Biomedical subjects

Michael Emch

Publications and source records attributed to Michael Emch.

9 recordsLinked to original sources

PubMed2006

Application of Poisson kriging to the mapping of cholera and dysentery incidence in an endemic area of Bangladesh.

BACKGROUND: Disease maps can serve to display incidence rates geographically, to inform on public health provision about the success or failure of interventions, and to make hypothesis or to provide evidences concerning disease etiology. Poisson kriging was recently introduced to filter the noise attached to rates recorded over sparsely populated administrative units. Its benefit over simple population-weighted averages and empirical Bayesian smoothers was demonstrated by simulation studies using county-level cancer mortality rates. This paper presents the first application of Poisson kriging to the spatial interpolation of local disease rates, resulting in continuous maps of disease rate estimates and the associated prediction variance. The methodology is illustrated using cholera and dysentery data collected in a cholera endemic area (Matlab) of Bangladesh. RESULTS: The spatial analysis was confined to patrilineally-related clusters of households, known as baris, located within 9 kilometers from the Matlab hospital to avoid underestimating the risk of disease incidence, since patients far away from the medical facilities are less likely to travel. Semivariogram models reveal a range of autocorrelation of 1.1 km for dysentery and 0.37 km for cholera. This result translates into a cholera risk map that is patchier than the dysentery map that shows a large zone of high incidence in the south-central part of the study area, which is quasi-urban. On both maps, lower risk values are found in the Northern part of the study area, which is also the most distant from the Matlab hospital. The weaker spatial continuity of cholera versus dysentery incidence rates resulted in larger kriging variance across the study area. CONCLUSION: The approach presented in this paper enables researchers to incorporate the pattern of spatial dependence of incidence rates into the mapping of risk values and the quantification of the associated uncertainty. Differences in spatial patterns, in particular the range of spatial autocorrelation, reflect differences in the mode of transmission of cholera and dysentery. Our risk maps for cholera and dysentery incidences should help identifying putative factors of increased disease incidence, leading to more effective prevention and remedial actions in endemic areas.

Bangladesh↗

PubMed2006

Relationship between neighbourhood-level killed oral cholera vaccine coverage and protective efficacy: evidence for herd immunity.

OBJECTIVES: The effectiveness of vaccines in populations must consider both direct and indirect protection. This study reanalyses data from a large individually randomized oral cholera vaccine trial that was conducted in rural Bangladesh from 1985 to 1990. A recent analysis of the results of that trial showed that the proportion of people in household clusters who received the vaccine was inversely related to placebo incidence during the first year of surveillance, which was attributed to herd immunity. METHODS: In this study we measure the relationship between neighbourhood-level oral cholera vaccine coverage and protective efficacy (PE) during a 2 year follow-up period, controlling for known effect modifiers. We link trial data to a household geographic information system to facilitate the neighbourhood-level analysis. Findings Neighbourhood-level PE can be partially explained by vaccine coverage after adjusting for ecological variables. CONCLUSIONS: The inverse relationship between vaccine coverage and efficacy illustrates that people living in high-coverage areas may be indirectly protected from cholera because people living around them are vaccinated.

Adult↗

PubMed2006

Efficacy calculation in randomized trials: global or local measures?

This study tests whether the effect of a vaccine trial varies in space and why. Analytical z-score maps identify unusually high- and low-efficacy values in a trial area. Relationships between neighborhood efficacy and ecological variables are measured to explain why efficacy varies in space. Efficacy was found to vary regardless of neighborhood size and the variation is related to several ecological determinants. Local efficacy measures can help public health practitioners make better decisions about when and where to vaccinate populations. The concepts offered in this study are pertinent for any health intervention trial, not just vaccines.

Adolescent↗

PubMed2005

Neighborhood size and local geographic variation of health and social determinants.

BACKGROUND: Spatial filtering using a geographic information system (GIS) is often used to smooth health and ecological data. Smoothing disease data can help us understand local (neighborhood) geographic variation and ecological risk of diseases. Analyses that use small neighborhood sizes yield individualistic patterns and large sizes reveal the global structure of data where local variation is obscured. Therefore, choosing an optimal neighborhood size is important for understanding ecological associations with diseases. This paper uses Hartley's test of homogeneity of variance (Fmax) as a methodological solution for selecting optimal neighborhood sizes. The data from a study area in Vietnam are used to test the suitability of this method. RESULTS: The Hartley's Fmax test was applied to spatial variables for two enteric diseases and two socioeconomic determinants. Various neighbourhood sizes were tested by using a two step process to implement the Fmaxtest. First the variance of each neighborhood was compared to the highest neighborhood variance (upper, Fmax1) and then they were compared with the lowest neighborhood variance (lower, Fmax2). A significant value of Fmax1 indicates that the neighborhood does not reveal the global structure of data, and in contrast, a significant value in Fmax2 implies that the neighborhood data are not individualistic. The neighborhoods that are between the lower and the upper limits are the optimal neighbourhood sizes. CONCLUSION: The results of tests provide different neighbourhood sizes for different variables suggesting that optimal neighbourhood size is data dependent. In ecology, it is well known that observation scales may influence ecological inference. Therefore, selecting optimal neigborhood size is essential for understanding disease ecologies. The optimal neighbourhood selection method that is tested in this paper can be useful in health and ecological studies.

Journal Article↗

PubMed2003

Use of a geographic information system for defining spatial risk for dengue transmission in Bangladesh: role for Aedes albopictus in an urban outbreak.

We used conventional and spatial analytical tools to characterize patterns of transmission during a community-wide outbreak of dengue fever and dengue hemorrhagic fever in Dhaka, Bangladesh in 2000. A comprehensive household-level mosquito vector survey and interview was conducted to obtain data on mosquito species and breeding as well as illness consistent with dengue. Clusters of dengue illnesses and high-density vector populations were observed in a distinct sector of the city. Dengue clusters are less identifiable in areas further away from major hospitals, suggesting that proximity to hospitals determines whether cases of dengue are diagnosed. Focusing on those areas relatively close to hospitals, we found a spatial association between dengue clusters and vector populations. Households reporting a recent dengue illness were more likely to have Aedes albopictus larvae present in the home when compared with households not reporting cases. Households reporting a recent dengue illness were also more likely to have a neighbor with Ae. albopictus present in the home. In contrast, the presence of Aedes aegypti within the premises as well as the homes of neighbors (within 50 meters) was not associated with dengue illness. Given that the breeding habitats for Ae. albopictus are somewhat distinct from those of Ae. aegypti, the findings of this study have implications for control of dengue transmission in this urban setting where much of the focus has been on indoor mosquito breeding and transmission. Public health officials may find the disease-environment map useful for planning targeted interventions because it displays areas where transmission is most intense.

Aedes↗

PubMed2002

The spatial epidemiology of cholera in an endemic area of Bangladesh.

This paper defines high-risk areas of cholera based on environmental risk factors of the disease in an endemic area of Bangladesh. The risk factors include proximity to surface water, high population density, and low educational status, which were identified in an earlier study by the authors. Cholera data were analyzed by spatially referenced extended household units for two time periods, 1983-1987 and 1992-1996. These periods were chosen because they had different dominant cholera agents. From 1983-1987 classical cholera was dominant and from 1992-1996 El Tor was dominant. By defining high-risk areas based on risk factors, this study builds a spatial risk model for cholera. The model is then evaluated based on the locations of observed cholera cases. The study also identifies the determinants of death due to cholera for the two different time periods dominated by the different cholera agents. The modeled risk areas that were based on the risk factors were found to correspond with actual distributions of cholera morbidity and mortality. The high-risk areas of the dominant cholera agents are relatively stable over time. However, from 1983-1987 El Tor cholera, which was not the dominant agent during that period, was not associated with high-risk areas, suggesting that the El Tor habitat may have changed over time. The case fatality rate for cholera was related to proximity to a diarrhea treatment hospital in the study area.

Bangladesh↗

PubMed2002

Spatial filtering using a raster geographic information system: methods for scaling health and environmental data.

Despite the use of geographic information systems (GIS) in academic research, it is still uncommon for public health officials to use such tools for addressing health and environmental issues. Complexities in methodological issues for addressing relationships between health and environment, investigating spatial variation of disease, and addressing spatial demand and supply of health care service, hinder the use of GIS in the health sector. This paper demonstrates simple spatial filtering methods for analyzing health and environmental data using a raster GIS. Computing spatial moving average rates reduces individual affects and creates a continuous surface of phenomena. Another spatial analytical method discussed is computation of exposure status surfaces including neighbors' influences weighted by distance decay. These methods describe how health and environmental data can be scaled in order to better address health problems. Spatial filtering methods are demonstrated using health and population surveillance data within a GIS that were collected for approximately 210,000 people in Matlab, Bangladesh.

Bangladesh↗

PubMed2002

Identifying environmental risk factors for endemic cholera: a raster GIS approach.

The bacteria that cause cholera are known to be normal inhabitants of surface water, however, the environmental risk factors for different biotypes of cholera are not well understood. This study identifies environmental risk factors for cholera in an endemic area of Bangladesh using a geographic information systems (GIS) approach. The study data were collected from a longitudinal health and demographic surveillance system and the data were integrated within a geographic information system database of the research area. Two study periods were chosen because they had different dominant biotypes of the disease. From 1992 to 1996 El Tor cholera was dominant and from 1983 to 1987 classical cholera was dominant. The study found the same three risk factors for the two biotypes of cholera including proximity to surface water, high population density, and poor educational level. The GIS database was used to measure the risk factors and spatial filtering techniques were employed. These robust spatial methods are offered as an example for future epidemiological research efforts that define environmental risk factors for infectious diseases.

Bangladesh↗

PubMedDate not supplied

Herd immunity conferred by killed oral cholera vaccines in Bangladesh: a reanalysis.

BACKGROUND: Decisions about the use of killed oral cholera vaccines, which confer moderate levels of direct protection to vaccinees, can depend on whether the vaccines also provide indirect (herd) protection when high levels of vaccine coverage are attained. We reanalysed data from a field trial in Bangladesh to ascertain whether there is evidence of indirect protection from killed oral cholera vaccines. METHODS: We analysed the first year of surveillance data from a placebo-controlled trial of B subunit-killed whole-cell and killed whole-cell-only oral cholera vaccines in children and adult women in Bangladesh. We calculated whether there was an inverse, monotonic trend for the relation between the level of vaccine coverage in a residential cluster and the incidence of cholera in individual vaccine recipients or placebo recipients residing in the cluster after controlling for potential confounding variables. FINDINGS: Vaccine coverage of the targeted population ranged from 4% to 65%. Incidence rates of cholera among placebo recipients were inversely related to levels of vaccine coverage (7.01 cases per 1000 in the lowest quintile of coverage vs 1.47 cases per 1000 in the highest quintile; p<0.0001 for trend). Receipt of vaccine by an individual and the level of vaccine coverage of the individual's cluster were independently related to a reduced risk of cholera. Moreover, after adjustment for the level of vaccine coverage of the cluster, vaccine protective efficacy remained significant (55% [95% CI 41-66], p<0.0001). INTERPRETATION: In addition to providing direct protection to vaccine recipients, killed oral cholera vaccines confer significant herd protection to neighbouring non-vaccinated individuals. Use of these vaccines could have a major effect on the burden of cholera in endemic settings.

Administration, Oral↗