Abstract
BackgroundIn Gabon, vector transmission has been poorly studied. Since the implementation of the Roll Back malaria recommendations, clinical studies have shown a decline in the burden of malaria in Libreville, the capital city of Gabon. To better understand the transmission dynamic in Libreville, an entomological survey was conducted in five districts of the city.MethodsMosquitoes were sampled by human landing collection during 1 year in five districts of Libreville: Alibandeng, Beauséjour, Camp des Boys and Sotega. Mosquitoes were identified morphologically and by molecular methods. The Plasmodium falciparum circumsporozoïte indices were measured by ELISA, and the entomological inoculation rates (EIR) were calculated for all areas. Molecular assessments of pyrethroid knock down resistance (kdr) and of insensitive acetylcholinesterase resistance were conducted.ResultsA total of 57,531 mosquitoes were caught during 341 person-nights (161 person-nights indoor and 180 person-nights outdoor) among which, 4,223 were Anopheles gambiae s.l. The average Human Biting Rate fell from 15.5 bites per person during the rainy season to 4.7 during the dry season. The An. gambiae complex population was composed of An. gambiae s.s molecular form S (99.5%), Anopheles melas (0.3%) and An. gambiae s.s. form M (0.2%). Thirty-three out of 4,223 An. gambiae s.l. were found to be infected by P. falciparum (CSP index = 0.78%). The annual EIR was estimated at 33.9 infected bites per person per year ranging from 13 in Alibandeng to 88 in Sotega. No insensitive AChE mutation was identified but both kdr-w and kdr-e mutations were present in An. gambiae molecular form S with a higher frequency of the kdr-w allele (76%) than the kdr-e allele (23.5%).ConclusionMalaria transmission in Libreville occurred mainly during the rainy season but also during the dry season in the five districts. Transmission level is high and seems to be very heterogeneous in the town. Interestingly, the highest EIR was recorded in the most central and urbanized quarter and the lowest in a peripheral area. The decrease of transmission usually seen from peri-urban areas to urban centers is probably more dependent of the socio-economic level of a quarter than of its location in the city. Urban malaria control programmes need to consider the socio economic level of an area rather than the location in the city in order to determine the areas most favourable to malaria transmission.
Highlights
In Gabon, vector transmission has been poorly studied
A preliminary study conducted between December 2006 and April 2007, during the rainy season in the French military base in Libreville (Camp de Gaulle), revealed the presence of Anopheles gambiae s.l. and showed a low transmission of four Plasmodium falciparum-infected bites per person per year [22]
The average biting rate for An. gambiae s.l. was 15.5 bites per person per night, with a peak of 40 bites per person per night; this fell to 4.7 bites per person per night during the dry season
Summary
In Gabon, vector transmission has been poorly studied. Since the implementation of the Roll Back malaria recommendations, clinical studies have shown a decline in the burden of malaria in Libreville, the capital city of Gabon. Gabon has a population, estimated at 1,534,381 inhabitants according to the Constitutional Court [16] Despite it has one of the lowest population densities of any country in Africa, it is not spared by the demographic change that is affecting the majority of the continent. A preliminary study conducted between December 2006 and April 2007, during the rainy season in the French military base in Libreville (Camp de Gaulle), revealed the presence of Anopheles gambiae s.l. and showed a low transmission of four Plasmodium falciparum-infected bites per person per year [22]. The military authorities have implemented an extensive anti-larval and anti-imago mosquito control in the camp, so the transmission level was relatively very low This is probably not the reflect of the situation in the surroundings neighborhoods as it has been already shown for the non-anopheline mosquito vectors during the Chikungunya outbreak in 2007, when their density was 12 times higher outside the camp [24]. Reliable entomological data on the levels of transmission of malaria in different districts in Libreville is scant, despite the fact that it contains a third of the country’s population
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