Brace yourself - Cholera is coming

Cholera is a bacterial infectious disease caused by pathogenic strains of the gram-negative bacteria Vibrio cholera. Firstly described in the 19^th century origin, functionality and consequences of Cholera are well investigated. Still thousands of cases of human cholera infection are reported each year. In the 70s of the 20^th century Rita Colwell discovered that V. cholerae strains occur naturally in coastal waters. She assumed that outbreaks of cholera epidemics are closely related to the distribution of natural plankton hosts in coastal areas. Today it is known that V. cholerae is spread and passed on not only by plankton but also by a wide range of other marine organisms. Hence, distribution of V. cholerae depends on oceanographic variables such as temperature, pH, salinity, plankton blooms and so forth. Regarding these variables it would be possible to characterize distribution areas and predict potential outbreaks. However, because of climate change it is also necessary to consider shifting of these variables. For instance, it has been established that increasing abundance and concentration of V. cholerae is linked to increased temperature. 

The study by Escobar et al. (2015) identified significant environmental variables linked to the occurrence of V. cholerae. The oceanographic variables were used to create a model which describes potential V. cholerae distribution areas. Further, the model was applied on current and future climate scenarios.

Firstly, they performed a literature review to accumulate all sites of V. cholerae presence. Twelve environmental variables were taken into account to build Environmental niche models (ENM). ENM are mathematical models which help to predict the distribution of species in geographic space considering their known distribution in environmental space. Principal components analysis (PCA) and jackknife test were used to identify and evaluate highly correlated environmental variables and their model performance.

Literature review yielded 15 sites in marine areas with V. cholerae reports worldwide. Most significant environmental variables which were used for the final ENM were Chlorophyll-a, maximum temperature, pH and mean salinity. Cholorphyll-a was the most powerful variable for explaining presence of V. cholerae. Appearance of V. cholerae is closely related to plankton blooms. The bacteria in estuaries and coastal waters are associated with plankton. Increased Chlorophyll-a concentration indicates increased potential for infection of humans.

Several regions were identified to be suitable for V. cholerae occurrence. Future models showed a pregnant increase of suitable regions worldwide. This may suggest that there is a higher level of risk of V. cholerae distribution in a future of global warming.

Since this is only a model, results of these findings have yet to be proven realistic. More information about distribution sites and environmental variables including not only abiotic factors, but also biotic factors are necessary to improve the model.

Although this is not a typical hands-on research paper, I think it gives an important overview about the future of Cholera disease. With increasing water temperature especially in coastal regions it is important to know which areas could be populated by the toxic bacteria in future events. Future studies should focus on the identification of new distribution areas. Has V. cholerae already moved somewhere else? Further, migration pathways should be more investigated to get a better picture of future spreading. 

Escobar L., Ryan S., Ibarra A., Finkelstein J., King C., Qiao H., Polhemus M (2015), Acta Tropica. A global map of suitability for coastal Vibrio cholerae under current and future climate conditions. doi:10.1016/j.actatropica.2015.05.028