Possible Syndemics in Spain During the First Half of the Twentieth Century Associated with Autochthonous Malaria

In epidemiological studies since the late twentieth century, the term syndemic has been coined to refer to epidemic outbreaks that occur together in the same space and time. Moreover, the concept of syndemic implies that these different outbreaks interact with negative synergies between them and have common triggering factors. In this study, we analyse possible syndemic outbreaks of infectious diseases in Spain during the first half of the twentieth century. We have therefore studied the evolution of mortality of the main infectious diseases from 1910 to 1945, when this type of disease was much more relevant than it is today. These data have allowed us to detect, at least, two syndemic episodes in which several infectious diseases were concentrated simultaneously. The first took place approximately since 1918 until 1923 and was associated with the Flu Pandemic of 1918 and the end of the First World War. The main diseases involved were malaria and tuberculosis, in addition to the aforementioned 1918 Flu. The second took place approximately between 1940 and 1944, coinciding with the end of the Spanish Civil War and the beginning of Second World War, and the main diseases involved were malaria, typhus and syphilis. Thus, between 1910 and 1940, we detected an R-sq between those who died of malaria and those who died of typhus and syphilis of over 0.42, although since 1920 until 1937 it was over 0.81. The initial causes of these two syndemic outbreaks were different; however, in both cases, large population movements and/or overcrowding of troops and civilians due to war or epidemics were involved. These massive population flows may explain why these two syndemics temporally overlapped with the two major outbreaks of autochthonous malaria in Spain during the twentieth century. Improving knowledge of the possible negative synergies of epidemic outbreaks of infectious diseases that coincide in space and time may help to prevent new syndemics if we learn to recognise their main drivers.