Do bednets including piperonyl butoxide offer additional protection against populations of Anopheles gambiae s.l. that are highly resistant to pyrethroids? An experimental hut evaluation in Burkina Fasov.

Malaria control is dependent on the use of longlasting insecticidal nets (LLINs) containing pyrethroids. A new generation of LLINs containing both pyrethroids and the synergist piperonyl butoxide (PBO) has been developed in response to increasing pyrethroid resistance in African malaria vectors, but questions remain about the performance of these nets in areas where levels of pyrethroid resistance are very high. This study was conducted in two settings in southwest Burkina Faso, Vallée du Kou 5 and Tengrela, where Anopheles gambiae s.l. (Diptera: Culicidae) mortality rates in World Health Organization (WHO) discriminating dose assays were < 14% for permethrin and < 33% for deltamethrin. When mosquitoes were pre-exposed to PBO in WHO tube assays, mortality rates increased substantially but full susceptibility was not restored. Molecular characterization revealed high levels of kdr alleles and elevated levels of P450s previously implicated in pyrethroid resistance. In cone bioassays and experimental huts, PBO LLINs outperformed the pyrethroid-only equivalents from the same manufacturers. Blood feeding rates were 1.6-2.2-fold lower and mortality rates were 1.69-1.78-fold greater in huts with PBO LLINs vs. non-PBO LLINs. This study indicates that PBO LLINs provide greater personal and community-level protection than standard LLINs against highly pyrethroid-resistant mosquito populations.

Swarming behaviour in natural populations of Anopheles gambiae and An. coluzzii: review of 4 years survey in rural areas of sympatry, Burkina Faso (West Africa).

The swarming behaviour of natural populations of Anopheles gambiae and An. coluzzii (formerly known as An. gambiae S and M forms, respectively) were investigated through longitudinal surveys conducted between July 2006 and October 2009 in two rural areas of south-western Burkina Faso where these forms are sympatric. In both sites, the majority of swarms were recorded above visual markers localised among houses. In Soumousso, a wooded area of savannah, 108 pairs caught in copula from 205 swarms were sampled; in VK7, a rice growing area, 491 couples from 250 swarms were sampled. If segregated swarms were the norm in both sites, many visual markers were shared by the two forms of An. gambiae. Furthermore, mixed swarms were collected annually in frequencies varying from one site to another, though no mixed inseminations were recorded, corroborating the low hybrid rate previously reported in the field. The occurrence of inter-specific mate-recognition mechanisms, which allow individuals to avoid hybridisation, is discussed.

Assortative mating in mixed swarms of the mosquito Anopheles gambiae s.s. M and S molecular forms, in Burkina Faso, West Africa.

The molecular form composition of Anopheles gambiae Giles s.s. (Diptera: Culicidae) mating swarms and the associated mating pairs (copulae) were investigated during two rainy seasons (July to October, 2005 and July to November, 2006) in the villages of Soumousso and Vallée du Kou (VK7). Although the habitats of these villages differ markedly, sympatric populations of M and S molecular forms of An. gambiae s.s. occur in both places periodically. The main aim was to assess the degree to which these molecular forms mate assortatively. In Soumousso, a wooded savannah habitat, the majority of swarm samples consisted of only S-form males (21/28), although a few M-form males were found in mixed M- and S-form swarms. In VK7, a rice growing area, the majority of swarm samples consisted of only M-form males (38/62), until October and November 2006, when there were nearly as many mixed-form as single-form swarms. Overall, ∼60% of M- and S-form swarms were temporally or spatially segregated; the two forms were effectively prevented from encountering each other. Of the remaining 40% of swarms, however, only about half were single-form and the rest were mixed-form. Of the 33 copulae collected from mixed-form swarms, only four were mixed-form pairs, significantly fewer than expected by random pairing between forms (χ(2) = 10.34, d.f. = 2, P < 0.01). Finally, all specimens of inseminated females were of the same form as the sperm contained within their spermatheca (n = 91), even for the four mixed-form copulae. These findings indicate that assortative mating occurs within mixed-form swarms, mediated most probably by close-range mate recognition cues.

Distribution of pyrethroid and DDT resistance and the L1014F kdr mutation in Anopheles gambiae s.l. from Burkina Faso (West Africa).

This study reports on the distribution of pyrethroid and DDT resistance and the L1014F knockdown resistance (kdr) mutation in Anopheles gambiae s.l. populations from 21 localities in three different climatic zones of Burkina Faso from August to October 2006. The susceptibility of these populations was assessed by bioassay using DDT (4%), permethrin (1%) and deltamethrin (0.05%). Anophelesgambiae were resistant to both permethrin and DDT in the Sudanian regions but were susceptible in the central and sahelian areas and susceptible to deltamethrin at all sites except Orodara, although mortality values in some populations were close to the resistance threshold. The kdr frequency varied from 0.4 to 0.97 in populations from the Sudanian region and was lower in populations from the Sudano-sahelian and sahelian areas (0.047 to 0.54). Compared to the last survey of kdr in An. gambiae populations conducted in 2000, the kdr frequency did not differ in the S form but had increased in the M form (0.6), with an extended distribution into the Sudano-sahelian region. The frequency of kdr was also found to have increased in An. arabiensis populations (0.28), where it was formerly reported in only a single specimen. These results have practical significance for malaria vector control programs.