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Bart G J Knols

Publications and source records attributed to Bart G J Knols.

At least 19 recordsLinked to original sources
PubMed2006

A reliable morphological method to assess the age of male Anopheles gambiae.

BACKGROUND: Release of genetically-modified (GM) or sterile male mosquitoes for malaria control is hampered by inability to assess the age and mating history of free-living male Anopheles. METHODS: Age and mating-related changes in the reproductive system of male Anopheles gambiae were quantified and used to fit predictive statistical models. These models, based on numbers of spermatocysts, relative size of sperm reservoir and presence/absence of a clear area around the accessory gland, were evaluated using an independent sample of mosquitoes whose status was blinded during the experiment. RESULTS: The number of spermatocysts in male testes decreased with age, and the relative size of their sperm reservoir increased. The presence of a clear area around accessory glands was also linked to age and mating status. A quantitative model was able to categorize males from the blind trial into age groups of young (< or = 4 days) and old (> 4 days) with an overall efficiency of 89%. Using the parameters of this model, a simple table was compiled that can be used to predict male age. In contrast, mating history could not be reliably assessed as virgins could not be distinguished from mated males. CONCLUSION: Simple assessment of a few morphological traits which are easily collected in the field allows accurate age-grading of male An. gambiae. This simple, yet robust, model enables evaluation of demographic patterns and mortality in wild and released males in populations targeted by GM or sterile male-based control programmes.

Age Factors↗
PubMed2006

Trapping of the malaria vector Anopheles gambiae with odour-baited MM-X traps in semi-field conditions in western Kenya.

BACKGROUND: The successful development of odour-baited trapping systems for mosquitoes depends on the identification of behaviourally active semiochemicals, besides the design and operating principles of such devices. A large variety of 'attractants' has been identified in laboratory investigations, yet few of these increase trap catches in the field. A contained system, intermediate between the laboratory and open field, is presented and previous reports that human foot odour induces behavioural responses of Anopheles gambiae confirmed. METHODS: The response of 3-5 day old female An. gambiae towards odour-baited counterflow geometry traps (MM-X model; American Biophysics Corp., RI) was studied in semi-field (screen house) conditions in western Kenya. Traps were baited with human foot odour (collected on socks), carbon dioxide (CO2, 500 ml min(-1)), ammonia (NH3), 1-octen-3-ol, or various combinations thereof. Trap catches were log (x+1) transformed and subjected to Latin square analysis of variance procedures. RESULTS: Apart from 1-octen-3-ol, all odour baits caused significant (P < 0.05) increases in trap catches over non-baited traps. Foot odour remained behaviourally active for at least 8 days after collection on nylon or cotton sock fabric. A synergistic response (P < 0.001) was observed towards the combination of foot odour and CO2, which increased catches of these odours alone by 3.8 and 2.7 times, respectively. CONCLUSION: These results are the first to report behavioural responses of an African malaria vector to human foot odour outside the laboratory, and further investigation of fractions and/or individual chemical components of this odour complex are called for. Semi-field systems offer the prospect of high-throughput screening of candidate kairomones, which may expedite the development of efficient trap-bait systems for this and other African mosquito species.

Ammonia↗
PubMed2006

Radiation-induced sterility for pupal and adult stages of the malaria mosquito Anopheles arabiensis.

BACKGROUND: In the context of the Sterile Insect Technique (SIT), radiation-induced sterility in the malaria mosquito Anopheles arabiensis Patton (Diptera: Culicidae) was studied. Male mosquitoes were exposed to gamma rays in the pupal or adult stage and dose-sterility curves were determined. METHODS: Pupae were irradiated shortly before emergence (at 22-26 hrs of age), and adults <24 hrs post emergence. Doses tested ranged between 0 and 100 Gy. The effects of irradiation on adult emergence, male survival, induced sterility and insemination capability were evaluated. Emergence and insemination data were analysed using independent t-tests against the control. Correlation analyses were performed for insemination rate and dose and insemination and fecundity. Male survival was analysed using Kaplan-Meier survival analyses. Finally, the calculated residual fertility values were inverse-normal transformed and linear regression analyses performed. RESULTS: Irradiation of pupae, for all doses tested, had no effect on adult emergence. Survival curves of males irradiated as pupae or adults were similar or even slightly higher than non-irradiated males. Overall, adults appeared to be slightly more susceptible to irradiation, although no significant differences for individual doses were observed. In the pupal stage, a significant negative correlation was found between insemination and dose, but the correlation-coefficient was associated with less than 25% of the total variation. A review of the literature indicated that An. arabiensis is more radiation resistant than other anopheline mosquitoes. CONCLUSION: The optimal dose for male insects to be released in an SIT programme depends on their level of sterility and competitiveness. The use of semi-sterilizing doses to produce more competitive insects is discussed. The most convenient developmental stage for mosquito irradiation on a mass-scale are pupae, but pupal irradiation resulted in a lower insemination rate at the highest dose compared to adult irradiation. On the basis of this study, a suitable dose range that includes semi-sterilizing doses is identified to initiate competitiveness experiments for males irradiated at both developmental stages.

Animals↗
PubMed2006

Ecologists can enable communities to implement malaria vector control in Africa.

BACKGROUND: Integrated vector management (IVM) for malaria control requires ecological skills that are very scarce and rarely applied in Africa today. Partnerships between communities and academic ecologists can address this capacity deficit, modernize the evidence base for such approaches and enable future scale up. METHODS: Community-based IVM programmes were initiated in two contrasting settings. On Rusinga Island, Western Kenya, community outreach to a marginalized rural community was achieved by University of Nairobi through a community-based organization. In Dar es Salaam, Tanzania, Ilala Municipality established an IVM programme at grassroots level, which was subsequently upgraded and expanded into a pilot scale Urban Malaria Control Programme with support from national academic institutes. RESULTS: Both programmes now access relevant expertise, funding and policy makers while the academic partners benefit from direct experience of community-based implementation and operational research opportunities. The communities now access up-to-date malaria-related knowledge and skills for translation into local action. Similarly, the academic partners have acquired better understanding of community needs and how to address them. CONCLUSION: Until sufficient evidence is provided, community-based IVM remains an operational research activity. Researchers can never directly support every community in Africa so community-based IVM strategies and tactics will need to be incorporated into undergraduate teaching programmes to generate sufficient numbers of practitioners for national scale programmes. Academic ecologists at African institutions are uniquely positioned to enable the application of practical environmental and entomological skills for malaria control by communities at grassroots level and should be supported to fulfil this neglected role.

Africa↗
PubMed2006

Use of carbon-13 as a population marker for Anopheles arabiensis in a sterile insect technique (SIT) context.

BACKGROUND: Monitoring of sterile to wild insect ratios in field populations can be useful to follow the progress in genetic control programmes such as the Sterile Insect Technique (SIT). Of the numerous methods for marking insects most are not suitable for use in mass rearing and mass release. Suitable ones include dye marking, genetic marking and chemical marking. METHODS: The feasibility of using the stable isotope of carbon, 13C, as a potential chemical marker for Anopheles arabiensis was evaluated in the laboratory. Labeled-13C glucose was incorporated into the larval diet in a powder or liquid form. The contribution of adult sugar feeding to the total mosquito carbon pool and the metabolically active carbon pool was determined by tracing the decline of the enrichment of the adult male mosquito as it switched from a labeled larval diet to an unlabeled adult diet. This decline in the adult was monitored by destructive sampling of the whole mosquito and analyzed using isotope ratio mass spectrometry. RESULTS: A two-pool model was used to describe the decline of the 13C-enrichment of adult mosquitoes. The proportion of the total adult carbon pool derived from the adult sugar diet over the life span of mosquitoes was determined and the ratio of structural carbon, with a low turnover rate to metabolically active non-structural carbon was assessed. The uptake and turnover of sugar in the metabolically active fraction suggests that after 3 days >70% of the active fraction carbon is derived from sugar feeding (increasing to >90% by day 7), indicating the high resource demand of male mosquitoes. CONCLUSION: It was possible to "fix" the isotopic label in adult An. arabiensis and to detect the label at an appropriate concentration up to 21 days post-emergence. The optimum labeling treatment would cost around 250 US$ per million mosquitoes. Stable isotope marking may thus aid research on the fate of released insects besides other population-based ecological studies.

Animal Feed↗
PubMed2006

The Farmer Field School: a method for enhancing the role of rural communities in malaria control ?

Malaria has strong linkages with agriculture, and farmers in malarious regions have a central position in creating or controlling the conditions that favour disease transmission. An interdisciplinary and integrated approach is needed to involve farmers and more than one sector in control efforts. It is suggested that malaria control can benefit from a complementary intervention in rural development, the Farmer Field School (FFS) on Integrated Pest Management (IPM). This is a form of education that uses experiential learning methods to build farmers' expertise, and has proven farm-level and empowerment effects. The benefits of incorporating malaria control into the IPM curriculum are discussed. An example of a combined health-agriculture curriculum, labeled Integrated Pest and Vector Management (IPVM), developed in Sri Lanka is presented. Institutional ownership and support for IPVM could potentially be spread over several public sectors requiring a process for institutional learning and reform.

Agriculture↗
PubMed2005

Infection of the malaria mosquito Anopheles gambiae with the entomopathogenic fungus Metarhizium anisopliae reduces blood feeding and fecundity.

The entomopathogenic fungus Metarhizium anisopliae is being considered as a biocontrol agent against adult African malaria vectors. In addition to causing significant mortality, this pathogen is known to cause reductions in feeding and fecundity in a range of insects. In the present study we investigated whether infection with M. anisopliae affected blood feeding and fecundity of adult female malaria vectors Anopheles gambiae Giles sensu stricto. Mosquitoes were contaminated with either a low or a moderately high dose of oil-formulated conidia of M. anisopliae, and offered a single human blood meal 48, 72, or 96 h later to assess feeding propensity and individual blood meal size. In a second experiment, individual fungus-infected females were offered a blood meal every third day (to a total of 8 gonotrophic cycles), and allowed to oviposit after each cycle in order to quantify feeding propensity and fecundity. Infected females took smaller blood meals and displayed reduced feeding propensity. It was found that mosquitoes, inoculated with a moderately high dose of fungal conidia, exhibited reduced appetite related to increasing fungal growth. Of the fungus-infected females, the proportion of mosquitoes taking the second blood meal was reduced with 51%. This was further reduced to 35.3% by the 4th blood meal. During 8 feeding opportunities, the average number of blood meals taken by uninfected females was 4.39, against 3.40 (low dose), and 2.07 (high dose) blood meals for the fungus-infected females. Moreover, infected females produced fewer eggs per gonotrophic cycle and had a lower life-time fecundity. Epidemiological models show that both blood feeding and fecundity are among the most important factors affecting the likelihood of a mosquito transmitting malaria, which suggests that this fungus may have potential as biocontrol agent for vector-borne disease control.

Animals↗
PubMed2005

Effect of larval crowding on mating competitiveness of Anopheles gambiae mosquitoes.

BACKGROUND: The success of sterile or transgenic Anopheles for malaria control depends on their mating competitiveness within wild populations. Current evidence suggests that transgenic mosquitoes have reduced fitness. One means of compensating for this fitness deficit would be to identify environmental conditions that increase their mating competitiveness, and incorporate them into laboratory rearing regimes. METHODS: Anopheles gambiae larvae were allocated to three crowding treatments with the same food input per larva. Emerged males were competed against one another for access to females, and their corresponding longevity and energetic reserves measured. RESULTS: Males from the low-crowding treatment were much more likely to acquire the first mating. They won the first female approximately 11 times more often than those from the high-crowding treatment (Odds ratio = 11.17) and four times more often than those from the medium-crowding treatment (Odds ratio = 3.51). However, there was no overall difference in the total number of matings acquired by males from different treatments (p = 0.08). The survival of males from the low crowding treatment was lower than those from other treatments. The body size and teneral reserves of adult males did not differ between crowding treatments, but larger males were more likely to acquire mates than small individuals. CONCLUSION: Larval crowding and body size have strong, independent effects on the mating competitiveness of adult male An. gambiae. Thus manipulation of larval crowding during mass rearing could provide a simple technique for boosting the competitiveness of sterile or transgenic male mosquitoes prior to release.

Animals↗
PubMed2005

An entomopathogenic fungus for control of adult African malaria mosquitoes.

Biological control of malaria mosquitoes in Africa has rarely been used in vector control programs. Recent developments in this field show that certain fungi are virulent to adult Anopheles mosquitoes. Practical delivery of an entomopathogenic fungus that infected and killed adult Anopheles gambiae, Africa's main malaria vector, was achieved in rural African village houses. An entomological inoculation rate model suggests that implementation of this vector control method, even at the observed moderate coverage during a field study in Tanzania, would significantly reduce malaria transmission intensity.

Animals↗
PubMed2005

Comparative performance of the Mbita trap, CDC light trap and the human landing catch in the sampling of Anopheles arabiensis, An. funestus and culicine species in a rice irrigation in western Kenya.

BACKGROUND: Mosquitoes sampling is an important component in malaria control. However, most of the methods used have several shortcomings and hence there is a need to develop and calibrate new methods. The Mbita trap for capturing host-seeking mosquitoes was recently developed and successfully tested in Kenya. However, the Mbita trap is less effective at catching outdoor-biting Anopheles funestus and Anopheles arabiensis in Madagascar and, thus, there is need to further evaluate this trap in diverse epidemiological settings. This study reports a field evaluation of the Mbita trap in a rice irrigation scheme in Kenya METHODS: The mosquito sampling efficiency of the Mbita trap was compared to that of the CDC light trap and the human landing catch in western Kenya. Data was analysed by Bayesian regression of linear and non-linear models. RESULTS: The Mbita trap caught about 17%, 60%, and 20% of the number of An. arabiensis, An. funestus, and culicine species caught in the human landing collections respectively. There was consistency in sampling proportionality between the Mbita trap and the human landing catch for both An. arabiensis and the culicine species. For An. funestus, the Mbita trap portrayed some density-dependent sampling efficiency that suggested lowered sampling efficiency of human landing catch at low densities. The CDC light trap caught about 60%, 120%, and 552% of the number of An. arabiensis, An. funestus, and culicine species caught in the human landing collections respectively. There was consistency in the sampling proportionality between the CDC light trap and the human landing catch for both An. arabiensis and An. funestus, whereas for the culicines, there was no simple relationship between the two methods. CONCLUSIONS: The Mbita trap is less sensitive than either the human landing catch or the CDC light trap. However, for a given investment of time and money, it is likely to catch more mosquitoes over a longer (and hence more representative) period. This trap can therefore be recommended for use by community members for passive mosquito surveillance. Nonetheless, there is still a need to develop new sampling methods for some epidemiological settings. The human landing catch should be maintained as the standard reference method for use in calibrating new methods for sampling the human biting population of mosquitoes.

Animals↗
PubMed2005

Redressing the sex imbalance in knowledge of vector biology.

The recent development of transgenic mosquitoes that are resistant to infection by the Plasmodium malarial parasite is a promising new tool in the fight against malaria. However, results of large-scale field releases of alternatively modified mosquitoes carried out during the 1970s and 1980s suggest that this approach could be difficult to implement in the field. These past attempts to control mosquito populations largely floundered as a result of our insufficient understanding of the behavioural ecology of released males. In spite of this, contemporary research on genetic control strategies has concentrated predominantly on molecular aspects, with little progress being made toward resolving key ecological uncertainties, male mosquito ecology being the most important. Here, we review the state of knowledge of male mosquito ecology, and highlight priorities for further research. Case studies of two crop pests, the Mediterranean fruit fly and melon fly, are given as examples of how knowledge of male ecology facilitates successful control in other species. Unless similar information becomes available for mosquitoes, any future genetic control strategy will risk failure.

Journal Article↗
PubMed2004

Autodissemination of the entomopathogenic fungus Metarhizium anisopliae amongst adults of the malaria vector Anopheles gambiae s.s.

BACKGROUND: The entomopathogenic fungus Metarhizium anisopliae is being considered as a biocontrol agent for adult African malaria vectors. In the laboratory, work was carried out to assess whether horizontal transmission of the pathogen can take place during copulation, as this would enhance the impact of the fungus on target populations when compared with insecticides. METHODS: Virgin female Anopheles gambiae sensu stricto were exposed to conidia whilst resting on fungus-impregnated paper. These females were then placed together for one hour with uncontaminated males in proportions of either 1:1 or 1:10 shortly before the onset of mating activity. RESULTS: Males that had acquired fungal infection after mating indicate that passive transfer of the pathogen from infected females does occur, with mean male infection rates between 10.7 +/- 3.2% and 33.3 +/- 3.8%. The infections caused by horizontal transmission did not result in overall differences in survival between males from test and control groups, but in one of the three experiments the infected males had significantly shorter life spans than uninfected males (P < 0.05). CONCLUSION: This study shows that autodissemination of fungal inoculum between An. gambiae s.s. mosquitoes during mating activity is possible under laboratory conditions. Field studies are required next, to assess the extent to which this phenomenon may augment the primary contamination pathway (i.e. direct contact with fungus-impregnated targets) of vector populations in the field.

Animals↗
PubMed2004

Entomopathogenic fungi for mosquito control: a review.

Fungal diseases in insects are common and widespread and can decimate their populations in spectacular epizootics. Virtually all insect orders are susceptible to fungal diseases, including Dipterans. Fungal pathogens such as Lagenidium, Coelomomyces and Culicinomyces are known to affect mosquito populations, and have been studied extensively. There are, however, many other fungi that infect and kill mosquitoes at the larval and/or adult stage. The discovery, in 1977, of the selective mosquito-pathogenic bacterium Bacillus thuringiensis Berliner israelensis (Bti) curtailed widespread interest in the search for other suitable biological control agents. In recent years interest in mosquito-killing fungi is reviving, mainly due to continuous and increasing levels of insecticide resistance and increasing global risk of mosquito-borne diseases. This review presents an update of published data on mosquito-pathogenic fungi and mosquito-pathogen interactions, covering 13 different fungal genera. Notwithstanding the potential of many fungi as mosquito control agents, only a handful have been commercialized and are marketed for use in abatement programs. We argue that entomopathogenic fungi, both new and existing ones with renewed/improved efficacies may contribute to an expansion of the limited arsenal of effective mosquito control tools, and that they may contribute in a significant and sustainable manner to the control of vector-borne diseases such as malaria, dengue and filariasis.

Animals↗
PubMed2004

The practical importance of permanent and semipermanent habitats for controlling aquatic stages of Anopheles gambiae sensu lato mosquitoes: operational observations from a rural town in western Kenya.

Control of aquatic-stage Anopheles is one of the oldest and most historically successful interventions to prevent malaria, but it has seen little application in Africa. Consequently, the ecology of immature afrotropical Anopheles has received insufficient attention. We therefore examined the population dynamics of African anopheline and culicine mosquitoes using operationally practicable techniques to examine the relative importance and availability of different larval habitats in an area of perennial malaria transmission in preparation for a pilot-scale larval control programme. The study was conducted in Mbita, a rural town on the shores of Lake Victoria in Western Kenya, over 20 months. Weekly larval surveys were conducted to identify the availability of stagnant water, habitat characteristics and larval densities. Adult mosquitoes were collected indoors at fortnightly intervals. Availability of aquatic habitats and abundance of mosquito larvae were directly correlated with rainfall. Adult mosquito densities followed similar patterns but with a time-lag of approximately 1 month. About 70% of all available habitats were man-made, half of them representing cement-lined pits. On average, 67% of all aquatic habitats on a given sampling date were colonized by Anopheles larvae, of which all identified morphologically were A. gambiae sensu lato. Natural and artificial habitats were equally productive over the study period and larval densities were positively correlated with presence of tufts of low vegetation and negatively with non-matted algal content. The permanence of a habitat had no significant influence on larval productivity. We conclude that A. gambiae is broadly distributed across a variety of habitat types, regardless of permanence. All potential breeding sites need to be considered as sources of malaria risk at any time of the year and exhaustively targeted in any larval control intervention.

Animals↗
PubMed2004

Comparative field evaluation of the Mbita trap, the Centers for Disease Control light trap, and the human landing catch for sampling of malaria vectors in western Kenya.

The mosquito sampling efficiency of a new bed net trap (the Mbita trap) was compared with that of the Centers for Disease Control miniature light trap (hung adjacent to an occupied bed net) and the human landing catch in western Kenya. Overall, the Mbita trap caught 48.7 +/- 4.8% (mean +/- SEM) the number of Anopheles gambiae Giles sensu lato caught in the human landing catch and 27.4 +/- 8.2% of the number caught by the light trap. The corresponding figures for Anopheles funestus Giles were 74.6 +/- 1.3% and 39.2 +/- 1.9%, respectively. Despite the clear differences in the numbers of mosquitoes caught by each method, both the Mbita trap and light trap catches were directly proportional to human landing catches regardless of mosquito density. No significant differences in parity or sporozoite incidence were observed between mosquitoes caught by the three methods for either An. gambiae s.l. or An. funestus. Identification of the sibling species of the An. gambiae complex by a polymerase chain reaction indicated that the ratio of An. gambiae Giles sensu stricto to An. arabiensis Patton did not vary according to the sampling method used. It is concluded that the Mbita trap is a promising tool for sampling malaria vector populations since its catch can be readily converted into equivalent human biting catch, it can be applied more intensively, it requires neither expensive equipment nor skilled personnel, and it samples mosquitoes in an exposure-free manner. Such intensive sampling capability will allow cost-effective surveillance of malaria transmission at much finer spatial and temporal resolution than has been previously possible.

Animals↗
PubMed2004

Rationalizing historical successes of malaria control in Africa in terms of mosquito resource availability management.

Environmental management of mosquito resources is a promising approach with which to control malaria, but it has seen little application in Africa for more than half a century. Here we present a kinetic model of mosquito foraging for aquatic habitats and vertebrate hosts that allows estimation of malaria transmission intensity by defining the availability of these resources as the rate at which individual mosquitoes encounter and use them. The model captures historically observed responses of malaria transmission to environmental change, highlights important gaps in current understanding of vector ecology, and suggests convenient solutions. Resource availability is an intuitive concept that provides an adaptable framework for models of mosquito population dynamics, gene flow, and pathogen transmission that can be conveniently parameterized with direct field measurements. Furthermore, the model presented predicts that drastic reductions of malaria transmission are possible with environmental management and elucidates an ecologic basis for previous successes of integrated malaria control in Africa before the advent of DDT or chloroquine. Environmental management for malaria control requires specialist skills that are currently lacking in sub-Saharan Africa where they are needed most. Infrastructure and human capacity building in clinical, public health, and environmental disciplines should therefore be prioritized so that growing financial support for tackling malaria can be translated into truly integrated control programs.

Animals↗