Yaw Afrane - Academia.edu (original) (raw)

Papers by Yaw Afrane

Research paper thumbnail of Genetic Diversity in Western Kenya Highlands

The American journal of tropical medicine and hygiene

ABSTRACT

Research paper thumbnail of Effects of microclimate changes caused by deforestation on the survivorship and reproductive fitness of Anopheles Gambiae in western Kenya highlands

The American journal of tropical medicine and hygiene

ABSTRACT

Research paper thumbnail of Life-table analysis of Anopheles Arabiensis in Western Kenya Highlands: effects of land covers on larval and adult survivorship

The American journal of tropical medicine and hygiene

In high-elevation areas in western Kenya, the abundance of Anopheles arabiensis is either very lo... more 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 > 1,500 m 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°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°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.

Research paper thumbnail of RNA-seq analyses of changes in the Anopheles gambiae transcriptome associated with resistance to pyrethroids in Kenya: identification of candidate-resistance genes and candidate-resistance SNPs

Parasites & vectors, 2015

The extensive use of pyrethroids for control of malaria vectors, driven by their cost, efficacy a... more The extensive use of pyrethroids for control of malaria vectors, driven by their cost, efficacy and safety, has led to widespread resistance. To favor their sustainable use, the World Health Organization (WHO) formulated an insecticide resistance management plan, which includes the identification of the mechanisms of resistance and resistance surveillance. Recognized physiological mechanisms of resistance include target site mutations in the para voltage-gated sodium channel, metabolic detoxification and penetration resistance. Such understanding of resistance mechanisms has allowed the development of resistance monitoring tools, including genotyping of the kdr mutation L1014F/S in the para gene. The sequence-based technique RNA-seq was applied to study changes in the transcriptome of deltamethrin-resistant and -susceptible Anopheles gambiae mosquitoes from the Western Province of Kenya. The resulting gene expression profiles were compared to data in the most recent literature to de...

Research paper thumbnail of Constructing a Genome-Wide LD Map of Wild A. gambiae Using Next-Generation Sequencing

BioMed Research International, 2015

Anopheles gambiae is the major malaria vector in Africa. Examining the molecular basis of A. gamb... more Anopheles gambiae is the major malaria vector in Africa. Examining the molecular basis of A. gambiae traits requires knowledge of both genetic variation and genome-wide linkage disequilibrium (LD) map of wild A. gambiae populations from malaria-endemic areas. We sequenced the genomes of nine wild A. gambiae mosquitoes individually using next-generation sequencing technologies and detected 2,219,815 common single nucleotide polymorphisms (SNPs), 88% of which are novel. SNPs are not evenly distributed across A. gambiae chromosomes. The low SNP-frequency regions overlay heterochromatin and chromosome inversion domains, consistent with the lower recombinant rates at these regions. Nearly one million SNPs that were genotyped correctly in all individual mosquitoes with 99.6% confidence were extracted from these high-throughput sequencing data. Based on these SNP genotypes, we constructed a genome-wide LD map for wild A. gambiae from malaria-endemic areas in Kenya and made it available through a public Website. The average size of LD blocks is less than 40 bp, and several large LD blocks were also discovered clustered around the para gene, which is consistent with the effect of insecticide selective sweeps. The SNPs and the LD map will be valuable resources for scientific communities to dissect the A. gambiae genome.

Research paper thumbnail of of Tropical Medicine and Hygiene Alterations in Plasmodium falciparum Genetic Structure Two Years after Increased Malaria Control Efforts in Western Kenya

The impact of malaria intervention measures (insecticide-treated net use and artemisinin combinat... more The impact of malaria intervention measures (insecticide-treated net use and artemisinin combination therapy) on malaria genetics was investigated at two sites in western Kenya: an endemic lowland and an epidemic high-land. The genetic structure of the parasite population was assessed by using microsatellites, and the prevalence of drug-resistant mutations was examined by using the polymerase chain reaction–restriction fragment length polymorphism method. Two years after intervention, genetic diversity remained high in both populations. A significant decrease in the prevalence of quintuple mutations conferring resistance to sulfadoxine-pyrimethamine was detected in both populations, but the mutation prevalence at codon 1246 of the Plasmodium falciparum multidrug resistance 1 gene had increased in the highland population. The decrease in sulfadoxine-pyrimethamine–resistant mutants is encouraging, but the increase in P. falciparum multidrug resistance 1 gene mutations is worrisome bec...

Research paper thumbnail of Low parasitemia in submicroscopic infections significantly impacts malaria diagnostic sensitivity in the highlands of Western kenya

PloS one, 2015

Asymptomatic malaria infections represent a major challenge in malaria control and elimination in... more Asymptomatic malaria infections represent a major challenge in malaria control and elimination in Africa. They are reservoirs of malaria parasite that can contribute to disease transmission. Therefore, identification and control of asymptomatic infections are important to make malaria elimination feasible. In this study, we investigated the extent and distribution of asymptomatic malaria in Western Kenya and examined how varying parasitemia affects performance of diagnostic methods including microscopy, conventional PCR, and quantitative PCR. In addition, we compared parasite prevalence rates and parasitemia levels with respect to topography and age in order to explore factors that influence malaria infection. Over 11,000 asymptomatic blood samples from children and adolescents up to 18 years old representing broad areas of Western Kenya were included. Quantitative PCR revealed the highest parasite positive rate among all methods and malaria prevalence in western Kenya varied widely...

Research paper thumbnail of Plasmodium falciparum genetic diversity in western Kenya highlands

The American journal of tropical medicine and hygiene, 2007

The present study examined the genetic diversity and population structure of Plasmodium falciparu... more The present study examined the genetic diversity and population structure of Plasmodium falciparum in western Kenya by analyzing the polymorphism of 12 microsatellite loci and two antigen loci. Malaria in highland areas is unstable and epidemic whereas malaria in lowland areas is endemic. Transmission intensity and malaria prevalence are substantially lower in the highlands than in the lowlands. Despite that the highland parasite populations exhibited reduced number of alleles, lower expected heterozygosity, and infection complexity in comparison to the surrounding lowland population, genetic diversity of the highland populations remained high in comparison to parasites from other meso-endemic regions. More than 70% of infections from western Kenya highland study sites were mixed genotype infections. Small but statistically significant genetic differentiation between highland and lowland populations was detected. These findings are discussed in the context of human travel and local ...

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

The American journal of tropical medicine and hygiene, 2006

Land use changes have been suggested as one of the causes for malaria epidemics in the African hi... more 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 ...

Research paper thumbnail of Active case surveillance, passive case surveillance and asymptomatic malaria parasite screening illustrate different age distribution, spatial clustering and seasonality in western Kenya

Malaria Journal, 2015

Background: Epidemiological characteristics of clinical malaria may differ from asymptomatic infe... more Background: Epidemiological characteristics of clinical malaria may differ from asymptomatic infections, thus both cross-sectional parasite screening and longitudinal clinical case surveillance are necessary for malaria transmission monitoring and control.

Research paper thumbnail of Insecticide-treated net (ITN) ownership, usage, and malaria transmission in the highlands of western Kenya

Parasites & Vectors, 2011

Background: Insecticide-treated bed nets (ITNs) are known to be highly effective in reducing mala... more Background: Insecticide-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. Methods: Cross-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.

Research paper thumbnail of Habitat stability and occurrences of malaria vector larvae in western Kenya highlands

Malaria Journal, 2009

Background: Although the occurrence of malaria vector larvae in the valleys of western Kenya high... more Background: Although the occurrence of malaria vector larvae in the valleys of western Kenya highlands is well documented, knowledge of larval habitats in the uphill sites is lacking. Given that most inhabitants of the highlands actually dwell in the uphill regions, it is important to develop understanding of mosquito breeding habitat stability in these sites in order to determine their potential for larval control.

Research paper thumbnail of Changing Patterns of Malaria Epidemiology between 2002 and 2010 in Western Kenya: The Fall and Rise of Malaria

PLoS ONE, 2011

Background: The impact of insecticide treated nets (ITNs) on reducing malaria incidence is shown ... more Background: The 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.

Research paper thumbnail of Utility of Health Facility-based Malaria Data for Malaria Surveillance

PLoS ONE, 2013

Background: Currently, intensive malaria control programs are being implemented in Africa to redu... more Background: Currently, intensive malaria control programs are being implemented in Africa to reduce the malaria burden. Clinical malaria data from hospitals are valuable for monitoring trends in malaria morbidity and for evaluating the impacts of these interventions. However, the reliability of hospital-based data for true malaria incidence is often questioned because of diagnosis accuracy issues and variation in access to healthcare facilities among sub-groups of the population. This study investigated how diagnosis and treatment practices of malaria cases in hospitals affect reliability of hospital malaria data.

Research paper thumbnail of Comparative Transcriptome Analyses of Deltamethrin-Resistant and -Susceptible Anopheles gambiae Mosquitoes from Kenya by RNA-Seq

PLoS ONE, 2012

Malaria causes more than 300 million clinical cases and 665,000 deaths each year, and the majorit... more 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. Fieldderived 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 andsusceptible 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.

Research paper thumbnail of Predation efficiency of Anopheles gambiae larvae by aquatic predators in western Kenya highlands

Parasites & Vectors, 2011

Background: The current status of insecticide resistance in mosquitoes and the effects of insecti... more Background: The current status of insecticide resistance in mosquitoes and the effects of insecticides on nontarget insect species have raised the need for alternative control methods for malaria vectors. Predation has been suggested as one of the important regulation mechanisms for malaria vectors in long-lasting aquatic habitats, but the predation efficiency of the potential predators is largely unknown in the highlands of western Kenya. In the current study, we examined the predation efficiency of five predators on Anopheles gambiae s.s larvae in 24 hour and semi-field evaluations. Methods: Predators were collected from natural habitats and starved for 12 hours prior to starting experiments. Preliminary experiments were conducted to ascertain the larval stage most predated by each predator species. When each larval instar was subjected to predation, third instar larvae were predated at the highest rate. Third instar larvae of An. gambiae were introduced into artificial habitats with and without refugia at various larval densities. The numbers of surviving larvae were counted after 24 hours in 24. In semi-field experiments, the larvae were counted daily until they were all either consumed or had developed to the pupal stage. Polymerase chain reaction was used to confirm the presence of An. gambiae DNA in predator guts. Results: Experiments found that habitat type (P < 0.0001) and predator species (P < 0.0001) had a significant impact on the predation rate in the 24 hour evaluations. In semi-field experiments, predator species (P < 0.0001) and habitat type (P < 0.0001) were significant factors in both the daily survival and the overall developmental time of larvae. Pupation rates took significantly longer in habitats with refugia. An. gambiae DNA was found in at least three out of ten midguts for all predator species. Gambusia affins was the most efficient, being three times more efficient than tadpoles. Conclusion: These experiments provide insight into the efficiency of specific natural predators against mosquito larvae. These naturally occurring predators may be useful in biocontrol strategies for aquatic stage An. gambiae mosquitoes. Further investigations should be done in complex natural habitats for these predators.

Research paper thumbnail of Topography as a modifier of breeding habitats and concurrent vulnerability to malaria risk in the western Kenya highlands

Parasites & Vectors, 2011

Background: Topographic parameters such as elevation, slope, aspect, and ruggedness play an impor... more Background: Topographic parameters such as elevation, slope, aspect, and ruggedness play an important role in malaria transmission in the highland areas. They affect biological systems, such as larval habitats presence and productivity for malaria mosquitoes. This study investigated whether the distribution of local spatial malaria vectors and risk of infection with malaria parasites in the highlands is related to topography. Methods: Four villages each measuring 9 Km 2 lying between 1400-1700 m above sea level in the western Kenya highlands were categorized into a pair of broad and narrow valley shaped terrain sites. Larval, indoor resting adult malaria vectors and infection surveys were collected originating from the valley bottom and ending at the hilltop on both sides of the valley during the rainy and dry seasons. Data collected at a distance of ≤500 m from the main river/stream were categorized as valley bottom and those above as uphill. Larval surveys were categorized by habitat location while vectors and infections by house location. Results: Overall, broad flat bottomed valleys had a significantly higher number of anopheles larvae/dip in their habitats than in narrow valleys during both the dry (1.89 versus 0.89 larvae/dip) and the rainy season (1.66 versus 0.89 larvae/dip). Similarly, vector adult densities/house in broad valley villages were higher than those within narrow valley houses during both the dry (0.64 versus 0.40) and the rainy season (0.96 versus 0.09). Asymptomatic malaria prevalence was significantly higher in participants residing within broad than those in narrow valley villages during the dry (14.55% vs. 7.48%) and rainy (17.15% vs. 1.20%) season. Malaria infections were wide spread in broad valley villages during both the dry and rainy season, whereas over 65% of infections were clustered at the valley bottom in narrow valley villages during both seasons. Conclusion: Despite being in the highlands, local areas within low gradient topography characterized by broad valley bottoms have stable and significantly high malaria risk unlike those with steep gradient topography, which exhibit seasonal variations. Topographic parameters could therefore be considered in identification of high-risk malaria foci to help enhance surveillance or targeted control activities in regions where they are most needed.

Research paper thumbnail of Evaluating larval mosquito resource partitioning in western Kenya using stable isotopes of carbon and nitrogen

Parasites & Vectors, 2013

Background: In sub-Saharan Africa, malaria, transmitted by the Anopheles mosquito, remains one of... more Background: In sub-Saharan Africa, malaria, transmitted by the Anopheles mosquito, remains one of the foremost public health concerns. Anopheles gambiae, the primary malaria vector in sub-Saharan Africa, is typically associated with ephemeral, sunlit habitats; however, An. gambiae larvae often share these habitats with other anophelines along with other disease-transmitting and benign mosquito species. Resource limitations within habitats can constrain larval density and development, and this drives competitive interactions among and between species. Methods: We used naturally occurring stable isotope ratios of carbon and nitrogen to identify resource partitioning among co-occurring larval species in microcosms and natural habitats in western Kenya. We used two and three source mixing models to estimate resource utilization (i.e. bacteria, algae, organic matter) by larvae.

Research paper thumbnail of Modest additive effects of integrated vector control measures on malaria prevalence and transmission in western Kenya

Malaria Journal, 2013

Background: The effect of integrating vector larval intervention on malaria transmission is unkno... more Background: The effect of integrating vector larval intervention on malaria transmission is unknown when insecticide-treated bed-net (ITN) coverage is very high, and the optimal indicator for intervention evaluation needs to be determined when transmission is low. Methods: A post hoc assignment of intervention-control cluster design was used to assess the added effect of both indoor residual spraying (IRS) and Bacillus-based larvicides (Bti) in addition to ITN in the western Kenyan highlands in 2010 and 2011. Cross-sectional, mass parasite screenings, adult vector populations, and cohort of active case surveillance (ACS) were conducted before and after the intervention in three study sites with two-to three-paired intervention-control clusters at each site each year. The effect of larviciding, IRS, ITNs and other determinants of malaria risk was assessed by means of mixed estimating methods.

Research paper thumbnail of Variation in exposure to Anopheles gambiae salivary gland peptide (gSG6-P1) across different malaria transmission settings in the western Kenya highlands

Malaria Journal, 2012

The existing metrics of malaria transmission are limited in sensitivity under low transmission in... more The existing metrics of malaria transmission are limited in sensitivity under low transmission intensity. Robust surveillance systems are needed as interventions to monitor reduced transmission and prevention of rapid reintroduction. Serological tools based on antibody responses to parasite and vector antigens are potential tools for transmission measurements. The current study sought to evaluate antibody responses to Anopheles gambiae salivary gland peptide (gSG6-P1), as a biomarker of human exposure to Anopheles bites, in different transmission settings and seasons. The comparison between anti-MSP-1 19 IgG immune responders and non-responders allowed exploring the robustness of the gSG6-P1 peptide as a surveillance tool in an area of decreasing malaria transmission.

Research paper thumbnail of Genetic Diversity in Western Kenya Highlands

The American journal of tropical medicine and hygiene

ABSTRACT

Research paper thumbnail of Effects of microclimate changes caused by deforestation on the survivorship and reproductive fitness of Anopheles Gambiae in western Kenya highlands

The American journal of tropical medicine and hygiene

ABSTRACT

Research paper thumbnail of Life-table analysis of Anopheles Arabiensis in Western Kenya Highlands: effects of land covers on larval and adult survivorship

The American journal of tropical medicine and hygiene

In high-elevation areas in western Kenya, the abundance of Anopheles arabiensis is either very lo... more 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 > 1,500 m 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°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°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.

Research paper thumbnail of RNA-seq analyses of changes in the Anopheles gambiae transcriptome associated with resistance to pyrethroids in Kenya: identification of candidate-resistance genes and candidate-resistance SNPs

Parasites & vectors, 2015

The extensive use of pyrethroids for control of malaria vectors, driven by their cost, efficacy a... more The extensive use of pyrethroids for control of malaria vectors, driven by their cost, efficacy and safety, has led to widespread resistance. To favor their sustainable use, the World Health Organization (WHO) formulated an insecticide resistance management plan, which includes the identification of the mechanisms of resistance and resistance surveillance. Recognized physiological mechanisms of resistance include target site mutations in the para voltage-gated sodium channel, metabolic detoxification and penetration resistance. Such understanding of resistance mechanisms has allowed the development of resistance monitoring tools, including genotyping of the kdr mutation L1014F/S in the para gene. The sequence-based technique RNA-seq was applied to study changes in the transcriptome of deltamethrin-resistant and -susceptible Anopheles gambiae mosquitoes from the Western Province of Kenya. The resulting gene expression profiles were compared to data in the most recent literature to de...

Research paper thumbnail of Constructing a Genome-Wide LD Map of Wild A. gambiae Using Next-Generation Sequencing

BioMed Research International, 2015

Anopheles gambiae is the major malaria vector in Africa. Examining the molecular basis of A. gamb... more Anopheles gambiae is the major malaria vector in Africa. Examining the molecular basis of A. gambiae traits requires knowledge of both genetic variation and genome-wide linkage disequilibrium (LD) map of wild A. gambiae populations from malaria-endemic areas. We sequenced the genomes of nine wild A. gambiae mosquitoes individually using next-generation sequencing technologies and detected 2,219,815 common single nucleotide polymorphisms (SNPs), 88% of which are novel. SNPs are not evenly distributed across A. gambiae chromosomes. The low SNP-frequency regions overlay heterochromatin and chromosome inversion domains, consistent with the lower recombinant rates at these regions. Nearly one million SNPs that were genotyped correctly in all individual mosquitoes with 99.6% confidence were extracted from these high-throughput sequencing data. Based on these SNP genotypes, we constructed a genome-wide LD map for wild A. gambiae from malaria-endemic areas in Kenya and made it available through a public Website. The average size of LD blocks is less than 40 bp, and several large LD blocks were also discovered clustered around the para gene, which is consistent with the effect of insecticide selective sweeps. The SNPs and the LD map will be valuable resources for scientific communities to dissect the A. gambiae genome.

Research paper thumbnail of of Tropical Medicine and Hygiene Alterations in Plasmodium falciparum Genetic Structure Two Years after Increased Malaria Control Efforts in Western Kenya

The impact of malaria intervention measures (insecticide-treated net use and artemisinin combinat... more The impact of malaria intervention measures (insecticide-treated net use and artemisinin combination therapy) on malaria genetics was investigated at two sites in western Kenya: an endemic lowland and an epidemic high-land. The genetic structure of the parasite population was assessed by using microsatellites, and the prevalence of drug-resistant mutations was examined by using the polymerase chain reaction–restriction fragment length polymorphism method. Two years after intervention, genetic diversity remained high in both populations. A significant decrease in the prevalence of quintuple mutations conferring resistance to sulfadoxine-pyrimethamine was detected in both populations, but the mutation prevalence at codon 1246 of the Plasmodium falciparum multidrug resistance 1 gene had increased in the highland population. The decrease in sulfadoxine-pyrimethamine–resistant mutants is encouraging, but the increase in P. falciparum multidrug resistance 1 gene mutations is worrisome bec...

Research paper thumbnail of Low parasitemia in submicroscopic infections significantly impacts malaria diagnostic sensitivity in the highlands of Western kenya

PloS one, 2015

Asymptomatic malaria infections represent a major challenge in malaria control and elimination in... more Asymptomatic malaria infections represent a major challenge in malaria control and elimination in Africa. They are reservoirs of malaria parasite that can contribute to disease transmission. Therefore, identification and control of asymptomatic infections are important to make malaria elimination feasible. In this study, we investigated the extent and distribution of asymptomatic malaria in Western Kenya and examined how varying parasitemia affects performance of diagnostic methods including microscopy, conventional PCR, and quantitative PCR. In addition, we compared parasite prevalence rates and parasitemia levels with respect to topography and age in order to explore factors that influence malaria infection. Over 11,000 asymptomatic blood samples from children and adolescents up to 18 years old representing broad areas of Western Kenya were included. Quantitative PCR revealed the highest parasite positive rate among all methods and malaria prevalence in western Kenya varied widely...

Research paper thumbnail of Plasmodium falciparum genetic diversity in western Kenya highlands

The American journal of tropical medicine and hygiene, 2007

The present study examined the genetic diversity and population structure of Plasmodium falciparu... more The present study examined the genetic diversity and population structure of Plasmodium falciparum in western Kenya by analyzing the polymorphism of 12 microsatellite loci and two antigen loci. Malaria in highland areas is unstable and epidemic whereas malaria in lowland areas is endemic. Transmission intensity and malaria prevalence are substantially lower in the highlands than in the lowlands. Despite that the highland parasite populations exhibited reduced number of alleles, lower expected heterozygosity, and infection complexity in comparison to the surrounding lowland population, genetic diversity of the highland populations remained high in comparison to parasites from other meso-endemic regions. More than 70% of infections from western Kenya highland study sites were mixed genotype infections. Small but statistically significant genetic differentiation between highland and lowland populations was detected. These findings are discussed in the context of human travel and local ...

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

The American journal of tropical medicine and hygiene, 2006

Land use changes have been suggested as one of the causes for malaria epidemics in the African hi... more 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 ...

Research paper thumbnail of Active case surveillance, passive case surveillance and asymptomatic malaria parasite screening illustrate different age distribution, spatial clustering and seasonality in western Kenya

Malaria Journal, 2015

Background: Epidemiological characteristics of clinical malaria may differ from asymptomatic infe... more Background: Epidemiological characteristics of clinical malaria may differ from asymptomatic infections, thus both cross-sectional parasite screening and longitudinal clinical case surveillance are necessary for malaria transmission monitoring and control.

Research paper thumbnail of Insecticide-treated net (ITN) ownership, usage, and malaria transmission in the highlands of western Kenya

Parasites & Vectors, 2011

Background: Insecticide-treated bed nets (ITNs) are known to be highly effective in reducing mala... more Background: Insecticide-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. Methods: Cross-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.

Research paper thumbnail of Habitat stability and occurrences of malaria vector larvae in western Kenya highlands

Malaria Journal, 2009

Background: Although the occurrence of malaria vector larvae in the valleys of western Kenya high... more Background: Although the occurrence of malaria vector larvae in the valleys of western Kenya highlands is well documented, knowledge of larval habitats in the uphill sites is lacking. Given that most inhabitants of the highlands actually dwell in the uphill regions, it is important to develop understanding of mosquito breeding habitat stability in these sites in order to determine their potential for larval control.

Research paper thumbnail of Changing Patterns of Malaria Epidemiology between 2002 and 2010 in Western Kenya: The Fall and Rise of Malaria

PLoS ONE, 2011

Background: The impact of insecticide treated nets (ITNs) on reducing malaria incidence is shown ... more Background: The 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.

Research paper thumbnail of Utility of Health Facility-based Malaria Data for Malaria Surveillance

PLoS ONE, 2013

Background: Currently, intensive malaria control programs are being implemented in Africa to redu... more Background: Currently, intensive malaria control programs are being implemented in Africa to reduce the malaria burden. Clinical malaria data from hospitals are valuable for monitoring trends in malaria morbidity and for evaluating the impacts of these interventions. However, the reliability of hospital-based data for true malaria incidence is often questioned because of diagnosis accuracy issues and variation in access to healthcare facilities among sub-groups of the population. This study investigated how diagnosis and treatment practices of malaria cases in hospitals affect reliability of hospital malaria data.

Research paper thumbnail of Comparative Transcriptome Analyses of Deltamethrin-Resistant and -Susceptible Anopheles gambiae Mosquitoes from Kenya by RNA-Seq

PLoS ONE, 2012

Malaria causes more than 300 million clinical cases and 665,000 deaths each year, and the majorit... more 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. Fieldderived 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 andsusceptible 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.

Research paper thumbnail of Predation efficiency of Anopheles gambiae larvae by aquatic predators in western Kenya highlands

Parasites & Vectors, 2011

Background: The current status of insecticide resistance in mosquitoes and the effects of insecti... more Background: The current status of insecticide resistance in mosquitoes and the effects of insecticides on nontarget insect species have raised the need for alternative control methods for malaria vectors. Predation has been suggested as one of the important regulation mechanisms for malaria vectors in long-lasting aquatic habitats, but the predation efficiency of the potential predators is largely unknown in the highlands of western Kenya. In the current study, we examined the predation efficiency of five predators on Anopheles gambiae s.s larvae in 24 hour and semi-field evaluations. Methods: Predators were collected from natural habitats and starved for 12 hours prior to starting experiments. Preliminary experiments were conducted to ascertain the larval stage most predated by each predator species. When each larval instar was subjected to predation, third instar larvae were predated at the highest rate. Third instar larvae of An. gambiae were introduced into artificial habitats with and without refugia at various larval densities. The numbers of surviving larvae were counted after 24 hours in 24. In semi-field experiments, the larvae were counted daily until they were all either consumed or had developed to the pupal stage. Polymerase chain reaction was used to confirm the presence of An. gambiae DNA in predator guts. Results: Experiments found that habitat type (P < 0.0001) and predator species (P < 0.0001) had a significant impact on the predation rate in the 24 hour evaluations. In semi-field experiments, predator species (P < 0.0001) and habitat type (P < 0.0001) were significant factors in both the daily survival and the overall developmental time of larvae. Pupation rates took significantly longer in habitats with refugia. An. gambiae DNA was found in at least three out of ten midguts for all predator species. Gambusia affins was the most efficient, being three times more efficient than tadpoles. Conclusion: These experiments provide insight into the efficiency of specific natural predators against mosquito larvae. These naturally occurring predators may be useful in biocontrol strategies for aquatic stage An. gambiae mosquitoes. Further investigations should be done in complex natural habitats for these predators.

Research paper thumbnail of Topography as a modifier of breeding habitats and concurrent vulnerability to malaria risk in the western Kenya highlands

Parasites & Vectors, 2011

Background: Topographic parameters such as elevation, slope, aspect, and ruggedness play an impor... more Background: Topographic parameters such as elevation, slope, aspect, and ruggedness play an important role in malaria transmission in the highland areas. They affect biological systems, such as larval habitats presence and productivity for malaria mosquitoes. This study investigated whether the distribution of local spatial malaria vectors and risk of infection with malaria parasites in the highlands is related to topography. Methods: Four villages each measuring 9 Km 2 lying between 1400-1700 m above sea level in the western Kenya highlands were categorized into a pair of broad and narrow valley shaped terrain sites. Larval, indoor resting adult malaria vectors and infection surveys were collected originating from the valley bottom and ending at the hilltop on both sides of the valley during the rainy and dry seasons. Data collected at a distance of ≤500 m from the main river/stream were categorized as valley bottom and those above as uphill. Larval surveys were categorized by habitat location while vectors and infections by house location. Results: Overall, broad flat bottomed valleys had a significantly higher number of anopheles larvae/dip in their habitats than in narrow valleys during both the dry (1.89 versus 0.89 larvae/dip) and the rainy season (1.66 versus 0.89 larvae/dip). Similarly, vector adult densities/house in broad valley villages were higher than those within narrow valley houses during both the dry (0.64 versus 0.40) and the rainy season (0.96 versus 0.09). Asymptomatic malaria prevalence was significantly higher in participants residing within broad than those in narrow valley villages during the dry (14.55% vs. 7.48%) and rainy (17.15% vs. 1.20%) season. Malaria infections were wide spread in broad valley villages during both the dry and rainy season, whereas over 65% of infections were clustered at the valley bottom in narrow valley villages during both seasons. Conclusion: Despite being in the highlands, local areas within low gradient topography characterized by broad valley bottoms have stable and significantly high malaria risk unlike those with steep gradient topography, which exhibit seasonal variations. Topographic parameters could therefore be considered in identification of high-risk malaria foci to help enhance surveillance or targeted control activities in regions where they are most needed.

Research paper thumbnail of Evaluating larval mosquito resource partitioning in western Kenya using stable isotopes of carbon and nitrogen

Parasites & Vectors, 2013

Background: In sub-Saharan Africa, malaria, transmitted by the Anopheles mosquito, remains one of... more Background: In sub-Saharan Africa, malaria, transmitted by the Anopheles mosquito, remains one of the foremost public health concerns. Anopheles gambiae, the primary malaria vector in sub-Saharan Africa, is typically associated with ephemeral, sunlit habitats; however, An. gambiae larvae often share these habitats with other anophelines along with other disease-transmitting and benign mosquito species. Resource limitations within habitats can constrain larval density and development, and this drives competitive interactions among and between species. Methods: We used naturally occurring stable isotope ratios of carbon and nitrogen to identify resource partitioning among co-occurring larval species in microcosms and natural habitats in western Kenya. We used two and three source mixing models to estimate resource utilization (i.e. bacteria, algae, organic matter) by larvae.

Research paper thumbnail of Modest additive effects of integrated vector control measures on malaria prevalence and transmission in western Kenya

Malaria Journal, 2013

Background: The effect of integrating vector larval intervention on malaria transmission is unkno... more Background: The effect of integrating vector larval intervention on malaria transmission is unknown when insecticide-treated bed-net (ITN) coverage is very high, and the optimal indicator for intervention evaluation needs to be determined when transmission is low. Methods: A post hoc assignment of intervention-control cluster design was used to assess the added effect of both indoor residual spraying (IRS) and Bacillus-based larvicides (Bti) in addition to ITN in the western Kenyan highlands in 2010 and 2011. Cross-sectional, mass parasite screenings, adult vector populations, and cohort of active case surveillance (ACS) were conducted before and after the intervention in three study sites with two-to three-paired intervention-control clusters at each site each year. The effect of larviciding, IRS, ITNs and other determinants of malaria risk was assessed by means of mixed estimating methods.

Research paper thumbnail of Variation in exposure to Anopheles gambiae salivary gland peptide (gSG6-P1) across different malaria transmission settings in the western Kenya highlands

Malaria Journal, 2012

The existing metrics of malaria transmission are limited in sensitivity under low transmission in... more The existing metrics of malaria transmission are limited in sensitivity under low transmission intensity. Robust surveillance systems are needed as interventions to monitor reduced transmission and prevention of rapid reintroduction. Serological tools based on antibody responses to parasite and vector antigens are potential tools for transmission measurements. The current study sought to evaluate antibody responses to Anopheles gambiae salivary gland peptide (gSG6-P1), as a biomarker of human exposure to Anopheles bites, in different transmission settings and seasons. The comparison between anti-MSP-1 19 IgG immune responders and non-responders allowed exploring the robustness of the gSG6-P1 peptide as a surveillance tool in an area of decreasing malaria transmission.