Comparative transcriptome analyses of deltamethrin-resistant and -susceptible Anopheles gambiae mosquitoes from Kenya by RNA-Seq.

Malaria causes more than 300 million clinical cases and 665,000 deaths each year, and the majority of the mortality and morbidity occurs in sub-Saharan Africa. Due to the lack of effective vaccines and wide-spread resistance to antimalarial drugs, mosquito control is the primary method of malaria prevention and control. Currently, malaria vector control relies on the use of insecticides, primarily pyrethroids. The extensive use of insecticides has imposed strong selection pressures for resistance in the mosquito populations. Consequently, resistance to pyrethroids in Anopheles gambiae, the main malaria vector in sub-Saharan Africa, has become a major obstacle for malaria control. A key element of resistance management is the identification of resistance mechanisms and subsequent development of reliable resistance monitoring tools. Field-derived An. gambiae from Western Kenya were phenotyped as deltamethrin-resistant or -susceptible by the standard WHO tube test, and their expression profile compared by RNA-seq. Based on the current annotation of the An. gambiae genome, a total of 1,093 transcripts were detected as significantly differentially accumulated between deltamethrin-resistant and -susceptible mosquitoes. These transcripts are distributed over the entire genome, with a large number mapping in QTLs previously linked to pyrethorid resistance, and correspond to heat-shock proteins, metabolic and transport functions, signal transduction activities, cytoskeleton and others. The detected differences in transcript accumulation levels between resistant and susceptible mosquitoes reflect transcripts directly or indirectly correlated with pyrethroid resistance. RNA-seq data also were used to perform a de-novo Cufflinks assembly of the An. gambiae genome.

Effects of co-habitation between Anopheles gambiae s.s. and Culex quinquefasciatus aquatic stages on life history traits.

BACKGROUND: The effective measures for the control of malaria and filariasis vectors can be achieved by targeting immature stages of anopheline and culicine mosquitoes in productive habitat. To design this strategy, the mechanisms (like biotic interactions with conspecifc and heterospecific larvae) regulating mosquito aquatic stages survivorship, development time and the size of emerging adults should be understood. This study explored the effect of co-habitation between An. gambiae s.s. and Cx. quinquefasciatus on different life history traits of both species under different densities and constant food supply in the habitats of the same size under semi-natural conditions. METHODS: Experiments were set up with three combinations; Cx. quinquefasciatus alone (single species treatment), An. gambiae s.s. alone (single species treatment); and An. gambiae s.s. with Cx. quiquefasciatus (co-habitation treatment) in different densities in semi field situation. RESULTS: The effect of co-habitation of An. gambiae s.s. and Cx. quinquefasciatus was found to principally affect three parameters. The wing-lengths (a proxy measure of body size) of An. gambiae s.s. in co-habitation treatments were significantly shorter in both females and males than in An. gambiae s.s single species treatments. In Cx. quinquefasciatus, no significant differences in wing-length were observed between the single species and co-habitation treatments. Daily survival rates were not significantly different between co-habitation and single species treatments for both An. gambiae s.s. and Cx. quinquefasciatus. Developmental time was found to be significantly different with single species treatments developing better than co-habitation treatments. Sex ratio was found to be significantly different from the proportion of 0.5 among single and co-habitation treatments species at different densities. Single species treatments had more males than females emerging while in co-habitation treatments more females emerged than males. In this study, there was no significant competitive survival advantage in co-habitation. CONCLUSION: These results suggest that co-habitation of An. gambiae s.s. and Cx. quinquefasciatus in semi-natural conditions affect mostly An. gambiae s.s. body size. Hence, more has to be understood on the effects of co-habitation of An. gambiae s.s. and Cx. quinquefasciatus in a natural ecology and its possible consequences in malaria and filariasis epidemiology.

High prevalence of asymptomatic plasmodium falciparum infections in a highland area of western Kenya: a cohort study.

BACKGROUND: Transmission of malaria in an area of hypoendemicity in the highlands of western Kenya is not expected to lead to rapid acquisition of immunity to malaria. However, the subpopulation of individuals with asymptomatic Plasmodium falciparum infection may play a significant role as an infection reservoir and should be considered in malaria-control programs. Determination of the spatiotemporal dynamics of asymptomatic subpopulations provides an opportunity to estimate the epidemiological importance of this group to malaria transmission. METHODS: Monthly parasitological surveys were undertaken for a cohort of 246 schoolchildren over 12 months. The prevalence of P. falciparum infection among 2,611 blood samples was analyzed by both microscopy and polymerase chain reaction, and infection durations were determined. RESULTS: Infection prevalence and duration (range, 1-12 months) decreased with age and altitude. The prevalence was high among pooled blood samples recovered from children aged 5-9 years (34.4%) and from those aged 10-14 years (34.1%) but was significantly lower among blood samples obtained from older children (9.1%). The prevalence decreased from 52.4% among pooled blood samples from children living at an altitude of approximately 1,430 m to 23.3% among pooled samples from children living at an altitude of 1,580 m. CONCLUSIONS: The prevalence of asymptomatic P. falciparum infection was high, with polymerase chain reaction analysis detecting a significantly greater number of infections, compared with microscopy. Our results are consistent with gradual acquisition of immunity with increasing age upon repeated infection, and they also show that the risk of malaria transmission is highly heterogeneous in the highland area. The results provide strong support for targeted malaria-control interventions.

Life-table analysis of Anopheles arabiensis in western Kenya highlands: effects of land covers on larval and adult survivorship.

In high-elevation areas in western Kenya, the abundance of Anopheles arabiensis is either very low or absent. The western Kenya highlands (an area with an elevation > 1,500m above sea level) have also been experiencing extensive deforestation, and deforestation has been suggested as one of the important factors that facilitate malaria transmission in the highlands. This study investigated whether climate conditions in the western Kenya highlands (Kakamega, elevation 1,500 m above sea level) were permissive to the development and survival of An. arabiensis and whether deforestation promoted An. arabiensis survivorship of immature and adult stages, using life-table analysis. We found that in larval habitats located in forested areas, only 4-9% of first-instar larvae developed into adults and the development length exceeded 20 days. Mean water temperature of aquatic habitats in the deforested area was 4.8-6.1 degrees C higher than that in the forested area, larval-to-adult survivorship was increased to 65-82%, and larval-to-adult development time was shortened by 8-9 days. The average indoor temperature in houses in the deforested area was 1.7-1.8 degrees C higher than in the forested area, and the relative humidity was 22-25% lower. The median survival time of adult mosquitoes in the deforested area was 49-55% higher than those in the forested area. The net reproductive rate of female mosquitoes in the deforested area was 1.7- to 2.6-fold higher than that in the forested area. Compared with previously published data on An. gambiae, the net reproductive rate of An. arabiensis was only 0.8-1.3% of Anopheles gambiae in the forested area and 2.3-2.6% in the deforested area. Therefore, the current ambient climate condition is less permissive to An. arabiensis than to An. gambiae in western Kenya highlands. However, environmental changes such as deforestation and global warming may facilitate the establishment of An. arabiensis populations in the highlands.

Effects of microclimatic changes caused by deforestation on the survivorship and reproductive fitness of Anopheles gambiae in western Kenya highlands.

Land use changes have been suggested as one of the causes for malaria epidemics in the African highlands. This study investigated the effects of deforestation-induced changes in indoor temperature on the survivorship and reproductive fitness of Anopheles gambiae in an epidemic prone area in the western Kenya highlands. We found that the mean indoor temperatures of houses located in the deforested area were 1.2 degrees C higher than in houses located in the forested area during the dry season and 0.7 degrees C higher during the rainy season. The mosquito mortality rate was highly age-dependent regardless of study site or season. Mosquitoes that were placed in houses in the deforested area showed a 64.8-79.5% higher fecundity than those in houses located in the forested area, but the median survival time was reduced by 5-7 days. Female mosquitoes in the deforested area showed a 38.5-40.6% increase in net reproductive rate and an 11.6-42.9% increase in intrinsic growth rate than those in the forested area. Significant increases in net reproductive rate and intrinsic growth rate for mosquitoes in the deforested area suggest that deforestation enhances mosquito reproductive fitness, increasing mosquito population growth potential in the western Kenya highlands. The vectorial capacity of An. gambiae under study was estimated at least 106% and 29% higher in the deforested area than in the forested area in dry and rainy seasons, respectively.

Population dynamics of malaria vectors in western Kenya highlands.

Studies were carried out at three sites in the highlands of western Kenya (Iguhu and Mbale in Kakamega and Vihiga districts, respectively, and Marani in Kisii district) and at one site in the western Kenya lowlands (Kombewa in Kisumu district) to determine the spatial-temporal dynamics of malaria vectors and intensity of malaria transmission from June 2003 to June 2004. At the highland sites, Anopheles gambiae Giles predominated, constituting >80% of the vector species, whereas An. funestus Giles made up <20%. In contrast, at the lowland site, An. funestus made up 68% of the vector species. The mean annual indoor resting densities of An. gambiae at Iguhu were 5.0 female mosquitoes per house per night, 14.2- and 26.3-fold greater than those at Mbale and Marani. During the main transmission season, the indoor resting densities of An. gambiae increased 4.1-, 10.1-, and 5.0-fold over the dry season period in Iguhu, Mbale, and Marani, respectively. The estimated annual entomological inoculation rate (EIR) at Iguhu was 16.6 infectious bites per person per year (ib/p/yr), 1.1 at Mbale, and 0.4 at Marani. This suggests high spatial variation in vector abundance and malaria transmission intensity. At the lowland site, Kombewa, the total annual EIR was 31.1 ib/p/yr and the indoor resting densities during the transmission season increased 7.1-fold in An. funestus and 18.5-fold in An. gambiae sensu lato over the dry season. The low level of transmission in the highlands suggests that it may be disrupted by vector control methods such as residual spraying.

Fitness consequences of Anopheles gambiae population hybridization.

BACKGROUND: The use of transgenic mosquitoes with parasite inhibiting genes has been proposed as an integral strategy to control malaria transmission. However, release of exotic transgenic mosquitoes will bring in novel alleles along with parasite-inhibiting genes that may have unknown effects on native populations. Thus it is necessary to study the effects and dynamics of fitness traits in native mosquito populations in response to the introduction of novel genes. This study was designed to evaluate the dynamics of fitness traits in a simulation of introduction of novel alleles under laboratory conditions using two strains of Anopheles gambiae: Mbita strain from western Kenya and Ifakara strain from Tanzania. METHODS: The dynamics of fitness traits were evaluated under laboratory conditions using the two An. gambiae strains. These two geographically different strains were cross-bred and monitored for 20 generations to score fecundity, body size, blood-meal size, larval survival, and adult longevity, all of which are important determinants of the vector's potential in malaria transmission. Traits were analysed using pair-wise analysis of variance (ANOVA) for fecundity, body size, and blood-meal size while survival analysis was performed for larval survival and adult longevity. RESULTS: Fecundity and body size were significantly higher in the progeny up to the 20th generation compared to founder strains. Adult longevity had a significantly higher mean up to the 10th generation and average blood-meal size was significantly larger up to the 5th generation, indicating that hybrids fitness is enhanced over that of the founder strains. CONCLUSION: Hybridization of the two mosquito populations used in this study led to increased performance in the fitness traits studied. Given that the studied traits are important determinants of the vector's potential to transmit malaria, these results suggest the need to release genetically modified mosquitoes that have the same or very similar backgrounds to the native populations.