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

Tropical Medicine Section
Does irrigated urban agriculture influence the transmission
of malaria in the city of Kumasi, Ghana?
Zusammenfassung
Urbane Landwirtschaft gewinnt in Großstädten Afrikas
eine zunehmend größere Bedeutung für die Versorgung
der Bevölkerung mit frischem Gemüse und
ist außerdem eine wichtige Erwerbsgrundlage. Für
die Millionenstadt Kumasi in Ghana wurde geschätzt,
dass etwa 90% des Bedarfs an Salat, Kohl
und Zwiebeln im Stadtgebiet selbst produziert werden.
Da eine wirtschaftliche und ganzjährige Produktion
nur möglich ist, wenn die Anbauflächen bewässert
werden (Abb. 1), besteht die Gefahr, dass Anopheles-Brutplätze
entstehen und sich das Malariarisiko
im Stadtgebiet erhöht. Dies wurde durch
Vergleichsuntersuchungen bestätigt, die zeigten,
dass Mückendichten und Malariaübertragung in
städtischen Gebieten mit Landwirtschaft deutlich
höher sind als in Gebieten ohne Landwirtschaft. Ein
ähnliches Bild ergab sich, wenn die Bevölkerung in
den Untersuchungsgebieten über die Häufigkeit von
Malariaanfällen und Anzahl von Krankheitstagen befragt
wurde. Beide Werte waren in urbanen Gebieten
mit Landwirtschaft deutlich erhöht.
Introduction
In fast growing cities in countries of Sub Saharan Africa
urban agriculture increases food supply and at the
same time contributes to improved nutrition, employment
and poverty alleviation. For the city of Kumasi,
Ghana, it is estimated that 90% of all lettuce, cabbage
and spring onions consumed in Kumasi are produced
in the city itself with the rest coming from peri-urban
and rural areas. In order to sustain the production of vegetables
around the year, these urban farms are irrigated,
making use of any water source available (Fig. 1).
Many urban vegetable farmers occupy lowlands or inland
valleys and dig shallow wells or construct conduits
to divert water from small streams onto their farms to
provide the needed water. To verify the possible impact
of irrigated urban agriculture on malaria transmission in
cities, we studied entomological parameters, self-reported
malaria episodes, and household-level data in the
city of Kumasi, Ghana.
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Figure 1: Irrigated vegetable farm in Kumasi. A dugout well connects
to a bed and furrow system providing breeding sites for
the malaria vector Anopheles gambiae s.s.
Project Description and Results
A comparison was made of the occurrence of Anopheles
breeding sites, numbers of mosquitoes caught during
night catches and infection rates of Anopheles with
Plasmodium in inner-city locations without irrigated
agriculture, inner-city locations with irrigated urban vegetable
production, and peri-urban locations with rainfed
agriculture (Fig. 2). In the rainy as well as in dry season
larvae of Anopheles spp. were found in the irrigation
systems of the urban farms. During night catches
in 2002 about 17000 mosquitoes were caught of which
2295 (13.5%) were anophelines. As expected, significantly
more mosquitoes, anophelines as well as culicines,
were caught in the rainy season than in the dry
season in all areas. Anopheles densities were significantly
higher in peri-urban and urban agricultural locations
compared to non-agricultural urban locations. Polymerase
chain reaction (PCR) analysis of Anopheles
gambiae sensu lato revealed that all specimens processed
were Anopheles gambiae sensu stricto. The results
of the entomological studies were in agreement with
the outcome of household interviews. Significantly
more episodes of malaria and subsequent days lost
due to illness were reported in peri-urban and urban
agricultural locations than in non-agricultural urban locations.
The increased malaria risk in urban agricultural
areas would have to be weighed against the benefits
that inner-city vegetable production brings to the households
and the local economy. The better economic situation
created through urban agriculture results in better
access to medical care and a better protection
against mosquito bites.