternative baits, especially human-derived
stimuli, may improve the results, enabling a more cost-effective mosquito
monitoring and surveillance tool, with less risks. The BG Sentinel trap, baited
with CO₂, was very efficient in collecting Culex mosquitoes.
The most important changes in the malaria control strategy in Suriname
as of 2005 were 1) the introduction of artemisinin-based combination
therapy as a new first line treatment for malaria caused by Plasmodium
falciparum, just prior to the onset of the new malaria program, and 2) the
mass-distribution of free long-lasting insecticide-treated nets (LLINs) to the
population at risk. Other measures included Indoor Residual Spraying, Active
Case Detection and training of on-site Malaria Service Deliverers in remote
areas. The effect of the control program on the mosquito populations was
assessed in a 4-year longitudinal vector study in three sentinel sites, using
human landing collections. Anopheles darlingi populations collapsed shortly
after onset of the malaria control program and did not recover during the
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following years. The possible impact of the LLINs on the mosquito population
is discussed. Our limited knowledge about the reasons for the vector
populations’ collapse and about An. darlingi ecology in general, prevent us
from making predictions about future population dynamics. An. darlingi
population densities remain low in the sentinel sites but a density increase
could be triggered at any time. A continued monitoring of the vector and
further studies with regard to its feeding behavior and ecology in general
will therefore be necessary.
The new malaria control strategy led to a significant decrease of malaria
in the villages in the interior of the country, enabling Suriname to reach the
Millennium Development Goal for malaria in 2007. The malaria situation
in Suriname changed to a state where the disease is almost completely
controlled in the stabile populations of the villages in the interior where
malaria incidence is down to near elimination levels. This is a significant
success, but can Suriname hold on to it? Transmission still occurs in the
mobile human populations in the forest, especially among the gold miners.
Additionally, the border region with French Guiana is vulnerable due to
cross-border movement of the people. The new challenge for Suriname is
to further control and possibly (locally) eliminate malaria by establishing
an integrated malaria control strategy with a strong malaria surveillance
system and prompt interventions in areas of renewed outbreaks. This will
require sufficient funds, dedication and regional collaboration.