BackgroundThe impact of insecticide treated nets (ITNs) on reducing malaria incidence is shown mainly through data collection from health facilities. Routine evaluation of long-term epidemiological and entomological dynamics is currently unavailable. In Kenya, new policies supporting the provision of free ITNs were implemented nationwide in June 2006. To evaluate the impacts of ITNs on malaria transmission, we conducted monthly surveys in three sentinel sites with different transmission intensities in western Kenya from 2002 to 2010.Methods and FindingsLongitudinal samplings of malaria parasite prevalence in asymptomatic school children and vector abundance in randomly selected houses were undertaken monthly from February 2002. ITN ownership and usage surveys were conducted annually from 2004 to 2010. Asymptomatic malaria parasite prevalence and vector abundances gradually decreased in all three sites from 2002 to 2006, and parasite prevalence reached its lowest level from late 2006 to early 2007. The abundance of the major malaria vectors, Anopheles funestus and An. gambiae, increased about 5–10 folds in all study sites after 2007. However, the resurgence of vectors was highly variable between sites and species. By 2010, asymptomatic parasite prevalence in Kombewa had resurged to levels recorded in 2004/2005, but the resurgence was smaller in magnitude in the other sites. Household ITN ownership was at 50–70% in 2009, but the functional and effective bed net coverage in the population was estimated at 40.3%, 49.4% and 28.2% in 2010 in Iguhu, Kombewa, and Marani, respectively.ConclusionThe resurgence in parasite prevalence and malaria vectors has been observed in two out of three sentinel sites in western Kenya despite a high ownership of ITNs. The likely factors contributing to malaria resurgence include reduced efficacy of ITNs, insecticide resistance in mosquitoes and lack of proper use of ITNs. These factors should be targeted to avoid further resurgence of malaria transmission.

BackgroundInsecticide-treated bed nets (ITNs) are known to be highly effective in reducing malaria morbidity and mortality. However, usage varies among households, and such variations in actual usage may seriously limit the potential impact of nets and cause spatial heterogeneity on malaria transmission. This study examined ITN ownership and underlying factors for among-household variation in use, and malaria transmission in two highland regions of western Kenya.MethodsCross-sectional surveys were conducted on ITN ownership (possession), compliance (actual usage among those who own ITNs), and malaria infections in occupants of randomly sampled houses in the dry and the rainy seasons of 2009.ResultsDespite ITN ownership reaching more than 71%, compliance was low at 56.3%. The compliance rate was significantly higher during the rainy season compared with the dry season (62% vs. 49.6%). Both malaria parasite prevalence (11.8% vs. 5.1%) and vector densities (1.0 vs.0.4 female/house/night) were significantly higher during the rainy season than during the dry season. Other important factors affecting the use of ITNs include: a household education level of at least primary school level, significantly high numbers of nuisance mosquitoes, and low indoor temperatures. Malaria prevalence in the rainy season was about 30% lower in ITN users than in non-ITN users, but this percentage was not significantly different during the dry season.ConclusionIn malaria hypo-mesoendemic highland regions of western Kenya, the gap between ITNownership and usage is generally high with greater usage recorded during the high transmission season. Because of the low compliance among those who own ITNs, there is a need to sensitize households on sustained use of ITNs in order to optimize their role as a malaria control tool.

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.

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Background Hypoendemic malaria transmission in western Kenya highlands is not expected to lead to rapid acquisition of immunity to malaria. However, the asymptomatic subpopulation may play a significant role as an infection reservoir that should be considered in malaria control programs. Determination of spatio-temporal dynamics of asymptomatic subpopulations provides an opportunity to estimate the epidemiological importance of this group to malaria transmission. Methods Monthly parasitological surveys were undertaken on a cohort of 246 children for 12 months. Plasmodium falciparum infection prevalence was analyzed by both microscopy and PCR, and infection durations were determined. Results Infection prevalence and duration (1–12 months) decreased with age and altitude. Prevalence among age groups 5–9 and 10–14 years was high (34.4% and 34.1%, respectively), but significantly lower in older children (9.1%). Prevalence decreased from (52.4%) at ~1,430 m to 23.3% at 1,580 m. Conclusions Prevalence of asymptomatic P. falciparum infections was high, with PCR detecting a significantly higher number of infections than microscopy. Our results are consistent with gradual acquisition of immunity with age upon repeated infection, and also show that malaria transmission risk is highly heterogeneous in the highland area. The results provide strong support for targeted control.

Climate change is expected to lead to latitudinal and altitudinal temperature increases. High elevation regions such as the highlands of Africa, and those that have temperate climate are most likely to be affected. The highlands of Africa generally exhibit low ambient temperatures. This restricts the distribution of Anopheles mosquitoes, the vectors of malaria, filariasis and O’nyong’nyong fever. The development and survival of larval and adult mosquitoes are temperature dependent, as are mosquito biting frequency and pathogen development rate. Given that various Anopheles species are adapted to different climatic conditions, changes in the climate could lead to changes in species composition in an area which may change the dynamics of mosquito-borne disease transmission. It is important to consider the effect of climate change on rainfall which is critical to the formation and persistence of mosquito breeding sites. In addition, environmental changes such as deforestation could increase local temperatures in the highlands; this could enhance the vectorial capacity of the Anopheles. This experimental data will be invaluable in facilitating the understanding of the impact of climate change on Anopheles.

Background: Wolbachia, a common insect endosymbiotic bacterium that can influence pathogen transmission and manipulate host reproduction, has historically been considered absent from the Anopheles (An.) genera, but has recently been found in An. gambiae s.l. populations in West Africa.  As there are numerous Anopheles species that have the capacity to transmit malaria, we analysed a range of species across five malaria endemic countries to determine Wolbachia prevalence rates, characterise novel Wolbachia strains and determine any correlation between the presence of Plasmodium, Wolbachia and the competing bacterium Asaia. Methods: Anopheles adult mosquitoes were collected from five malaria-endemic countries: Guinea, Democratic Republic of the Congo (DRC), Ghana, Uganda and Madagascar, between 2013 and 2017.  Molecular analysis was undertaken using quantitative PCR, Sanger sequencing, Wolbachia multilocus sequence typing (MLST) and high-throughput amplicon sequencing of the bacterial 16S rRNA gene.  Results: Novel Wolbachia strains were discovered in five species: An. coluzzii, An. gambiae s.s., An. arabiensis, An. moucheti and An. species A, increasing the number of Anopheles species known to be naturally infected. Variable prevalence rates in different locations were observed and novel strains were phylogenetically diverse, clustering with Wolbachia supergroup B strains.  We also provide evidence for resident strain variants within An. species A. Wolbachia is the dominant member of the microbiome in An. moucheti and An. species A but present at lower densities in An. coluzzii.  Interestingly, no evidence of Wolbachia/Asaia co-infections was seen and Asaia infection densities were shown to be variable and location dependent.  Conclusions: The important discovery of novel Wolbachia strains in Anopheles provides greater insight into the prevalence of resident Wolbachia strains in diverse malaria vectors.  Novel Wolbachia strains (particularly high-density strains) are ideal candidate strains for transinfection to create stable infections in other Anopheles mosquito species, which could be used for population replacement or suppression control strategies.

We investigated the effects of deforestation on microclimates and sporogonic development of Plasmodium falciparum parasites in Anopheles gambiae mosquitoes in an area of the western Kenyan highland prone to malaria epidemics. An. gambiae mosquitoes were fed with P. falciparum-infected blood through membrane feeders. Fed mosquitoes were placed in houses in forested and deforested areas in a highland area (1,500 m above sea level) and monitored for parasite development. Deforested sites had higher temperatures and relative humidities, and the overall infection rate of mosquitoes was increased compared with that in forested sites. Sporozoites appeared on average 1.1 days earlier in deforested areas. Vectorial capacity was estimated to be 77.7% higher in the deforested site than in the forested site. We showed that deforestation changes microclimates, leading to more rapid sporogonic development of P. falciparum and to a marked increase of malaria risk in the western Kenyan highland.