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Zhou G, Zhong D, Lee MC, Wang X, Atieli HE, Githure JI, Githeko AK, Kazura J, Yan G. Multi-Indicator and Multistep Assessment of Malaria Transmission Risks in Western Kenya. Am J Trop Med Hyg 2021; 104:1359-1370. [PMID: 33556042 DOI: 10.4269/ajtmh.20-1211] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/16/2020] [Accepted: 12/24/2020] [Indexed: 11/07/2022] Open
Abstract
Malaria risk factor assessment is a critical step in determining cost-effective intervention strategies and operational plans in a regional setting. We develop a multi-indicator multistep approach to model the malaria risks at the population level in western Kenya. We used a combination of cross-sectional seasonal malaria infection prevalence, vector density, and cohort surveillance of malaria incidence at the village level to classify villages into malaria risk groups through unsupervised classification. Generalized boosted multinomial logistics regression analysis was performed to determine village-level risk factors using environmental, biological, socioeconomic, and climatic features. Thirty-six villages in western Kenya were first classified into two to five operational groups based on different combinations of malaria risk indicators. Risk assessment indicated that altitude accounted for 45-65% of all importance value relative to all other factors; all other variable importance values were < 6% in all models. After adjusting by altitude, villages were classified into three groups within distinct geographic areas regardless of the combination of risk indicators. Risk analysis based on altitude-adjusted classification indicated that factors related to larval habitat abundance accounted for 63% of all importance value, followed by geographic features related to the ponding effect (17%), vegetation cover or greenness (15%), and the number of bed nets combined with February temperature (5%). These results suggest that altitude is the intrinsic factor in determining malaria transmission risk in western Kenya. Malaria vector larval habitat management, such as habitat reduction and larviciding, may be an important supplement to the current first-line vector control tools in the study area.
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Affiliation(s)
- Guofa Zhou
- 1Program in Public Health, University of California, Irvine, California
| | - Daibin Zhong
- 1Program in Public Health, University of California, Irvine, California
| | - Ming-Chieh Lee
- 1Program in Public Health, University of California, Irvine, California
| | - Xiaoming Wang
- 1Program in Public Health, University of California, Irvine, California
| | - Harrysone E Atieli
- 2School of Public Health and Community Development, Maseno University, Kisumu, Kenya
| | - John I Githure
- 3International Center of Excellence in Malaria Research, Tom Mboya University College, Homabay, Kenya
| | - Andrew K Githeko
- 4Centre for Global Health Research, Kenya Medical Research Institute, Kisumu, Kenya
| | - James Kazura
- 5Center for Global Health and Diseases, Case Western Reserve University, Cleveland, Ohio
| | - Guiyun Yan
- 1Program in Public Health, University of California, Irvine, California
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2
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Mwingira VS, Mboera LEG, Takken W. Synergism between nonane and emanations from soil as cues in oviposition-site selection of natural populations of Anopheles gambiae and Culex quinquefasciatus. Malar J 2021; 20:52. [PMID: 33478526 PMCID: PMC7819190 DOI: 10.1186/s12936-020-03575-0] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/21/2020] [Accepted: 12/31/2020] [Indexed: 11/10/2022] Open
Abstract
BACKGROUND Olfactory cues have been shown to have an important role in guiding gravid mosquito females to selected sites for egg laying. The objective of this study was to determine the influence of emanations from soil from a breeding site and the putative oviposition pheromone nonane on oviposition-site selection of natural populations of Anopheles gambiae sensu lato (s.l.) and Culex quinquefasciatus. METHODS This field-based study was conducted in Mvomero District in East-central Tanzania. In a dual-choice experimental set up, clay bowls were dug into the ground and filled with one of the following treatments: (i) distilled water + autoclaved soil (control), (ii) distilled water + soil from a natural mosquito breeding site, (iii) distilled water + nonane and (iv) distilled water + nonane + soil from a natural breeding site. Soil was dried and autoclaved or dried only before use. After five days of incubation, larvae were collected daily for 10 days. The median number of larvae per bowl per day was used as outcome measure. RESULTS Autoclaved soil had a significant attractive effect on oviposition behaviour of Cx. quinquefasciatus (median values ± s.e: 8.0 ± 1.1; P < 0.005) but no effect on An. gambiae (median value ± s.e: 0.0 ± 0.2; P = 0.18). Nonane and emanations from untreated soil significantly and positively influenced the selection of oviposition sites by both An. gambiae s.l. (median values ± s.e.: 12.0 ± 2.0 and 4.5 ± 1.5, respectively; P < 0.0001) and Cx. quinquefasciatus (median values ± s.e.: 19.0 ± 1.3 and 17.0 ± 2.0, respectively; P < 0.0001). A mixture of nonane and untreated soil caused a synergistic effect on oviposition behaviour in An. gambiae s.l. (median value ± s.e.: 23.5 ± 2.5; P < 0.0001) compared to either nonane (median values ± s.e.: 12.0 ± 2.0; P < 0.0001) or untreated soil alone (median value ± s.e.: 4.5 ± 1.5; P < 0.0001). A synergistic effect of nonane mixed with untreated soil was also found in Cx. quinquefasciatus (median value ± s.e.: 41.0 ± 2.1; P < 0.0001) compared to either nonane (median value ± s.e. 19.0 ± 1.3; P < 0.0001) or untreated soil alone (median value ± s.e.: 17.0 ± 2.0; P < 0.0001). The oviposition activity index for An. gambiae was 0.56 (P < 0.001) and for Cx. quinquefasciatus 0.59 (P < 0.0001). CONCLUSIONS The larval pheromone nonane and emanations from breeding-site soil both induced oviposition in wild An. gambiae s.l. and Cx. quinquefasciatus, with a synergistic effect when both stimuli were present simultaneously. This is the first study in which nonane is shown to cause oviposition under natural conditions, suggesting that this compound can potentially be exploited for the management of mosquito vectors.
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Affiliation(s)
- Victor S Mwingira
- Laboratory of Entomology, Wageningen University & Research, P.O. Box 16, 6700 AA, Wageningen, The Netherlands.,SACIDS Foundation for One Health, Sokoine University of Agriculture, Chuo Kikuu, P.O. Box 3297, Morogoro, Tanzania
| | - Leonard E G Mboera
- SACIDS Foundation for One Health, Sokoine University of Agriculture, Chuo Kikuu, P.O. Box 3297, Morogoro, Tanzania
| | - Willem Takken
- Laboratory of Entomology, Wageningen University & Research, P.O. Box 16, 6700 AA, Wageningen, The Netherlands.
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3
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Ng'habi K, Viana M, Matthiopoulos J, Lyimo I, Killeen G, Ferguson HM. Mesocosm experiments reveal the impact of mosquito control measures on malaria vector life history and population dynamics. Sci Rep 2018; 8:13949. [PMID: 30224714 PMCID: PMC6141522 DOI: 10.1038/s41598-018-31805-8] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/20/2018] [Accepted: 08/24/2018] [Indexed: 11/29/2022] Open
Abstract
The impact of control measures on mosquito vector fitness and demography is usually estimated from bioassays or indirect variables in the field. Whilst indicative, neither approach is sufficient to quantify the potentially complex response of mosquito populations to combined interventions. Here, large replicated mesocosms were used to measure the population-level response of the malaria vector Anopheles arabiensis to long-lasting insecticidal nets (LLINs) when used in isolation, or combined with insecticidal eave louvers (EL), or treatment of cattle with the endectocide Ivermectin (IM). State-space models (SSM) were fit to these experimental data, revealing that LLIN introduction reduced adult mosquito survival by 91% but allowed population persistence. ELs provided no additional benefit, but IM reduced mosquito fecundity by 59% and nearly eliminated all populations when combined with LLINs. This highlights the value of IM for integrated vector control, and mesocosm population experiments combined with SSM for identifying optimal combinations for vector population elimination.
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Affiliation(s)
- Kija Ng'habi
- Ifakara Health Institute, Environmental Health and Ecological Sciences, Ifakara, United Republic of Tanzania
- School of Health Sciences, University of Dar es Salaam, Dar es Salaam, Tanzania
| | - Mafalda Viana
- Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
| | - Jason Matthiopoulos
- Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, G12 8QQ, United Kingdom
| | - Issa Lyimo
- Ifakara Health Institute, Environmental Health and Ecological Sciences, Ifakara, United Republic of Tanzania
| | - Gerry Killeen
- Ifakara Health Institute, Environmental Health and Ecological Sciences, Ifakara, United Republic of Tanzania
- Liverpool School of Tropical Medicine, Department of Vector Biology, Liverpool, United Kingdom
| | - Heather M Ferguson
- Institute of Biodiversity, Animal Health and Comparative Medicine, College of Medical, Veterinary and Life Sciences, University of Glasgow, Glasgow, G12 8QQ, United Kingdom.
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4
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Killeen GF, Reed TE. The portfolio effect cushions mosquito populations and malaria transmission against vector control interventions. Malar J 2018; 17:291. [PMID: 30097031 PMCID: PMC6086012 DOI: 10.1186/s12936-018-2441-z] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/22/2018] [Accepted: 08/02/2018] [Indexed: 12/03/2022] Open
Abstract
BACKGROUND Portfolio effects were first described as a basis for mitigating against financial risk by diversifying investments. Distributing investment across several different assets can stabilize returns and reduce risks by statistical averaging of individual asset dynamics that often correlate weakly or negatively with each other. The same simple probability theory is equally applicable to complex ecosystems, in which biological and environmental diversity stabilizes ecosystems against natural and human-mediated perturbations. Given the fundamental limitations to how well the full complexity of ecosystem dynamics can be understood or anticipated, the portfolio effect concept provides a simple framework for more critical data interpretation and pro-active conservation management. Applied to conservation ecology purposes, the portfolio effect concept informs management strategies emphasizing identification and maintenance of key ecological processes that generate complexity, diversity and resilience against inevitable, often unpredictable perturbations. IMPLICATIONS Applied to the reciprocal goal of eliminating the least valued elements of global biodiversity, specifically lethal malaria parasites and their vector mosquitoes, simply understanding the portfolio effect concept informs more cautious interpretation of surveillance data and simulation model predictions. Malaria transmission mediated by guilds of multiple vectors in complex landscapes, with highly variable climatic and meteorological conditions, as well as changing patterns of land use and other human behaviours, will systematically tend to be more resilient to attack with vector control than it appears based on even the highest quality surveillance data or predictive models. CONCLUSION Malaria vector control programmes may need to be more ambitious, interpret their short-to-medium term assessments of intervention impact more cautiously, and manage stakeholder expectations more conservatively than has often been the case thus far.
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Affiliation(s)
- Gerry F Killeen
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Ifakara, United Republic of Tanzania.
- Vector Biology Department, Liverpool School of Tropical Medicine, Pembroke Place, Liverpool, L3 5QA, UK.
| | - Thomas E Reed
- School of Biological, Earth and Environmental Sciences, University College Cork, Western Road, Cork, Republic of Ireland
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5
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Killeen GF, Marshall JM, Kiware SS, South AB, Tusting LS, Chaki PP, Govella NJ. Measuring, manipulating and exploiting behaviours of adult mosquitoes to optimise malaria vector control impact. BMJ Glob Health 2017; 2:e000212. [PMID: 28589023 PMCID: PMC5444085 DOI: 10.1136/bmjgh-2016-000212] [Citation(s) in RCA: 49] [Impact Index Per Article: 7.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2016] [Revised: 12/15/2016] [Accepted: 12/19/2016] [Indexed: 11/04/2022] Open
Abstract
Residual malaria transmission can persist despite high coverage with effective long-lasting insecticidal nets (LLINs) and/or indoor residual spraying (IRS), because many vector mosquitoes evade them by feeding on animals, feeding outdoors, resting outdoors or rapidly exiting from houses after entering them. However, many of these behaviours that render vectors resilient to control with IRS and LLINs also make them vulnerable to some emerging new alternative interventions. Furthermore, vector control measures targeting preferred behaviours of mosquitoes often force them to express previously rare alternative behaviours, which can then be targeted with these complementary new interventions. For example, deployment of LLINs against vectors that historically fed predominantly indoors on humans typically results in persisting transmission by residual populations that survive by feeding outdoors on humans and animals, where they may then be targeted with vapour-phase insecticides and veterinary insecticides, respectively. So while the ability of mosquitoes to express alternative behaviours limits the impact of LLINs and IRS, it also creates measurable and unprecedented opportunities for deploying complementary additional approaches that would otherwise be ineffective. Now that more diverse vector control methods are finally becoming available, well-established entomological field techniques for surveying adult mosquito behaviours should be fully exploited by national malaria control programmes, to rationally and adaptively map out new opportunities for their effective deployment.
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Affiliation(s)
- Gerry F Killeen
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Ifakara and Dar es Salaam, United Republic of Tanzania.,Department of Vector Biology, Liverpool School of Tropical Medicine, Liverpool, UK
| | - John M Marshall
- Divisions of Biostatistics and Epidemiology, School of Public Health, University of California, Berkeley, California, USA
| | - Samson S Kiware
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Ifakara and Dar es Salaam, United Republic of Tanzania
| | | | - Lucy S Tusting
- Oxford Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, Oxford, UK
| | - Prosper P Chaki
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Ifakara and Dar es Salaam, United Republic of Tanzania
| | - Nicodem J Govella
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Ifakara and Dar es Salaam, United Republic of Tanzania
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Killeen GF, Tatarsky A, Diabate A, Chaccour CJ, Marshall JM, Okumu FO, Brunner S, Newby G, Williams YA, Malone D, Tusting LS, Gosling RD. Developing an expanded vector control toolbox for malaria elimination. BMJ Glob Health 2017; 2:e000211. [PMID: 28589022 PMCID: PMC5444090 DOI: 10.1136/bmjgh-2016-000211] [Citation(s) in RCA: 74] [Impact Index Per Article: 10.6] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/12/2016] [Revised: 11/30/2016] [Accepted: 12/11/2016] [Indexed: 11/21/2022] Open
Abstract
Vector control using long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) accounts for most of the malaria burden reductions achieved recently in low and middle-income countries (LMICs). LLINs and IRS are highly effective, but are insufficient to eliminate malaria transmission in many settings because of operational constraints, growing resistance to available insecticides and mosquitoes that behaviourally avoid contact with these interventions. However, a number of substantive opportunities now exist for rapidly developing and implementing more diverse, effective and sustainable malaria vector control strategies for LMICs. For example, mosquito control in high-income countries is predominantly achieved with a combination of mosquito-proofed housing and environmental management, supplemented with large-scale insecticide applications to larval habitats and outdoor spaces that kill off vector populations en masse, but all these interventions remain underused in LMICs. Programmatic development and evaluation of decentralised, locally managed systems for delivering these proactive mosquito population abatement practices in LMICs could therefore enable broader scale-up. Furthermore, a diverse range of emerging or repurposed technologies are becoming available for targeting mosquitoes when they enter houses, feed outdoors, attack livestock, feed on sugar or aggregate into mating swarms. Global policy must now be realigned to mobilise the political and financial support necessary to exploit these opportunities over the decade ahead, so that national malaria control and elimination programmes can access a much broader, more effective set of vector control interventions.
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Affiliation(s)
- Gerry F Killeen
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, United Republic of Tanzania
- Department of Vector Biology, Liverpool School of Tropical Medicine, Liverpool, UK
| | - Allison Tatarsky
- Malaria Elimination Initiative, Global Health Group, University of California, San Francisco, California, USA
| | - Abdoulaye Diabate
- Institut de Recherche en Sciences de la Santé/Centre Muraz, Bobo-Dioulasso, Burkina Faso
| | - Carlos J Chaccour
- Instituto de Salud Global, Barcelona Centre for International Health Research (CRESIB), Hospital Clínic, Universitat de Barcelona, Barcelona, Spain
- Instituto de Salud Tropical, Universidad de Navarra, Pamplona, Spain
| | - John M Marshall
- Divisions of Biostatistics and Epidemiology, School of Public Health, University of California, Berkeley, California, USA
| | - Fredros O Okumu
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, United Republic of Tanzania
- School of Public Health, University of the Witwatersrand, Johannesburg, South Africa
| | - Shannon Brunner
- Malaria Elimination Initiative, Global Health Group, University of California, San Francisco, California, USA
| | - Gretchen Newby
- Malaria Elimination Initiative, Global Health Group, University of California, San Francisco, California, USA
| | - Yasmin A Williams
- Malaria Elimination Initiative, Global Health Group, University of California, San Francisco, California, USA
| | - David Malone
- Innovative Vector Control Consortium, Liverpool, UK
| | - Lucy S Tusting
- Oxford Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, Oxford, UK
| | - Roland D Gosling
- Malaria Elimination Initiative, Global Health Group, University of California, San Francisco, California, USA
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Killeen GF, Kiware SS, Okumu FO, Sinka ME, Moyes CL, Massey NC, Gething PW, Marshall JM, Chaccour CJ, Tusting LS. Going beyond personal protection against mosquito bites to eliminate malaria transmission: population suppression of malaria vectors that exploit both human and animal blood. BMJ Glob Health 2017; 2:e000198. [PMID: 28589015 PMCID: PMC5444054 DOI: 10.1136/bmjgh-2016-000198] [Citation(s) in RCA: 56] [Impact Index Per Article: 8.0] [Reference Citation Analysis] [Abstract] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/26/2016] [Revised: 11/09/2016] [Accepted: 11/13/2016] [Indexed: 11/03/2022] Open
Abstract
Protecting individuals and households against mosquito bites with long-lasting insecticidal nets (LLINs) or indoor residual spraying (IRS) can suppress entire populations of unusually efficient malaria vector species that predominantly feed indoors on humans. Mosquitoes which usually feed on animals are less reliant on human blood, so they are far less vulnerable to population suppression effects of such human-targeted insecticidal measures. Fortunately, the dozens of mosquito species which primarily feed on animals are also relatively inefficient vectors of malaria, so personal protection against mosquito bites may be sufficient to eliminate transmission. However, a handful of mosquito species are particularly problematic vectors of residual malaria transmission, because they feed readily on both humans and animals. These unusual vectors feed often enough on humans to be potent malaria vectors, but also often enough on animals to evade population control with LLINs, IRS or any other insecticidal personal protection measure targeted only to humans. Anopheles arabiensis and A. coluzzii in Africa, A. darlingi in South America and A. farauti in Oceania, as well as A. culicifacies species E, A. fluviatilis species S, A. lesteri and A. minimus in Asia, all feed readily on either humans or animals and collectively mediate residual malaria transmission across most of the tropics. Eliminating malaria transmission by vectors exhibiting such dual host preferences will require aggressive mosquito population abatement, rather than just personal protection of humans. Population suppression of even these particularly troublesome vectors is achievable with a variety of existing vector control technologies that remain underdeveloped or underexploited.
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Affiliation(s)
- Gerry F Killeen
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Dar es Salaam, United Republic of Tanzania
- Department of Vector Biology, Liverpool School of Tropical Medicine, Liverpool, UK
| | - Samson S Kiware
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Dar es Salaam, United Republic of Tanzania
| | - Fredros O Okumu
- Environmental Health and Ecological Sciences Department, Ifakara Health Institute, Dar es Salaam, United Republic of Tanzania
- School of Public Health, University of the Witwatersrand, Johannesburg, South Africa
| | | | - Catherine L Moyes
- Oxford Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, Oxford, UK
| | | | - Peter W Gething
- Oxford Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, Oxford, UK
| | - John M Marshall
- Divisions of Biostatistics and Epidemiology, School of Public Health, University of California, Berkeley, California, USA
| | - Carlos J Chaccour
- Instituto de Salud Global, Barcelona Centre for International Health Research (CRESIB), Hospital Clínic, Universitat de Barcelona, Barcelona, Spain
- Instituto de Salud Tropical, Universidad de Navarra, Pamplona, Spain
| | - Lucy S Tusting
- Oxford Big Data Institute, Li Ka Shing Centre for Health Information and Discovery, University of Oxford, Oxford, UK
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Killeen GF, Govella NJ, Lwetoijera DW, Okumu FO. Most outdoor malaria transmission by behaviourally-resistant Anopheles arabiensis is mediated by mosquitoes that have previously been inside houses. Malar J 2016; 15:225. [PMID: 27093890 PMCID: PMC4837512 DOI: 10.1186/s12936-016-1280-z] [Citation(s) in RCA: 93] [Impact Index Per Article: 11.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/19/2015] [Accepted: 04/12/2016] [Indexed: 11/10/2022] Open
Abstract
Background Anopheles arabiensis is stereotypical of diverse vectors that mediate residual malaria transmission globally, because it can feed outdoors upon humans or cattle, or enter but then rapidly exit houses without fatal exposure to insecticidal nets or sprays. Methods Life histories of a well-characterized An. arabiensis population were simulated with a simple but process-explicit deterministic model and relevance to other vectors examined through sensitivity analysis. Results Where most humans use bed nets, two thirds of An. arabiensis blood feeds and half of malaria transmission events were estimated to occur outdoors. However, it was also estimated that most successful feeds and almost all (>98 %) transmission events are preceded by unsuccessful attempts to attack humans indoors. The estimated proportion of vector blood meals ultimately obtained from humans indoors is dramatically attenuated by availability of alternative hosts, or partial ability to attack humans outdoors. However, the estimated proportion of mosquitoes old enough to transmit malaria, and which have previously entered a house at least once, is far less sensitive to both variables. For vectors with similarly modest preference for cattle over humans and similar ability to evade fatal indoor insecticide exposure once indoors, >80 % of predicted feeding events by mosquitoes old enough to transmit malaria are preceded by at least one house entry event, so long as ≥40 % of attempts to attack humans occur indoors and humans outnumber cattle ≥4-fold. Conclusions While the exact numerical results predicted by such a simple deterministic model should be considered only approximate and illustrative, the derived conclusions are remarkably insensitive to substantive deviations from the input parameter values measured for this particular An. arabiensis population. This life-history analysis, therefore, identifies a clear, broadly-important opportunity for more effective suppression of residual malaria transmission by An. arabiensis in Africa and other important vectors of residual transmission across the tropics. Improved control of predominantly outdoor residual transmission by An. arabiensis, and other modestly zoophagic vectors like Anopheles darlingi, which frequently enter but then rapidly exit from houses, may be readily achieved by improving existing technology for killing mosquitoes indoors.
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Affiliation(s)
- Gerry F Killeen
- Environmental Health and Ecological Sciences Thematic Group, Ifakara Health Institute, Ifakara, Kilombero, Morogoro, United Republic of Tanzania. .,Department of Vector Biology, Liverpool School of Tropical Medicine, Liverpool, UK.
| | - Nicodem J Govella
- Environmental Health and Ecological Sciences Thematic Group, Ifakara Health Institute, Ifakara, Kilombero, Morogoro, United Republic of Tanzania
| | - Dickson W Lwetoijera
- Environmental Health and Ecological Sciences Thematic Group, Ifakara Health Institute, Ifakara, Kilombero, Morogoro, United Republic of Tanzania
| | - Fredros O Okumu
- Environmental Health and Ecological Sciences Thematic Group, Ifakara Health Institute, Ifakara, Kilombero, Morogoro, United Republic of Tanzania.,School of Public Health, University of the Witwatersrand, Johannesburg, Republic of South Africa
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9
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Brady OJ, Godfray HCJ, Tatem AJ, Gething PW, Cohen JM, McKenzie FE, Perkins TA, Reiner RC, Tusting LS, Sinka ME, Moyes CL, Eckhoff PA, Scott TW, Lindsay SW, Hay SI, Smith DL. Vectorial capacity and vector control: reconsidering sensitivity to parameters for malaria elimination. Trans R Soc Trop Med Hyg 2016; 110:107-17. [PMID: 26822603 PMCID: PMC4731004 DOI: 10.1093/trstmh/trv113] [Citation(s) in RCA: 109] [Impact Index Per Article: 13.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/22/2022] Open
Abstract
BACKGROUND Major gains have been made in reducing malaria transmission in many parts of the world, principally by scaling-up coverage with long-lasting insecticidal nets and indoor residual spraying. Historically, choice of vector control intervention has been largely guided by a parameter sensitivity analysis of George Macdonald's theory of vectorial capacity that suggested prioritizing methods that kill adult mosquitoes. While this advice has been highly successful for transmission suppression, there is a need to revisit these arguments as policymakers in certain areas consider which combinations of interventions are required to eliminate malaria. METHODS AND RESULTS Using analytical solutions to updated equations for vectorial capacity we build on previous work to show that, while adult killing methods can be highly effective under many circumstances, other vector control methods are frequently required to fill effective coverage gaps. These can arise due to pre-existing or developing mosquito physiological and behavioral refractoriness but also due to additive changes in the relative importance of different vector species for transmission. Furthermore, the optimal combination of interventions will depend on the operational constraints and costs associated with reaching high coverage levels with each intervention. CONCLUSIONS Reaching specific policy goals, such as elimination, in defined contexts requires increasingly non-generic advice from modelling. Our results emphasize the importance of measuring baseline epidemiology, intervention coverage, vector ecology and program operational constraints in predicting expected outcomes with different combinations of interventions.
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Affiliation(s)
- Oliver J Brady
- The Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK
| | | | - Andrew J Tatem
- Department of Geography and Environment, University of Southampton, Southampton, UK Fogarty International Center, NIH, Bethesda, MD, USA Flowminder Foundation, Stockholm, Sweden
| | - Peter W Gething
- Spatial Ecology and Epidemiology Group, Department of Zoology, Oxford University, Oxford, UK
| | | | | | - T Alex Perkins
- Fogarty International Center, NIH, Bethesda, MD, USA Department of Biological Sciences & Eck Institute for Global Health, University of Notre Dame, Notre Dame, IN, USA
| | - Robert C Reiner
- Fogarty International Center, NIH, Bethesda, MD, USA Department of Epidemiology & Biostatistics, Indiana University, Bloomington, IN, USA
| | - Lucy S Tusting
- Department of Disease Control, London School of Hygiene and Tropical Medicine, London, UK
| | - Marianne E Sinka
- The Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK Department of Zoology, University of Oxford, Oxford, UK
| | - Catherine L Moyes
- The Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK
| | | | - Thomas W Scott
- Fogarty International Center, NIH, Bethesda, MD, USA Department of Entomology and Nematology, University of California, Davis, CA, USA
| | - Steven W Lindsay
- School of Biological & Biomedical Sciences, Durham University, Durham, UK
| | - Simon I Hay
- The Wellcome Trust Centre for Human Genetics, University of Oxford, Oxford, UK Fogarty International Center, NIH, Bethesda, MD, USA Institute for Health Metrics and Evaluation, University of Washington, Seattle, WA, USA
| | - David L Smith
- Department of Zoology, University of Oxford, Oxford, UK Fogarty International Center, NIH, Bethesda, MD, USA Institute for Health Metrics and Evaluation, University of Washington, Seattle, WA, USA Sanaria Institute for Global Health and Tropical Medicine, Rockville, MD, USA
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10
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Stone C, Chitnis N, Gross K. Environmental influences on mosquito foraging and integrated vector management can delay the evolution of behavioral resistance. Evol Appl 2016; 9:502-17. [PMID: 26989441 PMCID: PMC4778105 DOI: 10.1111/eva.12354] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/03/2015] [Accepted: 12/21/2015] [Indexed: 11/29/2022] Open
Abstract
Along with the scaled‐up distribution of long‐lasting insecticidal nets for malaria control has become concern about insecticide resistance. A related concern regards the evolution of host‐seeking periodicity from the nocturnal to the crepuscular periods of the day. Why we observe such shifts in some areas but not others and which methods could prove useful in managing such behavioral resistance remain open questions. We developed a foraging model to explore whether environmental conditions affect the evolution of behavioral resistance. We looked at the role of the abundance of blood hosts and nectar sources and investigated the potential of attractive toxic sugar baits for integrated control. Higher encounter rates with hosts and nectar sources allowed behaviorally resistant populations to persist at higher levels of bed net coverage. Whereas higher encounter rates with nectar increased the threshold where resistance emerged, higher encounter rates of hosts lowered this threshold. Adding sugar baits lowered the coverage level of bed nets required to eliminate the vector population. In certain environments, using lower bed net coverage levels together with toxic sugar baits may delay or prevent the evolution of behavioral resistance. Designing sustainable control strategies will depend on an understanding of vector behavior expressed in local environmental conditions.
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Affiliation(s)
- Chris Stone
- Department of Statistics North Carolina State University Raleigh NC USA
| | - Nakul Chitnis
- Swiss Tropical and Public Health Institute Basel Switzerland; University of Basel Basel Switzerland
| | - Kevin Gross
- Department of Statistics North Carolina State University Raleigh NC USA
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Kiware SS, Corliss G, Merrill S, Lwetoijera DW, Devine G, Majambere S, Killeen GF. Predicting Scenarios for Successful Autodissemination of Pyriproxyfen by Malaria Vectors from Their Resting Sites to Aquatic Habitats; Description and Simulation Analysis of a Field-Parameterizable Model. PLoS One 2015; 10:e0131835. [PMID: 26186730 PMCID: PMC4505906 DOI: 10.1371/journal.pone.0131835] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2014] [Accepted: 06/07/2015] [Indexed: 11/25/2022] Open
Abstract
BACKGROUND Large-cage experiments indicate pyriproxifen (PPF) can be transferred from resting sites to aquatic habitats by Anopheles arabiensis--malaria vector mosquitoes to inhibit emergence of their own offspring. PPF coverage is amplified twice: (1) partial coverage of resting sites with PPF contamination results in far higher contamination coverage of adult mosquitoes because they are mobile and use numerous resting sites per gonotrophic cycle, and (2) even greater contamination coverage of aquatic habitats results from accumulation of PPF from multiple oviposition events. METHODS AND FINDINGS Deterministic mathematical models are described that use only field-measurable input parameters and capture the biological processes that mediate PPF autodissemination. Recent successes in large cages can be rationalized, and the plausibility of success under full field conditions can be evaluated a priori. The model also defines measurable properties of PPF delivery prototypes that may be optimized under controlled experimental conditions to maximize chances of success in full field trials. The most obvious flaw in this model is the endogenous relationship that inevitably occurs between the larval habitat coverage and the measured rate of oviposition into those habitats if the target mosquito species is used to mediate PPF transfer. However, this inconsistency also illustrates the potential advantages of using a different, non-target mosquito species for contamination at selected resting sites that shares the same aquatic habitats as the primary target. For autodissemination interventions to eliminate malaria transmission or vector populations during the dry season window of opportunity will require comprehensive contamination of the most challenging subset of aquatic habitats [Formula: see text] that persist or retain PPF activity (Ux) for only one week [Formula: see text], where Ux = 7 days). To achieve >99% contamination coverage of these habitats will necessitate values for the product of the proportional coverage of the ovipositing mosquito population with PPF contamination (CM) by the ovitrap-detectable rates of oviposition by wild mosquitoes into this subset of habitats [Formula: see text], divided by the titre of contaminated mosquitoes required to render them unproductive [Formula: see text], that approximately approach unity [Formula: see text]. CONCLUSIONS The simple multiplicative relationship between CM and [Formula: see text], and the simple exponential decay effect they have upon uncontaminated aquatic habitats, allows application of this model by theoreticians and field biologists alike.
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Affiliation(s)
- Samson S. Kiware
- Environmental Health and Ecological Sciences Thematic Group, Ifakara |Health Institute, P.O. Box 53, Ifakara, Tanzania
- Department of Mathematics, Statistics and Computer Science, Marquette University, Milwaukee, WI, 53201–1881, United States of America
| | - George Corliss
- Department of Mathematics, Statistics and Computer Science, Marquette University, Milwaukee, WI, 53201–1881, United States of America
| | - Stephen Merrill
- Department of Mathematics, Statistics and Computer Science, Marquette University, Milwaukee, WI, 53201–1881, United States of America
| | - Dickson W. Lwetoijera
- Environmental Health and Ecological Sciences Thematic Group, Ifakara |Health Institute, P.O. Box 53, Ifakara, Tanzania
- Liverpool School of Tropical Medicine, Vector Biology Department, Pembroke Place, Liverpool, L3 5QA, United Kingdom
| | - Gregor Devine
- QIMR Berghofer Medical Research Institute, Brisbane, Queensland, Australia
| | - Silas Majambere
- Environmental Health and Ecological Sciences Thematic Group, Ifakara |Health Institute, P.O. Box 53, Ifakara, Tanzania
- Liverpool School of Tropical Medicine, Vector Biology Department, Pembroke Place, Liverpool, L3 5QA, United Kingdom
| | - Gerry F. Killeen
- Environmental Health and Ecological Sciences Thematic Group, Ifakara |Health Institute, P.O. Box 53, Ifakara, Tanzania
- Liverpool School of Tropical Medicine, Vector Biology Department, Pembroke Place, Liverpool, L3 5QA, United Kingdom
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Killeen GF. Characterizing, controlling and eliminating residual malaria transmission. Malar J 2014; 13:330. [PMID: 25149656 PMCID: PMC4159526 DOI: 10.1186/1475-2875-13-330] [Citation(s) in RCA: 303] [Impact Index Per Article: 30.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/30/2014] [Accepted: 08/16/2014] [Indexed: 12/02/2022] Open
Abstract
Long-lasting insecticidal nets (LLINs) and indoor residual spraying (IRS) interventions can reduce malaria transmission by targeting mosquitoes when they feed upon sleeping humans and/or rest inside houses, livestock shelters or other man-made structures. However, many malaria vector species can maintain robust transmission, despite high coverage of LLINs/IRS containing insecticides to which they are physiologically fully susceptible, because they exhibit one or more behaviours that define the biological limits of achievable impact with these interventions: (1) Natural or insecticide-induced avoidance of contact with treated surfaces within houses and early exit from them, thus minimizing exposure hazard of vectors which feed indoors upon humans; (2) Feeding upon humans when they are active and unprotected outdoors, thereby attenuating personal protection and any consequent community-wide suppression of transmission; (3) Feeding upon animals, thus minimizing contact with insecticides targeted at humans or houses; (4) Resting outdoors, away from insecticide-treated surfaces of nets, walls and roofs. Residual malaria transmission is, therefore, defined as all forms of transmission that can persist after achieving full universal coverage with effective LLINs and/or IRS containing active ingredients to which local vector populations are fully susceptible. Residual transmission is sufficiently intense across most of the tropics to render malaria elimination infeasible without new or improved vector control methods. Many novel or improved vector control strategies to address residual transmission are emerging that either: (1) Enhance control of adult vectors that enter houses to feed and/or rest by killing, repelling or excluding them; (2) Kill or repel adult mosquitoes when they attack people outdoors; (3) Kill adult mosquitoes when they attack livestock; (4) Kill adult mosquitoes when they feed upon sugar or; (5) Kill immature mosquitoes in aquatic habitats. To date, none of these options has sufficient supporting evidence to justify full-scale programmatic implementation. Concerted investment in their rigorous selection, development and evaluation is required over the coming decade to enable control and, ultimately, elimination of residual malaria transmission. In the meantime, national programmes may assess options for addressing residual transmission under programmatic conditions through pilot studies with strong monitoring, evaluation and operational research components, similar to the Onchocerciasis Control Programme.
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Affiliation(s)
- Gerry F Killeen
- Ifakara Health Institute, Environmental Health and Ecological Sciences Thematic Group, Ifakara, Morogoro, United Republic of Tanzania.
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