Date Published: October 21, 2015
Publisher: Public Library of Science
Author(s): Alexander Kirpich, Thomas A. Weppelmann, Yang Yang, Afsar Ali, J. Glenn Morris, Ira M. Longini, Claudia Munoz-Zanzi. http://doi.org/10.1371/journal.pntd.0004153
Abstract: In the current study, a comprehensive, data driven, mathematical model for cholera transmission in Haiti is presented. Along with the inclusion of short cycle human-to-human transmission and long cycle human-to-environment and environment-to-human transmission, this novel dynamic model incorporates both the reported cholera incidence and remote sensing data from the Ouest Department of Haiti between 2010 to 2014. The model has separate compartments for infectious individuals that include different levels of infectivity to reflect the distribution of symptomatic and asymptomatic cases in the population. The environmental compartment, which serves as a source of exposure to toxigenic V. cholerae, is also modeled separately based on the biology of causative bacterium, the shedding of V. cholerae O1 by humans into the environment, as well as the effects of precipitation and water temperature on the concentration and survival of V. cholerae in aquatic reservoirs. Although the number of reported cholera cases has declined compared to the initial outbreak in 2010, the increase in the number of susceptible population members and the presence of toxigenic V. cholerae in the environment estimated by the model indicate that without further improvements to drinking water and sanitation infrastructures, intermittent cholera outbreaks are likely to continue in Haiti.
Partial Text: After a massive earthquake struck the island nation of Haiti in 2010, the introduction of an altered El Tor biotype of Vibrio cholerae O1 has led to one of the largest cholera outbreaks in recent history   . Almost four years after the identification of the first cholera cases, the transmission appears to have temporarily slowed, however the future of the cholera epidemic in Haiti remains uncertain . After the initial isolation of toxigenic V. cholerae O1 from surface water monitoring sites in the Ouest Department of Haiti in 2012 and 2013, there is evidence that the frequency of isolation from the environment has actually increased between 2013 and 2014  . In the absence of ongoing transmission, the presence of toxigenic V. cholerae O1 in the aquatic environment has left the international scientific community divided on the possibility that the causative bacterium has established environmental reservoirs in the surface waters of Haiti   . If this were to be the case, the goal of cholera elimination from the island of Hispaniola by 2022 would be more challenging, with the potential for cholera to become endemic in Haiti .
To reflect the basic differences in the modes of transmission, the model incorporates both the short cycle transmission from human-to-human and long cycle transmission from human-to-environment and environment-to-human. The short route relies on data suggesting that toxigenic V. cholerae assumes a short-lived hyperinfectious state immediately after passage from the human intestine . This facilitates rapid transmission of V. cholerae from one person to another, often related to personal hygiene practices within the household. Alternatively, transmission may occur when V. cholerae is acquired from contaminated drinking water or by contact with the aquatic environment. The presence of toxigenic V. cholerae in the aquatic environment may reflect contamination of water sources by feces from an infected individual, and/or the existence of an aquatic reservoir in which the microorganism can persist for months to years . Transmission through this aquatic route, while still having the potential for being relatively rapid, tends to involve more time than the short cycle transmission between humans.
The obtained model fit provided a good understanding of the dynamics of the epidemic over time. The visual summary of the model fit together with the adjusted reported cholera incidence is shown in Fig 3. First, the reported incidence was adjusted by rescaling to account for disease underreporting and plotted in orange in Fig 3 for better visual comparison with the model output. To produce the model realizations a different Gaussian white noise time series was generated for each set of 1000 parameter estimates obtained from the previous LSE fits. The corresponding model outputs are displayed in Fig 3. The transparency was tuned to display the density of the curves in each part of the graph and to improve visualization. The symptomatic cases produced by the model are displayed in dark green in panel A of Fig 3. The realizations of both the symptomatic (dark green) and total (light green) cases produced by the model are displayed for comparison in panel B of Fig 3 on a different scale. The total underlying realizations of the model that include both symptomatic and asymptomatic infections are much larger than the symptomatic realizations alone.
In this work, a dynamic model that incorporated the available environmental data was used to describe the transmission of cholera in Ouest Department of Haiti. The model output suggested the existence of a large environmental reservoir of toxigenic V. cholerae that reached a peak concentration early in 2012, with a subsequent slow decline (Fig 4). The presence of such an environmental reservoir was consistent with environmental studies conducted in the Leogane flood basin of the Ouest Department, which identified V. cholerae O1 in multiple river and estuarine ecosystems  . A similar trend was observed in the human susceptible compartment of the model, where the smallest number of susceptible population members was observed in early 2012, with a slow but steady increase since that time (Fig 4).