Climate change influence over tropical diseases dynamics

Tropical diseases incidence and geographic expansion monitoring should be part of epidemiological surveillance, focused on populations suffering or that will still suffer the impacts of climate change

Ulisses Confalonieri1; Júlia Menezes2

1Full Researcher at René Rachou Research Center, Oswaldo Cruz Foundation
2Doctoral student in Collective Health at René Rachou Research Center, Oswaldo Cruz Foundation

Global shifts in climate systems result from the build, in the high atmosphere, of the so-called greenhouse gases, from which the most important is carbon gas. These cases are a result of human activities, especially burning of fossil fuels (petroleum, coal) and plant biomass (deforestation). The result is an increase of solar radiation on the Earths surface, with its physical consequences (changes in temperature and rainfall patterns around the globe) and biological consequences (changes in phenology and species occurrence areas).

Until the end of the century, we expect the global average temperature to increase by 4.5ºC, influencing the occurrence of tropical diseases in new areas. Changed climates influence directly or indirectly the dynamics of tropical diseases. Directly, we can observe temperature and humiditys effect over infectious and parasitic agents and their vectors. Regarding temperature shifts, there is an ideal range (14ºC to 40ºC) in which the conditions are ideal for infection processes, however, subtle variations in the upper and lower limits can favor the occurrence of some diseases. A good example is the parasites reproductive cycle acceleration due to temperature increases. To some vectors, like Anopheles sp. and Aedes sp., warmer climates favor the development of immature forms and accelerate blood digestion in the vectors body, making them need to feed more frequently. This may contribute to an increased infection risk in areas with changed climate (e.g. Malaria). It can also geographically expand the distribution of a given disease (e.g. dengue fever) to places where it does not occur due to temperate climate, which prevents vector proliferation. Rainfall also has a direct influence over transmission dynamics, since it favors the formation of breeding spots and propitiates an increase of vegetal ecosystems, favoring an increase of host populations.

Some weather events can also cause infectious diseases outbreaks under indirect weather influence. If the climate change results in prolonged droughts, these could result in a human populations migration processes from the affected region and, this way, spatially dispersing a chronic infection previously restricted to a determined region. This seems to have happened in Brazilian Northeast during the 1980s and 1990s, when urban epidemics of cholera and visceral leishmaniasis (VL) were reported in some capitals immediately after successive years of drought. A similar mechanism took place in the early 1980s with Malaria outbreaks in Maranhão, when prolonged droughts triggered the migration of non-immunized populations to cities near the Amazon, areas of endemic transmission of the disease. Once the drought was over, the migrants would return home with the infection, causing a significant increase of imported malaria cases, which became more frequent than the autochthonous cases. As for floods, the relation between these events and leptospirosis cases were observed in Rio de Janeiro and Rio Grande do Norte, for example. Regarding the latter, epidemic outbreaks took place in years of heavy rainfall, above the historical average in 1985, 1986 and 1995.

Another factor for which climate change may influence tropical diseases is due to a greater frequency of natural phenomena as the El-Niño, which includes a periodic and irregular wind change and water temperature in the Pacific Ocean. The influence over the dynamics of infectious diseases can by diverse, since the phenomenon is associated to an increase of rainfall in some regions (e.g. São Francisco basin), while others, since the Amazon, the El-Niño is associated to a decreased rainfall. In Bahia, for example, the annual VL incidence seems to be influenced by the El-Niño episodes duration and frequency. Generally speaking, the years when the phenomenon occurred were associated to a lower VL incidence, while the following years (without the phenomenon) registered an increase in the diseases incidence.

Regardless of the expected shifts in tropical diseases dynamics or their occurrence areas, the fact is that climate change may require new ways to control and prevent these diseases in a close future. Whether in natural or in constructed environments, the environmental and climate changes expected for the next decades could create new ecological niches for both adult species and its immature forms. In face of this, monitoring incidence and geographical expansion of tropical diseases should be part of epidemiological surveillance, focused on populations suffering or that will still suffer the impacts of climate change.…