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A reduce and replace strategy for suppressing vector-borne diseases: insights from a stochastic, spatial model.

ABSTRACT: Two basic strategies have been proposed for using transgenic Aedes aegypti mosquitoes to decrease dengue virus transmission: population reduction and population replacement. Here we model releases of a strain of Ae. aegypti carrying both a gene causing conditional adult female mortality and a gene blocking virus transmission into a wild population to assess whether such releases could reduce the number of competent vectors. We find this "reduce and replace" strategy can decrease the frequency of competent vectors below 50% two years after releases end. Therefore, this combined approach appears preferable to releasing a strain carrying only a female-killing gene, which is likely to merely result in temporary population suppression. However, the fixation of anti-pathogen genes in the population is unlikely. Genetic drift at small population sizes and the spatially heterogeneous nature of the population recovery after releases end prevent complete replacement of the competent vector population. Furthermore, releasing more individuals can be counter-productive in the face of immigration by wild-type mosquitoes, as greater population reduction amplifies the impact wild-type migrants have on the long-term frequency of the anti-pathogen gene. We expect the results presented here to give pause to expectations for driving an anti-pathogen construct to fixation by relying on releasing individuals carrying this two-gene construct. Nevertheless, in some dengue-endemic environments, a spatially heterogeneous decrease in competent vectors may still facilitate decreasing disease incidence.

SUBMITTER: Okamoto KW

PROVIDER: S-EPMC3869666 | biostudies-literature | 2013

REPOSITORIES: biostudies-literature

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A reduce and replace strategy for suppressing vector-borne diseases: insights from a stochastic, spatial model.

Okamoto Kenichi W KW Robert Michael A MA Lloyd Alun L AL Gould Fred F

PloS one 20131220 12

Two basic strategies have been proposed for using transgenic Aedes aegypti mosquitoes to decrease dengue virus transmission: population reduction and population replacement. Here we model releases of a strain of Ae. aegypti carrying both a gene causing conditional adult female mortality and a gene blocking virus transmission into a wild population to assess whether such releases could reduce the number of competent vectors. We find this "reduce and replace" strategy can decrease the frequency of ...[more]

PMID: 24376506

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Project description:After a request from the European Commission, EFSA's Panel on Animal Health and Welfare summarised the main characteristics of 36 vector-borne diseases (VBDs) in https://efsa.maps.arcgis.com/apps/PublicGallery/index.html?appid=dfbeac92aea944599ed1eb754aa5e6d1. The risk of introduction in the EU through movement of livestock or pets was assessed for each of the 36 VBDs individually, using a semiquantitative Method to INTegrate all relevant RISK aspects (MINTRISK model), which was further modified to a European scale into the http://www3.lei.wur.nl/mintrisk/ModelMgt.aspx. Only eight of the 36 VBD-agents had an overall rate of introduction in the EU (being the combination of the rate of entry, vector transmission and establishment) which was estimated to be above 0.001 introductions per year. These were Crimean-Congo haemorrhagic fever virus, bluetongue virus, West Nile virus, Schmallenberg virus, Hepatozoon canis, Leishmania infantum, Bunyamwera virus and Highlands J. virus. For these eight diseases, the annual extent of spread was assessed, assuming the implementation of available, authorised prevention and control measures in the EU. Further, the probability of overwintering was assessed, as well as the possible impact of the VBDs on public health, animal health and farm production. For the other 28 VBD-agents for which the rate of introduction was estimated to be very low, no further assessments were made. Due to the uncertainty related to some parameters used for the risk assessment or the instable or unpredictability disease situation in some of the source regions, it is recommended to update the assessment when new information becomes available. Since this risk assessment was carried out for large regions in the EU for many VBD-agents, it should be considered as a first screening. If a more detailed risk assessment for a specific VBD is wished for on a national or subnational level, the EFSA-VBD-RISK-model is freely available for this purpose.

Project description:Many vector-borne diseases lack effective vaccines and medications, and the limitations of traditional vector control have inspired novel approaches based on using genetic engineering to manipulate vector populations and thereby reduce transmission. Yet both the short- and long-term epidemiological effects of these transgenic strategies are highly uncertain. If neither vaccines, medications, nor transgenic strategies can by themselves suffice for managing vector-borne diseases, integrating these approaches becomes key. Here we develop a framework to evaluate how clinical interventions (i.e., vaccination and medication) can be integrated with transgenic vector manipulation strategies to prevent disease invasion and reduce disease incidence. We show that the ability of clinical interventions to accelerate disease suppression can depend on the nature of the transgenic manipulation deployed (e.g., whether vector population reduction or replacement is attempted). We find that making a specific, individual strategy highly effective may not be necessary for attaining public-health objectives, provided suitable combinations can be adopted. However, we show how combining only partially effective antimicrobial drugs or vaccination with transgenic vector manipulations that merely temporarily lower vector competence can amplify disease resurgence following transient suppression. Thus, transgenic vector manipulation that cannot be sustained can have adverse consequences-consequences which ineffective clinical interventions can at best only mitigate, and at worst temporarily exacerbate. This result, which arises from differences between the time scale on which the interventions affect disease dynamics and the time scale of host population dynamics, highlights the importance of accounting for the potential delay in the effects of deploying public health strategies on long-term disease incidence. We find that for systems at the disease-endemic equilibrium, even modest perturbations induced by weak interventions can exhibit strong, albeit transient, epidemiological effects. This, together with our finding that under some conditions combining strategies could have transient adverse epidemiological effects suggests that a relatively long time horizon may be necessary to discern the efficacy of alternative intervention strategies.

Project description:Mongolia is an expansive nation, dominated by agriculture with livestock under nomadic herder care contributing significantly to the economy. Mongolian veterinarians service these herder's livestock and dogs, and are often the first point of contact for animal health advice, including ectoparasite prophylaxis. Dogs are competent reservoir and sentinel hosts for several zoonotic vector-borne diseases (VBD). These diseases in dogs can be dependent on the presence of other sylvatic or domestic reservoir hosts, the abundance of competent vectors and supporting environmental and climatic conditions. Therefore, VBD present a true One Health challenge. The direct and close association of nomadic herders with livestock and livestock protection dogs coupled with frequent relocation (associated with nomadic lifestyles) places all three host groups (herders, livestock and livestock protection dogs) at risk of acquiring VBD. Our study set out to investigate the overall knowledge, perceptions and practices of Mongolian veterinarians towards canine vector-borne diseases (CVBD). A hardcopy questionnaire was delivered through the Mongolian Veterinary Medical Association to a cohort of veterinarians representing 39% of Mongolia's total veterinary workforce with a 53% response rate. A total of 297 participants were included in the final study. The bulk of participants were livestock veterinarians, followed by mixed animal veterinarians. Overall Mongolian veterinarians' knowledge of CVBD were scored as low (58%; 0-3 points) or medium (32%; 4-6 points) on a ten-point scale. There was a significant discrepancy between self-rated and actual knowledge. Females had 1.7 (95% CI 1.1, 2.8) times higher knowledge compared with males and those veterinarians who had 3-5 canine consultations per day were also found to have higher knowledge (odds ratio 1.4, 95% CI 0.4, 4.5). Most veterinarians utilised two or less resources to source information on CVBD over the previous 12 months. The potential of climate-induced emergence of vector populations and their associated pathogens makes it imperative that veterinarians in Mongolia have the necessary resources and knowledge to be on the forefront of CVBD preparedness and mitigation. This study identifies the knowledge gaps and addresses the need for further resources for Mongolian veterinarians to effectively engage in a One Health approach for negating CVBD in animals and humans.

Dataset Information

A reduce and replace strategy for suppressing vector-borne diseases: insights from a stochastic, spatial model.

Publications

A reduce and replace strategy for suppressing vector-borne diseases: insights from a stochastic, spatial model.

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