The role of aquatic reservoir fluctuations in long-term cholera patterns

We propose and analyze an important extension of standard cholera epidemiological models, explicitly accounting for fluctuations of water availability to the human community under study. The seasonality of water input in the reservoir drives the variation of concentration of Vibrio cholerae. Two compartments are added to the Susceptible-Infected-Bacteria model. First, the recovered individuals, which, over many seasons, lose their immunity to the disease and replenish the Susceptible group. Second, the water volume of the reservoir, which determines bacterial dilution and, consequently, the probability of contracting cholera by ingesting contaminated water. By forcing the model with a seasonally varying hydrologic input, we obtain simulations that can be compared to available data for various regions of the World characterized by different hydrological and epidemiological regimes. The model is shown to satisfactorily reproduce important characteristics of disease insurgence and long-term persistence. Using bifurcation analysis of nonlinear systems, we also explore how different degrees of seasonality and values of the basic reproductive number can change the expected long-term epidemiological time series. We find that there exist parametric conditions where the model shows chaotic patterns - i.e. high unpredictability especially in the amplitude of prevalence peaks - which very much resemble actual data on long-term cholera insurgence. (C) 2011 Elsevier B. V. All rights reserved.

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

Hydroclimatology of dual-peak annual cholera incidence: Insights from a spatially explicit model

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

Cholera incidence in some regions of the Indian subcontinent may exhibit two annual peaks although the main environmental drivers that have been linked to the disease (e.g., sea surface temperature, zooplankton abundance, river discharge) peak once per year during the summer. An empirical hydroclimatological explanation relating cholera transmission to river flows and to the disease spatial spreading has been recently proposed. We specifically support and substantiate mechanistically such hypothesis by means of a spatially explicit model of cholera transmission. Our framework directly accounts for the role of a model river network in transporting and redistributing cholera bacteria among human communities as well as for spatial and temporal annual fluctuations of river flows. The model is forced by seasonal environmental drivers, namely river flow, temperature and chlorophyll concentration in the coastal environment, a proxy for Vibrio cholerae concentration. Our results show that these drivers may suffice to generate dual-peak cholera prevalence patterns for proper combinations of timescales involved in pathogen transport, hydrologic variability and disease unfolding. The model explains the possible occurrence of spatial patterns of cholera incidence characterized by a spring peak confined to coastal areas and a fall peak involving inland regions. Our modeling framework suggests insights on how environmental drivers concert the generation of complex spatiotemporal infections and proposes an explanation for the different cholera patterns (dual or single annual peaks) exhibited by regions that share similar hydroclimatological forcings. Citation: Bertuzzo, E., L. Mari, L. Righetto, M. Gatto, R. Casagrandi, I. Rodriguez-Iturbe, and A. Rinaldo (2012), Hydroclimatology of dual-peak annual cholera incidence: Insights from a spatially explicit model, Geophys. Res. Lett., 39, L05403, doi:10.1029/2011GL050723.

2012

Hydrological, Anthropogenic and Ecological Processes in Cholera Dynamics

Lorenzo Righetto

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.

EPFL2013

Hydrological, Anthropogenic and Ecological Processes in Cholera Dynamics

Lorenzo Righetto

EPFL2013

Hydroclimatology of dual-peak annual cholera incidence: Insights from a spatially explicit model

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

2012

Rainfall mediations in the spreading of epidemic cholera

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

Elsevier Sci Ltd2013