Research Group Heussler (Malaria I) - Bernhard-Nocht-Institut für ...

Parasitology Section
Simulium thyolense is the main vector of Onchocerca volvulus
in Malawi and southern Tanzania
Zusammenfassung
Die südlichsten Onchozerkose-Gebiete Afrikas liegen
in Süd-Tansania und Malawi und bilden scheinbar
isolierte Herde. Ein Vergleich dieser beiden Gebiete
hinsichtlich der Identität der potenziellen Überträgermücken
unter Anwendung cytotaxonomischer
und molekulargenetischer Methoden ergab, dass Simulium
thyolense die häufigste Art innerhalb der Onchozerkose-Gebiete
ist, während außerhalb der endemischen
Gebiete andere Arten häufiger waren.
Außerdem wurden alle am Menschen gefangenen
Mücken aus den verschiedenen Gebieten als S.
thyolense identifiziert, so dass diese Art vermutlich
alleiniger Überträger ist.
Summary
The southernmost foci of onchocerciasis in Africa are
found in southern Tanzania and Malawi and consist of
rather isolated areas. A comparison of these regions regarding
the identity of the potential blackfly vectors using
cytotaxonomic and molecular techniques revealed
that S. thyolense is the most abundant species within all
onchocerciasis foci, whilst outside the foci there were
other species prevailing. Furthermore, all biting female
flies from the different areas were identified as S. thyolense,
which suggests that this species is the only significant
vector.
Figure 1: Study area with foci of onchocerciasis.
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Introduction
Human onchocerciasis is an insect born disease. The
main African vectors are blackflies of the Simulium
damnosum complex. The East African endemic areas
of onchocerciasis are relatively small and isolated from
each other, and also differ in their ecology. These factors
favoured the evolution of a great biodiversity. The
S. damnosum complex particularly represents an example
for the differentiation of sibling species, which
hardly show morphological differences. One of the 50
or so known taxa is S. thyolense on which this study is
focussed. As in other medical important insects the taxonomic
knowledge is crucial for epidemiological considerations,
and vector control remains most effective
in terms of disease elimination. The southernmost focus
is located in Thyolo district/southern Malawi, with
extensions into the Mulanje Mts. and Mwanza district
(Fig. 1). In southern Tanzania the largest is the Ruvuma
focus, followed by the Tukuyu focus (Fig. 1). Concerning
the exact vector identity in these foci there only existed
incomplete or virtually no data so far but at least it
has already been suspected that Simulium thyolense
might play the major role as vector within the Thyolo
and Tukuyu foci.
Project Description and Results
The objective of this study was to assess the potential
isolation of some onchocerciasis foci and their vector
populations, respectively, by examining the distribution
of the vector and related cytoforms outside the foci to
look for possible re-invasion sources. In Tanzania this
was part of the ‘Tukuyu Focus Vector Elimination Project’.
The foci in southern Malawi were subject of independent
investigations. Altogether, during three surveys
in 2002/03 immature S. damnosum were collected
from their riverine breeding sites. Subsequently, they
were analysed by examining geographic variation in
their polytene chromosome inversions (cytotaxonomy),
morphology and DNA. In addition, biting female flies
were caught in the respective foci for comparison with
specimens obtained from the river samples. Cytotaxonomically,
the assumed vector S. thyolense can unequivocally
be identified by two diagnostic inversions
(Fig. 2 a+b). Within the onchocerciasis foci, this species
accounted for 80% of the analysed specimens, with almost
100 % in the Thyolo and Mulanje areas. Outside
the foci, the majority consisted of cytoforms other than
S. thyolense, which was almost or completely absent in
populations in central Malawi and the highlands north
of the Kipengere Range in Tanzania. A comparison between
the different populations of S. thyolense revealed
some marked chromosomal differences. First of all, a
distinct cytoform now called ‘Nyika’ form was found in
northern Malawi and at the border strip of Tanzania
(both outside any onchocerciasis areas). Furthermore,