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Maroua in northern Cameroon could soon be a mosquito-free zone. For
the past four weeks a 50-strong hit squad has been scouring the town in
search of cesspits and sewers where Culex quinque fasciatus lays its eggs.
Each potential pool of mosquito larvae has been sprayed with the team’s
secret weapon – a biological pesticide.

The WHO’s Tropical Disease Research programme backed this first large-scale
trial of the bacterium Bacillus sphaericus, whose spores contain a toxin
lethal to mosquito larvae.

The mobile anti-mosquito squad, led by French researchers, sprayed all
sources of stagnant water at the end of the dry season. They chose Maroua
because it is an ‘island’, separated from the nearest large town by more
than 200 kilometres of open country. Culex is essentially urban and cannot
survive in the savanna, so there is no pool of mosquitoes around the town.

‘By July, when people can be bitten over 300 times a night, we will
be able to assess the effectiveness of the pesticide,’ says the team’s leader,
Jean-Marc Hougard of ORSTOM, the French institute for scientific research
in development and cooperation. Already there are signs that the blitz may
be working. At one test site, where a typical sample yields around 90 mosquitoes,
the latest catch was a single elderly female.

Hougard does not expect to eradicate the mosquito completely. But if
the pesticide proves efficient, it will provide a cheap method of control.

Although Culex is not much more than a nuisance in Maroua, on the other
side of the continent in East Africa it transmits the worms that cause filariasis,
a debilitating parasitic disease. ‘If it works, we can repeat the experiment
where there is filariasis, in Dar es Salaam or other East African cities,’
says Hougard.

Unfortunately, the bacterium cannot be used against the Anopheles mosquitoes
that carry malaria, because they lay their eggs in transient pools and puddles
rather than permanent wet spots such as sewers.