Climate change not only causes temperatures to rise and extreme weather events to increase – climate impacts such as droughts, heat waves or floods also influence the risk of many infectious diseases. In an analysis of ten climate impacts and 378 known infectious diseases, researchers now quantify the extent of this influence. According to their results, climate impacts have already favored 58 percent of these diseases and led to more cases. Examples of this are pathogens that are transmitted by mosquitoes or ticks, but also pathogens that are spread via water or wild animals. At the same time, the worsening climatic conditions can also make people more susceptible to infections.
Whether tiger mosquito, Asian bush mosquito or exotic tick species: In recent years, several animal species that originally only occur in more southern regions have become established here and are considered potential carriers of infectious diseases. The increasingly mild winters and warm summers favor the survival of such vector species and the viruses or bacteria they transmit. As a result, tropical diseases such as dengue, chikungunya or spotted fever could also occur here in the future. There have already been first cases of West Nile fever, transmitted by mosquitoes, in Germany. But weather-related catastrophes can also promote diseases, for example by the spread of aquatic germs after floods or by flooded areas becoming mosquito breeding grounds.
More favorable conditions for 277 infectious diseases
Camilo Mora from the University of Hawaii in Manoa and his colleagues have now examined in more detail whether and how different climate impacts affect infectious diseases and their pathogens. For their meta-analysis, they evaluated the results of 830 studies from all over the world on all 375 known infectious diseases that are listed in official lists of health organizations. In addition to classic infectious diseases transmitted by microorganisms such as viruses, bacteria or fungi, they also recorded 40 diseases that were not communicable in the classic sense, including allergies in particular. “Their triggers can also be amplified by warming, flooding and storms and are becoming a serious public health problem for outbreaks of asthma, skin and respiratory diseases,” the scientists explain.
During the evaluation, the scientists found 3,213 documented cases of illness for which a connection with climate impacts could be proven. “These examples could be traced back to 286 diseases, 277 of which were aggravated by at least one climate-related factor,” report Mora and his team. “Only nine diseases caused by pathogens were negatively influenced by climate risks.” According to the analyzes, 58 percent of all diseases caused by pathogens have already been promoted by climate change and its consequences. There are many different ways in which this happens. “We have identified 1006 species that lead to cases of pathogenic diseases through climate impacts via various forms of transmission,” explains the team.
Effective in a thousand different ways
Warming has the greatest influence here: According to the data, it promotes 160 diseases, for example by expanding the distribution areas of pathogens and their carriers. The scientists cite diseases such as Zika, dengue, malaria, chikungunya, the plague or West Nile fever as examples of this, which are transmitted by vectors such as mosquitoes that have spread to previously cooler areas. The warming of the water in oceans and inland waterways can and does promote the spread of diseases, the team reports. Bacteria such as the cholera pathogen Vibrio cholerae or certain pathogenic amoebae can multiply better in warm water and lead to more outbreaks, even in areas where this previously did not occur at all or rarely. Changes in rainfall and flooding are linked to 122 and 121 different diseases, respectively, according to the data. This includes many diseases spread by mosquitoes or aquatic pathogens. Warmth and changes in precipitation can also create more favorable conditions for rodents and trigger mass breeding, which in turn can transmit pathogens such as hantavirus or plague to humans.
However, climate impacts also promote the transmission of diseases by bringing people and potential vectors into closer contact. This is the case, for example, when wildfires, droughts or floods bring bats, rodents and other wildlife closer to human settlements. Conversely, these climate impacts can lead to people driving their livestock into previously untouched habitats such as forests or using these habitats themselves for agriculture or as settlement areas. “Such climate-related land-use changes have encouraged human encroachment into wild areas and triggered numerous outbreaks of diseases such as Ebola, malaria, Tsutsugamushi fever, Lyme disease and Australian tick-borne typhus,” write Mora and his colleagues. At the same time, climate impacts such as heat or weather catastrophes can also weaken human defenses and make them more susceptible to infections.
“The sheer volume of pathogenic diseases and modes of transmission that are exacerbated by climate-related risks reveals the magnitude of the human health threat posed by climate change,” the scientists state. It is all the more important to slow down climate change by reducing greenhouse gas emissions. The virologist Renke Lühke from the Bernhard Nocht Institute for Tropical Medicine in Hamburg, who was not involved in the study, takes a similar view: “The study impressively shows that many different transmission paths have an influence on various pathogens. This complexity makes social adaptation very difficult, so that reducing greenhouse gas emissions must continue to be the main countermeasure.”
Source: Camilo Mora (University of Hawaii, Manoa) et al., Nature Climate Change, doi: 10.1038/s41558-022-01426-1