Relationship between malaria vector survival, infectivity and insecticide treated net use in western Kenya.

Background: Much effort and resources have been invested to control malaria transmission in Sub-Saharan Africa, but it remains a major public health problem. For the disease to be transmitted from one person to another, the female Anopheles vector must survive 10-14 days following an infective bite for the Plasmodiumgametocytes to develop into infectious sporozoites which can be transmitted to the next person during a bloodmeal. The goal of this investigation was to assess factors associated with wild-caught Anopheles survival and infection following host-seeking and indoor resting. Methods: The study was conducted in a longitudinal cohort of 75 households in 5 villages including a total of 755 household members in Bungoma County, Kenya. Monthly adult mosquito collection was conducted by attenuated aspiration in all the enrolled households, and the mosquitoes were reared in the insectary for 7 days. The daily mortality rate was determined through day 7, and all the mosquitoes were morphologically identified. Female Anopheline mosquitoes were dissected, and species-level members of the Anopheles gambiae complex were resolved by molecular methods. The abdomen for all samples were processed for P. falciparum detection by PCR. Results: Within a period of 25 months, the total number of culex and Anopheles mosquitoes collected indoors were 12,843 and 712 respectively. Anopheles gambiaeand Anopheles funestus were the major vectors though their population varied between different villages. 61.2% (n=436/712) of the Anopheles species survived up to day 7 with the lowest mortality rate recorded on day 5 of captivity. The survival rate also varied between the different Anophelesspecies. 683 of 712 mosquito abdomens were tested for P. falciparumdetection and 7.8% (53/683) tested positive for P. falciparum with An. funestus having a higher (10%) prevalence than An. gambaie s.s.(6.0%, p=0.095, Pearson Chi square test). The proportion of household members sleeping under a bednet the night before mosquito collection varied across time and village. An. funestus survival times were refractory to household ITN coverage and An. gambaie s.s. survival was reduced only under very high (>95%) ITN coverage. Conclusion: Despite ITN coverage, mosquitoes still acquired bloodmeals and P. falciparum infections. Survival differed across species and was inversely correlated with high ITN exposure in the household, but not oocyst development.

Ecological drivers of genetic connectivity for African malaria vectors Anopheles gambiae and An. arabiensis.

Anopheles gambiae and An. arabiensis are major malaria vectors in sub-Saharan Africa. Knowledge of how geographical factors drive the dispersal and gene flow of malaria vectors can help in combatting insecticide resistance spread and planning new vector control interventions. Here, we used a landscape genetics approach to investigate population relatedness and genetic connectivity of An. gambiae and An. arabiensis across Kenya and determined the changes in mosquito population genetic diversity after 20 years of intensive malaria control efforts. We found a significant reduction in genetic diversity in An. gambiae, but not in An. arabiensis as compared to prior to the 20-year period in western Kenya. Significant population structure among populations was found for both species. The most important ecological driver for dispersal and gene flow of An. gambiae and An. arabiensis was tree cover and cropland, respectively. These findings highlight that human induced environmental modifications may enhance genetic connectivity of malaria vectors.

Microbial larvicides for mosquito control: Impact of long lasting formulations of Bacillus thuringiensis var. israelensis and Bacillus sphaericus on non-target organisms in western Kenya highlands.

The microbial larvicides Bacillus thuringiensis var. israelensis and Bacillus sphaericus have been used extensively for mosquito control and have been found to be effective and safe to non-target organisms cohabiting with mosquito larvae. Recently developed long lasting microbial larvicides (LLML), although evading the previous challenge of short duration of activity, increase the risk of persistence of toxins in the treated larval habitats. This study monitored the impact of LLML FourStar® and LL3 on non-target organisms cohabiting with mosquito larvae in an operational study to control malaria vectors in western Kenya highlands. A total of 300 larval habitats were selected in three highland villages. The habitats were first monitored for 5 weeks to collect baseline data on non-target organisms cohabiting with mosquito larvae and then randomized into two treatment arms (respective FourStar® and LL3) and one control arm. Non-target organisms were sampled weekly for 5 months after treatment to assess the impact of LLML intervention. Before treatment, the mean density of all non-target organisms combined in the control, LL3 and FourStar® treated habitats was 1.42, 1.39 and 1.49 individuals per habitat per sampling occasion, respectively. Following treatment, this density remained fairly unchanged for 21 weeks at which time it was 1.82, 2.11, and 2.05 for the respective control, LL3 and FourStar® treated habitats. Statistical analysis revealed that LL3 and FourStar® did not significantly alter abundance, richness or diversity of the 11 taxa studied, when comparing the intervention and control larval habitats. However, both FourStar® and LL3 significantly reduced the density of malaria vectors. In conclusion, one round of label rate application of FourStar® or LL3 in natural larval habitats did not alter richness, abundance or diversity of the monitored aquatic non-target organisms cohabiting with mosquito larvae to an ecologically significant level.

Efficacy and persistence of long-lasting microbial larvicides against malaria vectors in western Kenya highlands.

BACKGROUND: Chemical-based malaria vector control interventions are threatened by the development of insecticide resistance and changes in the behavior of the vectors, and thus require the development of alternative control methods. Bacterial-based larvicides have the potential to target both insecticide resistant and outdoor-biting mosquitoes and are safe to use in the environment. However, the currently available microbial larvicide formulations have a short duration of activity requiring frequent re-applications which increase the cost of control interventions. This study was designed to evaluate the efficacy and duration of activity of two long-lasting formulations of Bacillus thuringiensis var. israelensis (Bti) and Bacillus sphaericus (Bs) (LL3 and FourStar®) under field conditions in western Kenya highlands. METHODS: Three sites were selected for this study in the highlands of western Kenya. In each site, one hundred anopheline larval habitats were selected and assigned to one of three arms: (i) LL3; (ii) FourStar®; and (iii) untreated control larval habitats. Four types of larval habitats were surveyed: abandoned gold mines, drainage canals, fish ponds and non-fish ponds. The habitats were sampled for mosquito larvae by using a standard dipping technique and collected larvae were recorded according to the larval stages of the different Anopheles species. The larvicides were applied at manufacturers' recommended dosage of 1 briquette per 100 square feet. Both treatment and control habitats were sampled for mosquito larvae immediately before treatment (day 0), and then at 24 hours, 3 days and weekly post-treatment for 5 months. RESULTS: Overall larval density in treatment habitats was significantly reduced after application of the two microbial larvicides as compared to the control habitats. Post-intervention reduction in anopheline larval density by LL3 was 65, 71 and 84% for 1 day, 2 weeks and 4 weeks, respectively. FourStar® reduced anopheline larval density by 60, 66 and 80% for 1 day, 2 weeks and 4 weeks, respectively. Comparisons between the treatments reveal that LL3 and FourStar® were similar in efficacy. A higher reduction in Anopheles larval density was observed in the abandoned goldmines, while drainage canals had the lowest reduction. CONCLUSIONS: Both LL3 and FourStar® long-lasting microbial larvicides were effective in reducing immature stages of An. gambiae complex and An. funestus group species, with significant reductions lasting for three months post-application.

Molecular Characterization Reveals Diverse and Unknown Malaria Vectors in the Western Kenyan Highlands.

The success of mosquito-based malaria control is dependent upon susceptible bionomic traits in local malaria vectors. It is crucial to have accurate and reliable methods to determine mosquito species composition in areas subject to malaria. An unexpectedly diverse set of Anopheles species was collected in the western Kenyan highlands, including unidentified and potentially new species carrying the malaria parasite Plasmodium falciparum. This study identified 2,340 anopheline specimens using both ribosomal DNA internal transcribed spacer region 2 and mitochondrial DNA cytochrome oxidase subunit 1 loci. Seventeen distinct sequence groups were identified. Of these, only eight could be molecularly identified through comparison to published and voucher sequences. Of the unidentified species, four were found to carry P. falciparum by circumsporozoite enzyme-linked immunosorbent assay and polymerase chain reaction, the most abundant of which had infection rates comparable to a primary vector in the area, Anopheles funestus. High-quality adult specimens of these unidentified species could not be matched to museum voucher specimens or conclusively identified using multiple keys, suggesting that they may have not been previously described. These unidentified vectors were captured outdoors. Diverse and unknown species have been incriminated in malaria transmission in the western Kenya highlands using molecular identification of unusual morphological variants of field specimens. This study demonstrates the value of using molecular methods to compliment vector identifications and highlights the need for accurate characterization of mosquito species and their associated behaviors for effective malaria control.

Mosquito species abundance and diversity in Malindi, Kenya and their potential implication in pathogen transmission.

Mosquitoes (Diptera: Culicidae) are important vectors of human disease-causing pathogens. Mosquitoes are found both in rural and urban areas. Deteriorating infrastructure, poor access to health, water and sanitation services, increasing population density, and widespread poverty contribute to conditions that modify the environment, which directly influences the risk of disease within the urban and peri-urban ecosystem. The objective of this study was to evaluate the mosquito vector abundance and diversity in urban, peri-urban, and rural strata in Malindi along the Kenya coast. The study was conducted in the coastal district of Malindi between January and December 2005. Three strata were selected which were described as urban, peri-urban, and rural. Sampling was done during the wet and dry seasons. Sampling in the wet season was done in the months of April and June to cover the long rainy season and in November and December to cover the short rainy season, while the dry season was between January and March and September and October. Adult mosquito collection was done using Pyrethrum Spray Collection (PSC) and Centers for Disease Control and Prevention (CDC) light traps inside houses and specimens were identified morphologically. In the three strata (urban, peri-urban, and rural), 78.5% of the total mosquito (n = 7,775) were collected using PSC while 18.1% (n = 1,795) were collected using the CDC light traps. Using oviposition traps, mosquito eggs were collected and reared in the insectary which yielded 329 adults of which 83.8% (n = 276) were Aedes aegypti and 16.2% (n = 53) were Culex quinquefasciatus. The mosquito distribution in the three sites varied significantly in each collection site. Anopheles gambiae, Anopheles funestus and Anopheles coustani were predominant in the rural stratum while C. quinquefasciatus was mostly found in urban and peri-urban strata. However, using PSC and CDC light trap collection techniques, A. aegypti was only found in urban strata. In the three strata, mosquitoes were mainly found in high numbers during the wet season. Further, A. gambiae, C. quinquefasciatus, and A. aegypti mosquitoes were found occurring together inside the houses. This in turn exposes the inhabitants to an array of mosquito-borne diseases including malaria, bancroftian filariasis, and arboviruses (dengue fever, Yellow fever, Rift Valley fever, Chikungunya fever, and West Nile Virus). In conclusion, our findings provide useful information for the design of integrated mosquito and disease control programs in East African environments.

Wide-scale application of Bti/Bs biolarvicide in different aquatic habitat types in urban and peri-urban Malindi, Kenya.

Larval control is a major component in mosquito control programs. This study evaluated the wide-scale application of Bti/Bs biolarvicide (Bacillus thuringiensis var. israelensis [Bti] and Bacillus sphaericus [Bs]) in different aquatic habitats in urban and peri-urban Malindi, Kenya. This study was done from June 2006 to December 2007. The urban and peri-urban area of Malindi town was mapped and categorized in grid cells of 1 km(2). A total of 16 1-km(2) cells were selected based on presence Community Based Organization dealing with malaria control within the cells. Each of the 16 1-km(2) cells was thoroughly searched for the presence of potential larval habitats. All habitats, whether positive or negative for larvae, were treated and rechecked 24 h (1 day), 6 days, and 10 days later for the efficacy of Bti/Bs. Weekly larval sampling was done to determine the mosquito larval dynamics in the aquatic habitats during the study period. Morphological identification of the mosquito larvae showed that Anopheles gambiae s.l. Giles was the most predominant species of the Anopheles and while in the culicines, Cx. quinquefasciatus Say was the predominant species. Anopheles larvae were all eliminated in habitats within a day post-application. For culicine larvae, 38.1% (n=8) of the habitat types responded within day 1 post-treatment and all the larvae were killed, they turned negative during the days of follow-up. Another 38.1% (n=8) of the habitat types had culicine larvae but turned negative by day 6, while three habitats (14.3%) had larvae by 6th day but turned negative by 10th day. However during this Bti/Bs application studies, two habitat types, house drainage and cesspits (9.5%), remained positive during the follow-up although the mosquito larvae were significantly reduced. Both early and late instars of Anopheles larvae immediately responded to Bti/Bs application and reached 100% mortality. The early and late instars of culicine responded to the Bti/Bs application but not as fast as the Anopheles larval instars. The early instars Culex, responded with 90.8% mortality at day 1 post-treatment, and the mortality was 99.9% at day 10. Similarly, the late instars Culex followed the same trend and exhibited same mortalities. The weekly sampling in the aquatic habitats showed that there was a 36.3% mosquito larval reduction in the aquatic habitats over the 18-months study period. In conclusion, Bti/Bs biolarvicide are useful in reducing the mosquito larval densities in a wide range of habitats which have a direct impact of adult mosquito populations.

Heavy metals in mosquito larval habitats in urban Kisumu and Malindi, Kenya, and their impact.

Concentrations and distribution of cadmium, chromium, copper, iron, lead, manganese and zinc in mosquito larval habitats in urban Kisumu and Malindi, Kenya and their effect on the presence of Anopheles gambiae, Aedes aegypti, Culex quinquefasciatus and Anopheles funestus larvae were investigated. Manganese and iron were the most prevalent heavy metals in water of larval habitats in urban Kisumu and Malindi, respectively. Iron was the most prevalent heavy metal in bottom sediments in larval habitats in both cities. The highest concentrations of all heavy metals, except cadmium and iron, were recorded in the poorly planned-well drained stratum in the two cities. All heavy metals were more concentrated in human-made than in natural larval habitats. Copper was positively associated with the presence of Ae. aegypti, and lead was associated with the presence of An. gambiae and Ae. aegypti in urban Kisumu. Absence of significant correlation between the other metals and mosquito species in both cities, despite relatively high concentrations, suggest that the local larval populations, including key malaria vectors have adapted to the detected levels of these metals.

Efficacy of vectobac DT and culinexcombi against mosquito larvae in unused swimming pools in Malindi, Kenya.

The efficacy and persistence of 2 bacterial larvicides, Vectobac-DT (Bacillus thuringiensis israelensis [Bti]) and CulinexCombi (Bti and Bacillus sphaericus [Bs]), were tested against Anopheles gambiae and Culex quinquefasciatus in temporarily unused swimming pools with rainwater in Malindi, Kenya. Pre- and posttreatment larval densities were recorded by sampling with the standard WHO dipping technique for 8 consecutive days. The larvicides were applied to the pools with a knapsack sprayer. The data showed that Vectobac-DT was highly effective against early instars of An. gambiae with 89% reduction within 24 h but not as effective against the early stages of Cx. quinquefasciatus with reduction of only 46%. CulinexCombi resulted in high mortalities to early instars of both species with over 97% reduction within 24 h, but showed a drastic reduction 48 h after application. Both Vectobac-DT and CulinexCombi were highly effective against late instars of both species, whereby Vectobac-DT persisted much longer than CulinexCombi. Anopheles gambiae was found to be more susceptible to both larvicides than Cx. quinquefasciatus. By their high efficacy and good persistence against mosquito larvae, both Vectobac-DT and CulinexCombi can be recommended for use in integrated mosquito control programs.

Ecological limitations on aquatic mosquito predator colonization in the urban environment.

Urban malaria cases are becoming common in Africa as more people move into cities and industrialization proceeds. While many species of Anopheles mosquitoes vector malaria in rural areas, only a few are found within cities. The success of anthropophilic species in cities, such as members of the An. gambiae complex, may be explained by limitations on colonization by predator species in urban environments. Habitats that are temporal or structurally simple have lower predator survivorship in a variety of ecosystems, but these have not been investigated previously in an urban area. Areas within and around the Kenyan coastal town of Malindi were previously sampled for the presence of standing water using a geographic sampling strategy with probability proportional to size sampling of planned well-drained, unplanned poorly-drained, planned poorly-drained, unplanned well-drained, and peri-urban locations. Standing aquatic habitats in these areas were reassessed. During monthly sampling, presence/absence of mosquitoes and predator taxa were noted, as were ecological habitat variables: structural complexity and presence of water. Lambda statistics were calculated to associate predator guilds, habitat types, location variables, and ecological variables. All predator guilds found in habitats were strongly associated with habitat type, as were the structural complexity and temporal nature of the habitats. Types of habitat were heterogeneously distributed throughout Malindi, with swimming pools as a common habitat type in planned urban areas and tire track pools a common habitat type in peri-urban areas of Malindi. Predator colonization of aquatic habitats in Malindi was strongly influenced by habitat type, and not associated with location characteristics. Ecological variables were affected by the type of habitats, which are co-associated with planning and drainage in Malindi. While habitat types are distributed heterogeneously within Malindi, habitats with low predation pressure are available for mosquito colonization in both urban and peri-urban areas. The temporal, peri-urban tire track pools and the structural simplicity of urban swimming pools may discourage predator colonization, thereby increasing the probability of malaria vectors in these areas of Malindi. Future studies should evaluate habitats for use in malaria surveillance and experimentally test the effects of structural complexity and temporal nature of urban habitats on the densities of mosquito larvae and their aquatic predators.