Malaria affects around 600 million people in the world and leads to an annual death toll of over 2 million. It is the world’s most widespread parasitic disease. It is caused by Plasmodium falciparum, a pathogen transmitted to humans by a mosquito.
In Africa, where malaria is endemic, mosquitoes of the Anopheles genus are the only vectors of the disease. The many studies which have been devoted to them have led to the characterization of different species and the identification, among these, of vector species. To date, four groups of vectors of the genus Anopheles have been recorded on the African continent: Anopheles gambiae, A. funestus, A. nili and A. moucheti, each comprising a set of species, morphologically very similar although genetically different.
As part of the PAL+ programme, initiated by the Ministry of Research in 1999, IRD and OCEAC (Organisation de Coordination pour la lutte contre les endémies en Afrique Centrale) scientists in southern Cameroon studied the morphology and genetics of mosquitoes of this genus. They focused especially on the Anopheles nili group, with a geographical distribution that mainly covers Central Africa.
Investigations on larvae and adults collected along the banks of the River Ntem, in the South of Cameroon, found evidence of morphological variations between specimens, making it difficult to classify them within one of the three known species of the A. nili group. This observation suggests the existence of a new variant called "Oveng form", from the name of the village where it was collected. Molecular biology techniques provided genetic confirmation of the differences observed and raised the "Oveng form" to the rank of species, with the name A. ovengensis. The discovery of the parasite P. falciparum in this new species shows that it is a new vector of malaria. A more detailed study under way should shed more light on its role in transmission of the disease.
This identification method, which combines taxonomy and genetics, was also applied, by the same team, to the study of another group of the Anopheles genus, A. funestus, in Cameroon. The researchers have thus found evidence of a new species, close to A. rivulorum, which is one of the nine already described within this group. As no presence of the parasite P. falciparum was detected, this new species, termed A. rivulorum-like, is not considered to be a vector. Research is currently continuing in order to provide more detailed information on the biology and geographical distribution of this new species.
These results emphasize the important place of taxonomy for studying malaria vectors. The knowledge acquired on these vectors will help understand better the epidemiology of the disease. The characterization and geographical distribution of each mosquito vector, the latter’s infestation rate and the mechanisms it deploys in transmission of the parasite to humans lead to improvements in anti-vector campaigns and hence to more effective prevention and control strategies. Similar studies are already under way in other regions of Africa with the long-term aim of establishing a comprehensive database that will hold information on the Anopheles vectors of malaria.