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El uso de Sistemas de Información Geografica y Sensores Remotos (SR) en Salud Pública Bogotá, Colombia, 27 al 30 de Marzo de 2006 surveillance as a tool for targeting. In contrast, for vectors such as T dimidiata or (at least in Venezuela) R prolixus where eradication is not a feasible target, long term control is required. A range of strategies are then possible (Schofield 2000). One obvious policy is to spray only the houses surveyed that are found positive. However, it may be cost-effective in some villages to spray houses surveyed and found negative (as false negatives are possible) and/or to spray houses that are not surveyed at all (e.g. if a high proportion of a random sample of houses surveyed in a village are positive, then it is probable that a high proportion of the remaining houses are also likely to be positive). At one extreme, one can implement an eradication strategy in which all houses are sprayed even if only one is found infested (the stated strategy in Venezuela). Alternatively, as in parts of Colombia, the decision whether or not to spray negative or un-surveyed houses can be made contingent on the proportion of houses found positive amongst those surveyed. Two approaches have been taken to evaluate the relative impact of different Chagas disease vector control strategies: field intervention trials and modelling. For example, two trials (Acevedo et al., 2000; Oliveira-Filho et al., 2000) have directly compared the impact of blanket spraying (all houses) versus focal spraying (only houses found to be infested); however, it is clearly inappropriate to assume generalisability of the results beyond trial conditions. In reality, there is a wide series of decisions that need to be made and in order to estimate their cost- effectiveness under a range of conditions, a modelling approach provides flexibility. In addition to the choice of chemical used for house spraying, each Chagas vector control policy can be defined by at least six parameters whose values can vary in response to local conditions: (1) the proportion of houses to be surveyed, (2) the proportion of surveyed houses found positive to be sprayed, (3) the proportion of surveyed houses found negative to be sprayed, (4) the proportion of un-surveyed houses to be sprayed, (5) the sensitivity of the survey method, and (6) the specificity of the survey method. The rationale for spraying apparently uninfested houses is not because of any residual effect reducing the risk of house invasion following spraying, but rather because sampling is not 100% sensitive. Sensitivity potentially varies with sampling methodology, bug behaviour, house design and the level of infestation. Nevertheless, there is good evidence that people living in houses with detectable infestations are at higher risk of infection than people living in apparently uninfested 26
El uso de Sistemas de Información Geografica y Sensores Remotos (SR) en Salud Pública Bogotá, Colombia, 27 al 30 de Marzo de 2006 houses (e.g. Mott et al., 1978; Piesman et al., 1985; De Andrade et al., 1995; Gurtler et al., 1998; Sosa-Jurado et al., 2004). In the 15 departments of Colombia where the national survey was undertaken, it appears that incidence rates in apparently infested houses are on average about 2%/year compared to 0.2% in apparently uninfested houses, confirming that (1) there is some advantage to target spraying to houses with detectable infestations, and (2) a risk map for triatomine vectors should reflect the geographic distribution of current transmission rates. Such a risk map, if suitably validated by comparison with independent data (eg King et al 2004), could then be used to determine which strategy should be implemented in each locality to make best use of limited resources. Risk maps for triatomine vectors can be generated by a range of statistical techniques, but they generally involve some combination of (1) interpolation from the data collected in villages which have been surveyed and (2) statistical associations with environmental features – such as temperature, rainfall, land cover, and socioeconomic variables. To date all risk maps that have been described for Chagas disease vectors have been static (Gorla 2002, Costa et al 2002, Peterson et al 2002, Beard et al 2003, Dumonteil & Gorbiere 2004, Lopez- Cardenas et al 2005). Whereas a decision-support system for Chagas disease control programmes would benefit most from a dynamic risk map, where predictions of risk are allowed to vary each year according to changing conditions, most notably in response to house spraying. Such a dynamic map could allow re- infestation rates of sprayed villages to vary geographically in response to variability in local environmental conditions (whether vegetation- or household-related). Evaluations of the effectiveness of house spraying against T. dimidiata infestations in Guatemala, for example, indicate some geographic clustering of locations where spraying has been less effective (Hashimoto K. & Codron-Rosales C., pers. comm.). In conclusion, it is worth emphasising that the introduction of Geographic Information Systems into Chagas disease control programmes should not only aid in the development of evidence-based control strategies (i.e. policy), but will also provide immediate help by improving the quality of data management and so improve operational capability (i.e. practice). There is considerable scope to make 27
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Page 7 and 8: PROLOGO El uso de Sistemas de Infor
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Page 31 and 32: Summary El uso de Sistemas de Infor
Page 33 and 34: References El uso de Sistemas de In
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Infes tacion CGN+Ba + Cc+A4 CGN El
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El uso de Sistemas de Información
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HABITAT DE COLECTA El uso de Sistem
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1. ANEXOS No. MUNICIPIO AÑOS VERED
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Rhodnius prolixus Panstrongylus gen
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1. Esquema de la presentación El u
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Demand of healthcare for Chronic Ch
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Regional Hospital of Soata Departme
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APENDICE 3: Participantes en el pan
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Bogotá, Colombia, 27 al 30 de Marz
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Bibliografía Bogotá, Colombia, 27
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