Introduction
Climate change, including global warming and its effects on the Earth's weather patterns, is largely caused by increased greenhouse gas emissions as a result of human activity, with other ‘anthropogenic’ factors such as deforestation and increased atmospheric aerosol levels playing additional roles.(1) The mean annual temperature in South Africa has increased by at least 1°C over the past 50 years, which is more than the global average.(2) Climate change has already had a profound impact on the physical environment, ecosystems, and human societies across the globe, and its effects are predicted to increase in concert with the ongoing global warming in the decades to come. Humans can be affected by climate change in many different and interlocking ways, including food insecurity, water insecurity, and extreme weather conditions like heat waves and floods. However, one major impact on humans is predicted to be more indirect: climate change can shift the patterns of infectious diseases, in most cases for the worse.
The extent of this risk was recently qualified in a systematic review by Mora et al.(3) Drawing on three complementary search strategies, the authors found over 3213 instances in the published literature where specific climate hazards (such as floods or global warming) impacted known pathogenic diseases. This included over 1000 unique pathogenic pathways. Overall, 286 unique infectious diseases were impacted – a number that makes up over three quarters (76%) of the major infectious diseases listed in authoritative international databases (figure 1).(4) Worryingly, 78% of the affected diseases were aggravated by climate hazards in all documented instances, 19% were sometimes aggravated and sometimes diminished, and only 3% were exclusively diminished.
Effect of climate change on infectious diseases. (Data source: Mora C et al. Nat Clim Chang. 2022 Aug 8;1-7).
Conceptually, the types of infectious diseases most influenced by climate change are the ones where the pathogens spend significant periods outside the human body and in the environment.(5) In decreasing order of importance, these are the diseases spread by vectors, water, aerosols, fomites, and food.(3) Diseases that are spread directly from human to human (e.g. sexually-transmitted infections or most human respiratory viruses) are far less likely to be directly affected by climate, although indirect effects may still intervene as a result of human displacement, living conditions, and general state of health.(5)
Vector-borne diseases
A vector is an organism that acts as an intermediary in the transmission of an infectious agent. These are typically invertebrates, and the two most important are mosquitos and ticks.
Most vectors are highly sensitive to the prevailing climate. Mosquito reproduction, survival and biting rates are all increased by warmer temperatures, and higher seasonal temperatures have been linked to increased transmission of arboviral diseases such as dengue, zika, and chikungunya.(6,7) Mosquito breeding is also impacted by changes in rainfall patterns. Increased rainfall provides more stagnant water pools for mosquitos to lay their eggs in, but abnormally dry conditions can paradoxically also increase mosquito-borne disease rates, probably because of the increased human-mosquito contact that is associated with the temporary storage of scarce water, as well as by forming pools of stagnant water in places where water flowed before.(8)
In addition to altering vector behaviours, the traditional geographical ranges of the vectors can also be shifted by climate change. As a result of the progressive increase in average global temperature, vectors such as mosquitos may find hospitable habitats at higher latitudes and higher altitudes than previously. Lastly, warmer temperatures may extend the disease transmission period of certain diseases, such as malaria.
Detailed investigations of these changes however, quickly reveal complexities. Aedes mosquitos (responsible for transmitting dengue, yellow fever, chikungunya, and Zika, amongst other diseases) consist of two main disease-transmitting species: Ae. albopictus and Ae. aegypti. The latter are more heat-tolerant than the former. As a result, while rising global temperatures have been predicted to potentially expose hundreds of millions of new people to Aedes-borne viral diseases in formerly temperate areas, the transmission potential in the tropics from Ae. albopictus may actually decrease as the temperature in tropical regions becomes too hot for year-long transmission periods.(9) A similarly complex, heterogenous pattern is predicted for malaria, a disease whose transmission by Anopheles mosquitoes is essentially limited to environmental temperatures of between 17 and 34°C.(10) As temperatures rise, the most suitable areas for malaria transmission in Africa (central and western Africa) may actually become less suitable, with the population most at risk shifting eastwards over time, away from the western coastal regions of Africa and towards Uganda.(11) Overall, however, malaria's geographical range in Africa is predicted to expand, and areas such as South Africa (where temperatures are, for the most part, too cold to sustain malarial transmission currently) may become new targets for malaria to expand into.(11) Ominously, higher temperatures have already been linked to increased malarial transmission in Limpopo.(12)
Ticks transmit several important infectious diseases, including Crimean Congo haemorrhagic fever, tick-borne encephalitis, and various rickettsial diseases. As with mosquitos, warmer temperatures will open up new geographical niches for them to move into. Rising temperatures have been linked to a 50-fold increase in tick-borne encephalitis incidence in northern Russia from 1980-2009. The sharp rise in cases was strongly associated with the northward expansion of the vector, Ixodes persulcatus, in synchrony with warming annual air temperatures.(13) Similarly, Lyme disease has been increasing in Europe at least partly due to warming temperatures, which have led to greater numbers of the tick vector, Ixodes ricinus, as well as an expansion of its range into higher latitudes.(5) Similar invasion by tick vectors into areas that were previously too cold for them has been documented in Sweden, Canada, and the Czech Republic.(14)
Water-borne diseases
Extreme weather conditions frequently lead to an increase in water-borne illnesses, either as a result of flooding or, paradoxically, drought.(5) Floods disrupt sanitation systems, leading to contamination of water sources and the environment. During a drought, there is a lack of water for sufficient hygiene, and droughts also disrupt the normal flow of rivers and diminish the size of standing water sources, concentrating water-borne pathogens within them.
Among the most important water-borne infectious diseases affected by climate change is cholera, caused by the bacterium Vibrio cholerae. Cholera is acquired primarily by ingesting contaminated water or food, and its incidence is on the rise due to several different factors associated with climate change. V. cholerae is commonly found in coastal ecosystems and is affected by rising sea levels because of climbing temperatures.(1) This encroachment of saltwater inland increases the distribution and prevalence of marine bacteria such as V. cholerae. (15) Warm water of moderate salinity is ideal for this bacterium, and so are increased temperatures and precipitation, with a subsequent reduction in salinity, resulting in an accelerated growth rate.(15) A 2000-2001 cholera outbreak in KwaZulu Natal was linked to unusually elevated sea temperatures.(16) During natural disasters, cholera dissemination may occur also following the introduction of V. cholerae into an area by displaced populations or aid workers, facilitated by overcrowding and limited access to safe water.(17) A cholera outbreak occurred in Zimbabwe in 1992 following a severe drought in the region and was exacerbated by a subsequent influx of refugees due to the drought.(18)
Other Vibrio species are similarly affected; V. vulnificus and V. parahaemolyticus cause soft tissue infections through the exposure of wounds to contaminated water, and infection can range from mild to fatal. Heat waves in northern Europe over the last 30 years have been associated with increased Vibrio soft tissue infections. The highest number of annual Vibrio-associated soft tissue infections ever reported in Sweden and Finland occurred in 2014, a year that corresponded with the most severe heatwave the region had experienced.(19)
An increase in soft tissue infections secondary to other bacteria and fungi have also been associated with climate change events. Increasing temperatures and flooding after a hurricane, tsunami or heavy rain are most frequently associated with these infections.(20) A retrospective analysis found that cases of cellulitis increased post-Typhoon Morakot in Taiwan in 2009 and that infections associated with limb immersion into flood water were more likely to be secondary to polymicrobial or Gram-negative bacteria.(21)
The spread of other water-borne diarrhoeal diseases apart from cholera can also be influenced by higher global temperatures. Possible explanations for this include more picnics and outdoor cooking, increased water-related activities, improved conditions for pathogen growth, and drought. Drought can lead to increased human or animal faecal-based fertilisers being used to try to maintain crops despite limited water and a lack of hygiene associated with water shortages.(22) On the other hand, heavy rainfall can disturb sediment leading to the accumulation of faecal bacteria.(23)
Foodborne, airborne and direct contact infectious diseases
Foodborne diseases are those that are acquired by ingesting food contaminated by pathogenic organisms. The most concerning of these pathogens are those that have low infectious doses, persist in the environment, and can withstand extreme environmental conditions. One of these bacterial species is Salmonella, whose reproduction rate increases as temperature increases within the range of 7-35°C. Increasing environmental temperatures, together with heavy rainfall and flooding, are therefore associated with increased Salmonella infections.(1) Similarly, Campylobacter infections are also increased with rising temperatures.(24)
Vibrio vulnificus and parahaemolyticus can be acquired through the ingestion of contaminated seafood. An increase in the area and suitability of their environment, as outlined above, will contribute to an increase in these foodborne infections.(19)
Wind and dust storms can be a factor in the transmissibility of airborne infections. Avian influenza outbreaks have been linked to regions downwind of Asian dust storms, involving Japan and South Korea in particular, and Influenza A concentrations were found to be significantly higher on dust storm days compared to normal days.(23)
Coccidioides immitis, an infectious dimorphic fungus found only in the Americas, has a well-established association with dust storms, with outbreaks occurring afterwards. It has also been shown that on dust storm days, the concentration of cultivatable bacteria and fungi is increased, whether this increases infection of these pathogens is not yet known.(25)
Sudden large changes in temperature have been associated with influenza epidemics in northern mid-latitude regions, specifically the United States, China, France and Italy.(25) These large temperature changes quantified by ‘rapid weather variability’ are, through climate projection models, estimated to increase in the future. If this is accurate, the influenza epidemic risk will increase by 20%–50% in these regions.(26)
Soil-transmitted helminths and hookworms are also affected by climate change. Higher temperatures increase the rate of hookworm egg and larval development, which decreases the time to infectivity. Increased precipitation and humidity extend the survival period of larvae, as their desiccation is prevented.(22) These factors may lead to increased hookworm infections.
Lastly, Ebola is an example of a zoonotic infection impacted by climate change and can be acquired through direct contact. Fruit bats are migrating and settling in towns and cities because of climate variability, increasing their contact with humans and leading to an increased risk for Ebola infection. Extreme weather can rob farmers of cultivatable land, forcing them to venture into forested areas to look for suitable land to farm, thus bringing them closer to wild animals.(27)
Infectious diseases diminished by climate change
Although most infectious diseases are expected to worsen with climate change, a handful of diseases may diminish or attenuate instead. Several of these are respiratory diseases (such as those caused by respiratory syncytial virus and pneumococcus) which have long been associated with colder weather conditions, partly due to indoor clustering of people as well as favourable environmental effects on the stability of infectious droplets and aerosols.
The fate of certain helminths in warming, more volatile world may also be imperilled. A good example of this is schistosomiasis (bilharzia), a disease that depends on freshwater snails to complete its pathogen's lifecycle. Climate change is expected to lead to several adverse impacts on the range and survival of these snails, including decreased reproduction from warmer water temperatures and shrunken range from droughts.(28) In addition, while warmer temperatures may increase the production of infectious schistosomal cercaria, this comes at the expense of the snail host's survival, and these cercariae may also not survive as long. Overall, a decrease in the bilharzia transmission area within Africa of perhaps 13-18% has been modelled to occur by 2080.(29) South Africa may not reap this benefit though, as global warming is expected to expand local transmission zones further south, into areas currently too cold to sustain schistosomiasis transmission.(29)
Conclusion
The effects of climate change on infectious diseases are likely to be profound, with over three quarters of the major infectious diseases potentially being impacted. Alarmingly, the published literature suggests a sharp asymmetry to this impact – the vast majority are likely to worsen rather than diminish. Although this may reflect a degree of research and publication bias (more effort is likely to have gone into understanding potential climate risks than benefits), the overall trajectory is nonetheless sobering. A warming climate, and more frequent and extreme climate hazards, are likely to exacerbate infectious diseases, whether spread by vectors, water, air, or food. And climate-driven shifts in the geographical range may mean that diseases which are on the verge of control could shift to geographical regions that are unprepared for the new challenge.
There is still much uncertainty in these prognostications, as the extent of climate change over the coming decades can still be tempered by our collective actions. And we are only beginning to understand the intricate, interlocking ways in which climate can affect the complex organisms that cause infectious diseases. In addition to intensifying our efforts to minimize climate change and ploughing increased resources into studying its effects, we also need to redouble our efforts to control and contain – and, where possible, eliminate and eradicate – as many infectious diseases of public health importance as possible. Climate change may well tend to set diseases like malaria, dengue, and cholera on a worse course, but there is much we can still do in response. Efforts such as improving access to treatment, controlling vectors, providing mass chemoprophylaxis, and developing more efficacious vaccines are perhaps more needed than ever and offer yet other ways to combat climate change's pernicious effects.
