Spatially explicit modelling of cholera epidemics

Enrico Bertuzzo, Flavio Finger, Marino Gatto, Allyn Christine Knox, Lorenzo Mari, Andrea Rinaldo
2013
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Résumé

Epidemiological models can provide crucial understanding about the dynamics of infectious diseases. Possible applications range from real-time forecasting and allocation of health care resources to testing alternative intervention mechanisms such as vaccines, antibiotics or the improvement of sanitary conditions. We apply a spatially explicit model to the cholera epidemic that struck Haiti in October 2010 and is still ongoing. The dynamics of susceptibles as well as symptomatic and asymptomatic infectives are modelled at the scale of local human communities. Dissemination of Vibrio cholerae through hydrological transport and human mobility along the road network is explicitly taken into account, as well as the effect of rainfall as a driver of increasing disease incidence. The model is calibrated using a dataset of reported cholera cases. We further model the long term impact of several types of interventions on the disease dynamics by varying parameters appropriately. Key epidemiological mechanisms and parameters which affect the efficiency of treatments such as antibiotics are identified. Our results lead to conclusions about the influence of different intervention strategies on the overall epidemiological dynamics.

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https://infoscience.epfl.ch/record/198543?ln=fr

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Rainfall mediations in the spreading of epidemic cholera

Enrico Bertuzzo, Marino Gatto, Lorenzo Mari, Lorenzo Righetto, Andrea Rinaldo

Following the empirical evidence of a clear correlation between rainfall events and cholera resurgence that was observed in particular during the recent outbreak in Haiti, a spatially explicit model of epidemic cholera is re-examined. Specifically, we test a multivariate Poisson rainfall generator, with parameters varying in space and time, as a driver of enhanced disease transmission. The relevance of the issue relates to the key insight that predictive mathematical models may provide into the course of an ongoing cholera epidemic aiding emergency management (say, in allocating life-saving supplies or health care staff) or in evaluating alternative management strategies. Our model consists of a set of dynamical equations (SIRB-like i.e. subdivided into the compartments of Susceptible, Infected and Recovered individuals, and including a balance of Bacterial concentrations in the water reservoir) describing a connected network of human communities where the infection results from the exposure to excess concentrations of pathogens in the water. These, in turn, are driven by rainfall washout of open-air defecation sites or cesspool overflows, hydrologic transport through waterways and by mobility of susceptible and infected individuals. We perform an a posteriori analysis (from the beginning of the epidemic in October 2010 until December 2011) to test the model reliability in predicting cholera cases and in testing control measures, involving vaccination and sanitation campaigns, for the ongoing epidemic. Even though predicting reliably the timing of the epidemic resurgence proves difficult due to rainfall inter-annual variability, we find that the model can reasonably quantify the total number of reported infection cases in the selected time-span. We then run a multi-seasonal prediction of the course of the epidemic until December 2015, to investigate conditions for further resurgences and endemicity of cholera in the region with a view to policies which may bring to the eradication of the disease in Haiti. The projections, although strongly depending on still uncertain epidemiological processes, show an endemic, seasonal pattern establishing in the region, which can be better forestalled by an improvement of the sanitation system only, rather than by vaccination alone. We thus conclude that hydrologic drivers and water resources management prove central to prediction, emergency management and long-term control of epidemic cholera. (C) 2013 Elsevier Ltd. All rights reserved.

Elsevier Sci Ltd2013

Chargement

Modelling cholera epidemics: the role of waterways, human mobility and sanitation

Enrico Bertuzzo, Marino Gatto, Lorenzo Mari, Lorenzo Righetto, Andrea Rinaldo

We investigate the role of human mobility as a driver for long-range spreading of cholera infections, which primarily propagate through hydrologically controlled ecological corridors. Our aim is to build a spatially explicit model of a disease epidemic, which is relevant to both social and scientific issues. We present a two-layer network model that accounts for the interplay between epidemiological dynamics, hydrological transport and long-distance dissemination of the pathogen Vibrio cholerae owing to host movement, described here by means of a gravity-model approach. We test our model against epidemiological data recorded during the extensive cholera outbreak occurred in the KwaZulu-Natal province of South Africa during 2000-2001. We show that long-range human movement is fundamental in quantifying otherwise unexplained inter-catchment transport of V. cholerae, thus playing a key role in the formation of regional patterns of cholera epidemics. We also show quantitatively how heterogeneously distributed drinking water supplies and sanitation conditions may affect large-scale cholera transmission, and analyse the effects of different sanitation policies.

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The present Thesis deals with understanding, measuring and modelling epidemic cholera. The relevance of the endeavour stems from the fact that mathematical epidemiology, properly guided by model-guided field validation, is a reliable and powerful tool to monitor and predict ongoing epidemics in time for action, and to save lives through evaluation of the effectiveness of mitigation policies or the deployment of medical staff and supplies. In recent years, waterborne diseases – and cholera in particular – have consolidated their role as a major threat for developing countries, where sanitation conditions are poor and the vulnerability to extreme events is highest. Several important features controlling the dynamics of occurrence and spreading of the disease in a region are studied in the present Thesis, from both theoretical and experimental perspective. Understanding the basic processes that regulate cholera infection is key to build reliable prediction tools. In this Thesis several driving mechanisms of cholera are investigated, with the objective of connecting together hydro-climatology, ecology and epidemiology in a comprehensive framework. First, the role of water volume fluctuations is analyzed partly through a bifurcation study in a mostly theoretical assessment. Such work is then particularized in a field campaign carried out in rural Bangladesh, where hydro-climatological variables and Vibrio cholerae concentrations have been monitored for more than a year in one of the ponds constituting the local water reservoir. Concomitantly, a procedure for the detection of Vibrio cholerae, based on flow cytometry, is tested in the field. Further emphasis is also given to the role of human mobility in disseminating the disease among different communities. In particular, the contribution of human mobility in the dispersal of vibrios along the hydrologic network is specifically analyzed. All the knowledge collected in these studies is then used to add essential details to a modeling framework that is applied to the dramatic case of the Haiti epidemic. A spatially explicit model, taking into account both hydrological transport and human mobility, is developed to simulate the spreading of the disease since its onset. It is also shown that the resurgence of the disease, coinciding with the rainy season of June-July 2011, can only be reproduced if hydrological forcings are considered. The framework is tested by forcing it with synthetic rainfall scenarios and projecting epidemiological outputs. It is shown that the model can quantify correctly the number of cases in a given time span, even when calibrated with limited information. This result allows then to use it as a tool to assess a priori the effectiveness of intervention policies, such as vaccination and sanitation. The effect of these two is tested both in the short- and in the long-term, with different results. Such endeavour represents the ultimate goal of the work presented in this Thesis – albeit further effort is needed to link together public health management and mathematical epidemiology in this field.

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Chargement

Rainfall mediations in the spreading of epidemic cholera

Enrico Bertuzzo, Marino Gatto, Lorenzo Mari, Lorenzo Righetto, Andrea Rinaldo

Elsevier Sci Ltd2013

EPFL2013