An international team of researchers, which includes the climatologist Xavier Rodó, head of the Climate Research Laboratory at the Parc Científic de Barcelona (PCB, Barcelona Science Park), has published a study that shows how climatic variations, in certain circumstances, affect the development of an infectious disease. In this study, the researchers have developed a mathematical model that allows them to describe the evolution of cholera and to improve the predictions of epidemic outbreaks. The results of this study have been published in this week's issue of the scientific journal Nature.

Many infectious diseases, such as cholera, malaria and dengue, follow annual cycles but also present inter-annual variation. For example, in some years the disease has a greater incidence and epidemics develop while in others the incidence rate is lower than usual. These temporal dynamics depend both on internal factors, such as demographic evolution, the social characteristics of populations, the proportion of individuals that are immune, and health policies and vaccination campaigns, and external environmental factors, such as those related to the climate. One of the objectives of the research performed by this group of scientists is to establish a way to quantitatively measure the relation between climatic factors and disease development.

This study presents the results of including the two types of factors, climatic variability and variations in population immunity, in a single equation that provides an explanation of both the common dynamics of diseases like cholera, that is to say, their annual cycle, and the inter-annual variations of the same. This model has been based on a previous equation formulated to describe the dynamics of cholera on the Indian Subcontinent but which can be extrapolated to other regions and diseases with similar dynamics.

The study has been conducted using a time series of climatic data collected over forty years in Matlab (Bangladesh). The results show a high correlation between inter-annual variations in the transmission of the disease and the climatic patterns of monsoons, the flow of the Brahmaputra river, the floods in Bangladesh and the surface temperature in the Bay of Bengal, as well as the phenomenon known as “El Niño” (ENSO, “El Niño” Southern Oscillation), which regulates inter-annual climatic dynamics. Furthermore, the study shows that, in the case of cholera, the current belief that climatic control of the disease is regulated by coastal environments, is unsubstantiated. On the basis of their results, the researchers conclude that the climatic effect is linked to extreme situations. In this regard, drier and hotter years lead to population concentration around scarce water masses while the combination of torrential rains and overflowing rivers cause floods that destroy or alter the distribution systems for drinking water. Both circumstances lead to significant bacterial growth and therefore increase the probability of contact between the pathogen and the already stressed human population.

This new model incorporates climate as a determinant factor in the dynamics of these diseases and allows scientists to study how this climatic pressure occurs. In addition, it also opens up the possibility of improving epidemic predictions by allowing the anticipation of the possible effects of climatic variation on the emergence or re-emergence of certain infectious diseases that have similar dynamics to those of cholera.