Background
Urbanization is increasing in Africa resulting in a change in the epidemiology of malaria [
1]. One characteristic of African cities is the maintenance in urban areas of traditional rural practices, such as housing and food crops, that create favorable conditions for malaria transmission [
2‐
4]. Marked intra-city variations in the burden of malaria exist due to differences in urbanization level, in equipment, in housing and in socio-economic level between districts [
5]. Entomological inoculation rates can vary from 0 to 54 infective bites per man per year between Ouagadougou and Dar es Salaam and can double between the centre and periphery of a same city [
6]. Currently, 40 African cities have more than one million inhabitants and in 2003, 39% of Africans lived in cities [
7]. Keiser
et al. estimated urban malaria morbidity to be between 25 and 100 million cases, amounting to between six and 28% of the total annual incidence [
6]. According to the United Nations (UN) projections, by 2025, over 800 million people, about 54% of the continent's population, will live in urban areas (1). Thus, urban malaria has become an emerging public health problem in Africa and has become the subject of many studies to better understand its determinants, suggest preventive measures and appropriate control [
8‐
15].
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. Due to a rural exodus, the majority (about 80%) of the population is urban or semi-urban [
16]. Libreville alone has 579,577 inhabitants: 36.36% of the total population and 47.21% of the urban population [
16]. Libreville is flanked by the Atlantic Ocean to the west and is irrigated by numerous rivers to the east. In Gabon, the burden of malaria has often been determined from clinical and laboratory data, by morbidity and mortality measurements, particularly at the Libreville Hospital Centre and in the Medical Research Unit of the Albert Schweitzer Hospital in Lambaréné [
17,
18]. Vector transmission has been poorly studied [
19‐
21]. 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]. This same study confirmed the high prevalence of molecular markers of resistance to pyrethroids described by Pinto
et al. in 2006 and showed the presence of molecular markers of resistance to organophosphates and carbamates [
23]. 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. However, 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.
Malaria in Libreville is a major public health problem as in other Central African capitals. In the 2000s,
P. falciparum was responsible for 15% to 40% of medical care for fever in children under 11 years old [
25]. During the same period, the prevalence of infection with
P. falciparum and anemia were respectively 53.6% and 53% among pregnant women [
26]. In 2005, artemisinin-based combination therapy (ACT) was adopted for uncomplicated malaria, long-lasting nets impregnated with pyrethroids (deltamethrin) were distributed to high risk groups (pregnant women and children under 5 years of age) and a strategy of intermittent preventive treatment with sulphadoxine-pyrimethamine for pregnant women (IPTp-SP) was made available in all health centers in the country [
27]. Since then, clinical studies have shown a decline in the burden of malaria in Libreville both in pregnant women and in children [
25,
27]. Currently, data on parasite infection in febrile children aged 5 to 10 years old suggests a direct impact of this strategy on the level of exposure to malaria [
25]. To better understand the transmission dynamics of malaria in Libreville, an entomological survey was conducted from December 2008 to January 2010 in five districts.
Discussion
The nocturnal mosquito biting rate was intense essentially due to Culex quinquefasciatus in both areas of Libreville but varied according to the location in the city. The Cx. quinquefasciatus aggressiveness ranged from 73 bites per night in Beauséjour to 250 bites per night in Akébé-Poteau. The densities of Cx. quinquefasciatus were higher in the centre of the city in the most urbanized areas, suggesting difficulties in waste management, and were lower in the peripheral areas of Alibandeng and Camp des Boys. Stegomya spp. population densities were low during the survey, but night captures are not appropriate for such diurnal species. The abundance of Aedes aegypti and Aedes albopictus seemed to vary according to the districts signifying that the risk of chikungunya or dengue transmission is probably not homogenous throughout Libreville. In Akébé-Poteau, a more urbanized area with less vegetation, Ae. aegypti was more present than Ae. albopictus. In the others districts, where gardens and other green areas are more important the two species were caught with equal frequency.
Three members of the
An. gambiae complex were present in Libreville:
An. melas, An. gambiae s.s. molecular form M and
An. gambiae s.s. molecular form S. The presence of
An. melas and
An. gambiae s.s. molecular form M was recorded for the first time in Libreville. Only
An. gambiae s.s. molecular form S, that represented more than 99% of the anopheline population, was involved in malaria transmission, which is contrary to what is observed in Port Gentil, the second main city of the country, where
An. melas is an important malaria vector [
22]. The level of transmission varied between the districts and the annual EIR ranged from 13 to 87
P. falciparum infected bites per person per year. Differences between districts can be explained in part by the variations in
An. gambiae s.s. molecular form S biting rates for three of the districts however, in Alibandeng, where the annual aggressiveness was one of the highest, the lower level of transmission is possibly due to a low CSP index and to a higher exophagic behaviour of
An. gambiae s.l..
A meta-analysis of studies of malaria transmission in sub-Saharan Africa found a linear negative relationship between the level of malaria transmission and the level of urbanization: transmission decreased from rural to peri-urban areas and from peri-urban areas to urban centre [
34] and these findings have been confirmed in the field [
35]. In Libreville, the situation appears different as the highest EIR was in the most central and urbanized area and the lowest in a peripheral area. In urban settings, malaria risk heterogeneity is due to diversity in degrees and types of urbanization, density of human population, quality of water and waste management, vector control measures, household factors and access to health care [
7,
14,
34,
36,
37] or human migration patterns which could import parasites from rural areas [
38‐
40]. The occurrence of malaria in African cities has been linked to agricultural practices [
5,
41‐
45], distance from breeding sites [
46‐
50] and vegetation cover [
50]. In Libreville, some of these factors could explain the differences between the intermediate area (Beauséjour), the peripheral area (Alibandeng) and the central zone (Sotega) and the apparent inversion of the usual gradient of transmission from peri-urban areas to urban centre. Households with low socio-economic status and poor housing conditions have already been identified as risk factors for urban malaria. In the central area, districts like Sotega which have slum-like conditions characterized by a high density of population, a low socio-economic level and difficulties in waste management, as seen by the high biting rate of
Cx quinquefasciatus. Contrary to the usual scheme described in most of African cities, the peripheral and intermediate areas are more affluent, less densely populated and inhabitants have a higher socio economic status than those of the central area of Sotega. The gradient of transmission described from peri-urbans areas to urban centre is probably more dependent of the socio-economic status of the area than of the location within a city. Urban malaria control programmes thus need to consider living conditions in a given area rather than the location in a city to determine areas favourable to malaria transmission. In 2007, a preliminary study conducted in the French military camp showed a low transmission of malaria in Libreville [
22]. However, the authors identified two limitations: first, a single study in one area is not sufficient to assess the global level of transmission and second, that the vector control programme implemented in the camp all year around could have minimized the level of malaria transmission. This current study included the district Camp des Boys due to its proximity to the French military camp of Libreville, and found that the level of transmission was ten fold higher with a measured annual EIR of 39.2 infected bites per person per year vs. the estimated annual EIR of 3.7 infected bites per person per year in the French military camp. It is the first study showing the effectiveness of the vector control programme implemented against malaria vectors in the French camps in Africa [
24].
This study also showed that most of malaria transmission occurred during the rainy season. As the CSP indexes were not statistically different between the rainy and the dry seasons, this variation is due to the climatic conditions that are more favourable to
An. gambiae s.l. populations during the rainy period. Rainfalls provide larvae breeding sites, allowing an increase in the density of the populations and a higher level of hygrometry increases the longevity of the populations and therefore their vector capacity. The predominant presence of
S form is further evidence of the importance of rain-dependent temporary breeding sites in Libreville [
51].
The evolution of the proportion of
P. falciparum infected persons in patients consulting for fever is proof of the link between malaria transmission and rainfall. However, many people spend their summer holidays in the inlands, and the increase of malaria transmission seen in October after the return of people could be explained by the importation of
P. falciparum to the city by returning vacationers and students [
38‐
40]. Preliminary work conducted in 2007 showed that the aggressiveness of
An. gambiae s.l. was linked to rainfall; this study showed that malaria transmission in Libreville is clearly linked to rainfall. Malaria transmission primarily occurred during the rainy season, but transmission persisted during the dry season, though it was six times lower (4.5 infected bites per person during the dry season vs. 29.5 infected bites per person during the rainy season).
As An. gambiae s.l. have been caught both indoors and outdoors, malaria transmission can occur both indoors and outdoors. During the dry season, An. gambiae s.l. seemed to modify its host-seeking behaviour and to penetrate more easily in the houses. This modification of the biting behaviour could participate to the maintenance of malaria transmission during the dry season. Considering the four districts followed during the two seasons, An. gambiae s.l. was endophagic in one of them (Sotega). Interestingly, malaria transmission was higher in this district during the dry season.
The human night biting pattern supports the efficacy of impregnated nets in malaria prevention. Nevertheless, some biting activity takes place at the end of night (14% of bites from 5:00 to 7:00 a.m.) when people don't use any protection. As recommended by WHO, repellents are used by French forces stationed in sub-Saharan countries to complete protection, outside of the period of use of mosquito nets even in the early morning [
5,
52‐
54]. A successful trial in Amazonia has showed the efficiency of this strategy using a "natural" locally produced repellent [
55]. However, plant extracts have limited protection, for a short duration "natural" repellents traditionally used in Africa can be proposed [
56‐
58].
As previously described, in Libreville, both kdr-w and kdr-e mutations in
An. gambiae molecular form S were present [
23,
59] with a higher frequency of the kdr-w allele (76%) than the kdr-e allele (23.5%) as reported in neighbouring countries [
59‐
62]. As those mutations have been tightly linked with resistant phenotypes, the effectiveness of the current distribution programme of pyrethroid-impregnated bed nets in Libreville has to be assessed however none consensus has been reached on the impact of kdr mutations on the efficacy of ITNs [
63‐
69]. No insensitive AChE mutations were found in the five districts, suggesting that the molecular resistance to organophosphates and carbamates insecticides in
An.gambiae s.s. form S has not infiltrated Libreville and that these compounds are still a viable alternative.
Competing interests
The authors declare that they have no competing interests.
Authors' contributions
JRM was responsible for the supervision of data collection, analysis, interpretation and production of the final manuscript and revisions. TC contributed to the supervision of data collection, the data analysis, and interpretation. FJ contributed to the supervision of data collection, to the data analysis. CC contributed to the data analysis and production of final manuscript. EP contributed to the data analysis. LG contributed to the data analysis. MK contributed to overall scientific management, analysis, interpretation and preparation of the final manuscript and revisions. FP was responsible for overall scientific management, analysis, interpretation and preparation of the final manuscript and revisions. All authors read and approved the final manuscript.