[Agricultural activities and epidemiology of malaria in Soudano-Sahelian zone in Cameroon]

We have comparatively studied the dynamics of malaria transmission in the villages of Mokolo-Douvar located in the rural area with traditional agriculture and Gounougou irrigated rice area, in 2004 August and November and 2006 May and October, to assess vectors biting habits, and malaria inoculation rate and malaria parasite prevalence in cohort of children from 0 to 15 years. Mosquitoes were collected by landing catches on volunteers and by pyrethrum spray collections. A total of 5961 Anopheles were collected. Seven Anopheles species were identified: Anopheles gambiae s.s., Anopheles arabiensis, Anopheles funestus, Anopheles pharoensis, Anopheles rufipes, Anopheles ziemanni and Anopheles squamosus. A. arabiensis was the major species (56.2%) and the main malaria vector in both study sites, followed by A. funestus (32.6%). Malaria transmission was high in the irrigated area of Gounougou (1.42 infection bites per man per night) whereas in the non-irrigated zone of Mokolo-Douvar, it was below detection level during the rainy season (0,245 ib/h/n). In Gounougou, a total of 655 children were examined. The mean plasmodic index was 21.1%. Our findings confirm that changes in irrigated rice agriculture influence malaria transmission dynamics, and call for control measures that are readily adapted to local eco-epidemiological settings.

Evidence for subdivision within the M molecular form of Anopheles gambiae

The principal vector of malaria in sub-Saharan Africa, Anopheles gambiae is subdivided into two molecular forms M and S. Additionally, several chromosomal forms, characterized by the presence of various inversion polymorphisms, have been described. The molecular forms M and S each contain several chromosomal forms, including the Savanna, Mopti and Forest forms. The M and S molecular forms are now considered to be the reproductive units within A. gambiae and it has recently been argued that a low recombination rate in the centromeric region of the X chromosome has facilitated isolation between these forms. The status of the chromosomal forms remains unclear however. Therefore, we studied genetic differentiation between Savanna S, Forest S, Forest M and Mopti M populations using microsatellites. Genetic differentiation between Savanna S and Forest S populations is very low (F(ST) = 0.0053 +/- 0.0049), even across large distances. In comparison, the Mopti M and Forest M populations show a relatively high degree of genetic differentiation (F(ST) = 0.0406 +/- 0.0054) indicating that the M molecular form may not be a single entity, but could be subdivided into at least two distinct chromosomal forms. Previously it was proposed that inversions have played a role in the origin of species within the A. gambiae complex. We argue that a possible subdivision within the M molecular form could be understood through this process, with the acquisition of inversions leading to the expansion of the M molecular form into new habitat, dividing it into two distinct chromosomal forms.

Impact of insecticide-treated bed nets implementation on the genetic structure of Anopheles arabiensis in an area of irrigated rice fields in the Sahelian region of Cameroon

Variation at 12 microsatellite loci was investigated to assess the impact of the implementation of insecticide-treated bed nets (ITNs) on the genetic structure of Anopheles arabiensis in Simatou, a village surrounded by irrigated rice fields in the Sahelian area of Cameroon. The An. arabiensis population of Simatou was sampled twice before ITN implementation, and twice after. Effective population size estimates (N(e)) were similar across each time point, except for the period closely following ITN introduction where a nonsignificant reduction was recorded. Hence, we believe that ITN implementation resulted in a temporary bottleneck, rapidly followed by a demographic expansion. The genetic diversity of the population was not significantly affected since different genetic parameters (allele number, observed and expected heterozygosities) remained stable. Low estimates of genetic differentiation between the populations from Simatou and Lagdo, separated by 300 km, suggested extensive gene flow among populations of An. arabiensis in the Sahelian region of Cameroon. A decrease in the susceptibility to deltamethrin was observed following ITN introduction, but no kdr mutation was detected and a metabolic resistance mechanism is probably involved. The temporary effect of ITNs on the genetic structure of An. arabiensis population suggests that, to optimize the success of any control programme of this species based on ITNs, the control area should be very large and the programme should be implemented for a long period of time.

An unusual distribution of the kdr gene among populations of Anopheles gambiae on the island of Bioko, Equatorial Guinea

In West Africa, Anopheles gambiae exists in discrete subpopulations known as the M and S molecular forms. Although these forms occur in sympatry, pyrethroid knock-down resistance (kdr) is strongly associated with the S molecular form. On the island of Bioko, Equatorial Guinea we found high frequencies of the kdr mutation in M form individuals (55.8%) and a complete absence of kdr in the S form. We also report the absence of the kdr allele in M and S specimens from the harbour town of Tiko in Cameroon, representing the nearest continental population to Bioko. The kdr allele had previously been reported as absent in populations of An. gambiae on Bioko. Contrary to earlier reports, sequencing of intron-1 of this sodium channel gene revealed no fixed differences between M form resistant and susceptible individuals. The mutation may have recently arisen independently in the M form on Bioko due to recent and intensive pyrethroid application.

Comparison of three sampling methods of man-biting anophelines in order to estimate the malaria transmission in a village of south Cameroon

In order to get an accurate measure of malaria transmission by mean of human bait attraction, three methods of catches for human-bail mosquitoes were compared in a Cameroonian village of high anopheline density. These three methods were 1) the classical human landing catches where the man was in the same time bait and catcher, 2) the single-nets which acted as a trap and 3) the double-nets where the outer net acted as a trap and the inner avoided mosquito bites. The anopheline densities per man with human landings were 1.6 time higher than those obtained with single-nets, these later being 4.7 times higher than those obtained with double-nets. In the three methods, the results were similar for the anophelines species catched and for their respective proportions. The samples of Anopheles nili and An. gambiae had comparative parity rates and sporozoitic indexes. From these results, in order to estimate the malaria transmission it can be envisaged to change the standard human-bait catch for the human-baited single bed-net catch; on the contrary the double-net have to be discarded because of their poor results.

Efficacy of insecticide impregnated bed-nets to control malaria in a rural forested area in southern Cameroon

Due to current spreading of chemoresistant strains of Plasmodium falciparum malaria control must incorporate vector control programmes. Due to well known constraints house sprayings cannot be performed as before. Personal protection can be developed and a large scale use of insecticide treated bed-nets appeared to be very useful to reduce man-vector contact in Asia, South America and West and East Africa. No trial has been done in forest Central Africa where transmission is permanent. We performed such a trial in the southern part of Cameroon (using deltamethrin, at 25 mg/m2) and obtained similar data to those observed in The Gambia, Burkina Faso and Tanzania with a noteworthy reduction of both transmission and high parasitaemia of P. falciparum (respectively 78% and 75%) meaning a drop of malaria morbidity.

Importance of low dispersion of Anopheles gambiae (Diptera: Culicidae) on malaria transmission in hilly towns in south Cameroon

The dispersion of Anopheles gambiae Giles s.l. from larval breeding places was examined in the towns of Yaounde and Edea, Cameroon. The study areas are characterized by hilly reliefs of approximately 40 m and by larval breeding places in the swampy valleys. Mosquito density was estimated by night-biting indoor catches carried out during 1 yr in three districts, with four collecting houses in each, along a transect from 0 (at the valley bottom) to 350 m (at the hilltop). In each district, the human-biting rate decreased gradually from valley bottom to hilltop. In two districts, the observed densities were zero at 200 and 250 m from breeding places. In the third district where the habitat is less dense, there was a 50% reduction in biting mosquitoes at 350 m from valley bottom. Our studies confirm that dense habitat in the urban area is associated with a low degree of vector dispersion. It emphasizes that slope is another factor which limits dispersion. These findings explain why malaria transmission is heterogenous (range, 0-45 infective bites per man per year) inside the districts and occurs mainly around swamps in the valleys. These data are important when considering the uncontrolled urbanization which is occurring in the undeveloped areas in the valleys.

Anthropophilic mosquitoes and malaria transmission at Edea, Cameroon

An entomological study was carried out during 1990 in the town of Edea in the south of Cameroon to study anthropophilic mosquitoes with special reference to malaria transmission. Man-biting mosquitoes were caught regularly during one night each month in two different districts: Bilalang which is a well planned suburb with 160 houses on a hill-top, provided with a piped water supply; and Pongo which is a densely urbanised suburb in a valley. From 188 man-nights 1030 mosquitoes were collected, comprising 700 Culex quinquefasciatus (68%), 262 Anopheles gambiae (25%) and others species (7%) belonging to the genus Anopheles, Mansonia, Culex and Aedes. The estimated annual biting rates of mosquitoes were 811 bites per man in Bilalang and 2,866 in Pongo. The estimated yearly malaria inoculation rates were 3.8 and 30.2 infective bites per man in Bilalang and Pongo, respectively. In different parts of Pongo district much variation existed; extreme values of the estimated yearly inoculation rate were zero and 86.3 in two houses 200 m apart, located on the top of a hill and in the bottom of a valley, respectively. This study is one of the first conducted on malaria transmission in a moderate sized African town; it shows that the mosquito populations are typically urban and differ greatly from rural ones.