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

Medical Microbiology Section
Elimination of the Vector Simulium neavei from
an Onchocerciasis Focus in Western Uganda
Zusammenfassung
Überträgerbekämpfung war für lange Zeit die einzige
Möglichkeit, die Onchocerciasis wirksam zu
bekämpfen. Inzwischen wurde sie in den meisten
Onchocerciasis-Herden durch die einmal jährliche
Behandlung der Bevölkerung mit dem Medikament
Ivermectin abgelöst. Überträgerbekämpfung bleibt
aber dann eine kostengünstige Alternative, wenn in
kleineren, isolierten Herden die Aussicht besteht,
den Überträger auszurotten. Versucht wurde dies im
Itwara Onchocercias-Herd in Westuganda (Fig. 1).
Überträger ist dort die Kriebelmücke Simulium neavei,
deren Larven sich ausschließlich auf Süßwasserkrabben
entwickeln und nur gefunden werden können,
wenn man Krabben fängt und diese untersucht
(Abb. 2). Ab 1995 wurde damit begonnen, die Larven
des Überträgers durch Einleiten des Insektizids
Temephos in die Brutflüsse zu bekämpfen. Schon
Ende 1996 war der gesamte westliche Teil des Gebietes
überträgerfrei (Abb. 3), sodass dort die Bekämpfungsmaßnahmen
eingestellt werden konnten.
Im östlichen Teil des Projektgebietes wurde das Projekt
ab 1997 mehrfach durch Rebellenaktivitäten behindert.
Eine gründliche Nachuntersuchung in 2003
ergab, dass die Übertragung im gesamten Gebiet
unterbrochen war, aber noch vereinzelt Larven auf
Krabben gefunden wurden. Anfang 2004 wurden
nochmals 3814 Süßwasserkrabben von 40 Plätzen
untersucht. Der Überträger Simulium neavei wurde
nicht mehr gesehen.
Introduction
Vector control was for many years the only means to
control onchocerciasis and has been successfully employed
in the savanna areas of West Africa by the WHO
Onchocerciasis Control Programme (1974 to 2002).
Meanwhile vector control has been replaced in most of
the onchocerciasis foci by the community directed
treatment with ivermectin (CDTI), which became available
at the end of the 1980ties. However, vector control
remains a valid option in small and isolated foci, where
a realistic chance exists that the vector can be eliminated
in a limited period of time. Such situations exist in
Uganda, where an estimated 1.2 million people are infected
in a chain of isolated foci, mostly near the western
boundaries of the country (Fig. 1).
The main vector in Uganda is the blackfly Simulium
neavei, the larvae of which develop attached to river
crabs in an obligate phoretic association (Fig. 2). They
can only be found by trapping and examining crabs.
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Project Description and Results
The vector elimination project is being carried out in the
Itwara Onchocerciasis Focus in western Uganda, which
covers an area of about 600 km 2 (Fig. 3). Onchocerciasis
was hyperendemic in the area with a population
at risk of about 70000. Since the area is economically
important (large tea plantations) control of onchocerciasis
was always seen as an important task. Community
based treatments with ivermectin were inaugurated by
GTZ in 1991. Entomological studies were begun at the
same time to investigate whether treatments with ivermectin
alone would be able to reduce or even interrupt
the transmission in the area. Vector flies were caught
by vector collectors at four catching sites and dissected
for infections with stages of the parasite Onchocerca
volvulus. Infection rates of the fly population were found
to be very high (>40%). After four years the infection
rates had decreased, but the transmission remained intense.
Plans were then made to enhance the effect of
the ivermectin by vector control. The vector S. neavei
was found to have its breeding sites in three river systems,
in the Sogohi-Wamise in the Iwara Forest Reserve
to the west and further east in the Siisa and Aswa
Rivers. After preliminary trials carried out to test the efficiency
of the larvicide temephos (Abate®) against the
larvae of S. neavei a project to eliminate the vector from
the whole focus by treating all breeding sites was begun
in 1995. At the end of 1996 S. neavei had disappeared
from the Sogohi-Wamise river system and vector
control could be suspended. This sub-focus is now
vector free since more than 7 years. Activities on the Siisa
and Aswa rivers were temporarily hindered by inse-
Figure 1: Onchocerciasis foci in Uganda, where Simulium
neavei is the vector, and location of the Itwara Onchocerciasis
Focus (arrow).