Discovery of the hitherto-unknown variety, provisionally dubbed Oveng Form after the village where it was found, is likely to make the fight against the malaria in Cameroon even more difficult, researchers say, although more research is needed.

It joins four other species already known in the central African country, all of them resistant to common anti-malaria drugs.

Malaria is Africa's biggest killer alongside HIV/AIDS.

A team of French and Cameroonian scientists made the discovery after collecting samples of Anopheles mosquitoes from five localities including Oveng village - which lies between two rivers near the border with Gabon and Equatorial Guinea.

"It is then that we discovered that the samples from Oveng village were different from existing varieties in terms of morphology and behaviour," Parfait Herman Awono-Ambene, one of the scientists who carried out the research, told Reuters on Friday.

Mosquitoes from the Anopheles group transmit malaria - which kills roughly 3000 people every day - to humans along rivers in Africa. The results of the study were published in the July issue of the Journal of Medical Entomology.

"The interesting thing about Oveng Form is that it is hardly found inside houses though it bites human beings just like others and contains the malaria-carrying agent Plasmodium falciparum," Awono-Ambene said, referring to the most life-threatening form of the disease.

He said the mosquito found in Oveng becomes very active at dusk, feeding on people who live near or along the banks of the two rivers.

More research

More research will need to be carried out to determine whether this variety is only present in Oveng village, but Awono-Ambene said the mosquito was also likely to exist in neighbouring Gabon and Equatorial Guinea.

Malaria costs Africa about $12bn a year in lost income.

In Cameroon, it represents 35% to 40% of deaths in hospitals and is responsible for 40% of deaths among children aged between zero and five.

The country's public health authorities are promoting the use of impregnated nets to fight the disease, but adequate nets are not always easily available and at 3500 CFA francs ($6.50) they are often too expensive for the average household.