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several autochthonous cases have been reported from malaria–free places. If
the predicted global climate change or other environmental modification would
cause a large increase in mosquito vectorial capacity, malaria re-emergence in
Europe could become possible. To assess how environmental driven factors
may be linked to the risk of re-introducing malaria in Portugal, one must start by
characterising the current status of its former vectors. By studying the
receptivity and infectivity of present-day mosquito populations, it will be possible
to identify factors that may trigger disease emergence and spreading, as well as
to provide entomological data to be used in the identification of environmental
induced changes of epidemiological significance. Aiming at contributing to these
goals, this study has focused on the following objectives: (i) to estimate
Anopheles atroparvus
Van Thiel, 1927 vectorial capacity towards malaria and
analyse other bioecological parameters with relevance to the introduction of the
disease; (ii) to determine
An. atroparvus
vector competence for tropical strains
of
Plasmodium falciparum
Welch, 1897
.
The region of Comporta presents a
unique setting to assess the vector capacity and competence of
An. atroparvus
from Portugal. It was a former malaria hyperendemic region, where
P.
falciparum
was the most prevalent malaria parasite. It is a semi-rural area with
vast numbers of mosquito breeding sites and a highly mobile human population
due mainly to tourism. It is also located fairly close to Lisbon which allows
frequent visits to the study area. Nine would be the maximum estimated number
of new daily inoculations that could occur if an infective human host would be
introduced in the area. This estimate was obtained for a sporogonic cycle of 11
days (compatible with
P. vivax
development under optimal conditions) and the
highest man biting rate obtained in this study (38 bites
per
person
pe
r day). This
value of C is similar to some obtained for other malaria vectors. However, due
to the overestimation of most of the computed variables, one can foresee that
the receptivity of the area to the re-emergence of the disease is very limited.
With the exception of August 2001, the threshold of C=1 was only surpassed
during winter/spring months, when parous rates were above 0.95 but
abundances were lowest.
Out of 2,207
An. atroparvus
that were sent to Nijmegen Medical Centre to be
artificially infected with the tropical strains of
P. falciparum
, more than 790
specimens took one or two infected blood meals.
Anopheles atroparvus
females
infection was successful in a single experiment. These specimens took two
infective feeds with a seven days interval. Blood fed females were kept always
at 26ºC with the exception of a 19 hours period that occurred two hours after
the second blood meal and during which mosquitoes were placed at 21ºC. Out
of the 37 mosquitoes that were dissected, five presented oocysts in their
midguts. Prevalence of infection was 13.5% and the mean number of oocysts
per
infected female was 14, ranging between 2 to 75 oocysts
per
infected
midgut. It was confirmed that
An. atroparvus
is, at the most, a low competent
vector regarding tropical strains of
P. falciparum
. Artificial infection experiments
were not carried out beyond the oocysts phase, thus no conclusion can be
drawn regarding sporozoite formation and invasion of salivary glands.
Nevertheless,
An. atroparvus
complete refractoriness to tropical
P. falciparum
strains seems less certain than at the beginning of this study.
This study has produced an update on the bionomics of
An. atroparvus
in
Portugal and, for the first time, a comprehensive assessment of its vectorial
capacity and competence for the transmission of human malaria parasites. It
was also attempted to determine if the biology and behaviour of this species
has suffered any major switches since the time malaria was an endemic
disease in Portugal. The results obtained in this study support the idea that the