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| Effects of climate change on human health |
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Climate change is influencing thetransmission andburden of manyinfectious diseases worldwide.[1] Rising temperatures, shifting rainfall patterns, and more frequentextreme weather events affect howpathogens,vectors and disease hosts interact. These changes are altering the geographic ranges and seasonal activity of disease-carrying organisms such as mosquitoes and ticks, and influence the growth and survival ofbacteria and other pathogens in food and water systems.[1][2][3]: 9
Infectious diseases that are sensitive to climate can be grouped into:vector-borne diseases (transmitted viamosquitos,ticks etc.),waterborne diseases (transmitted throughviruses or bacteria in water), andfood-borne diseases (spread through pathogens in food).[4]: 1107 In 2022 scientists stated a clear observation that "the occurrence of climate-relatedfood-borne andwaterborne diseases has increased."[2]: 11
Vector-borne diseases likedengue fever,malaria,tick-borne diseases,leishmaniasis,zika fever,chikungunya andEbola are especially sensitive to climatic conditions. Warmer and wetter conditions expand suitable habitats for vectors, enabling them to survive in areas that were previously too cold or dry.[4]: 1045 [3] As temperatures rise at higher elevations and latitudes, transmission risks are expected to increase in parts of North America, Europe, and highland regions of Africa and Asia.[4]: 1094 [5] For example, the range of ticks that transmitLyme disease andtick-borne encephalitis has expanded, and further warming could lengthen their active seasons.[4]: 1094
Climate change also affects waterborne and food-borne diseases by influencingwater quality,sanitation, andmicrobial ecology. Warmer waters and increased flooding promote the growth and spread of bacteria such asVibrio cholerae, which causescholera, and other pathogens responsible forgastroenteritis and wound infections.Drought and poor access to clean water increase the risk of contamination and exposure todiarrheal diseases,typhoid, andhepatitis A.[4]: 1107 [3]: 12
The health impacts of these climate-related risks are unevenly distributed.Low-income countries and communities with highsocio-economic constraints and limited healthcare, infrastructure and sanitation are the most vulnerable.[6] Nearly one in three people globally lack access to safe drinking water, which amplifies exposure to waterborne pathogens and related illnesses.[7] These conditions can also affect mental and social well-being and place additional strain onpublic health systems.
Withoutmitigation andadaptation measures, climate-related infectious disease risks will continue to rise. Limitinggreenhouse gas emissions, strengtheningdisease surveillance,vector control,vaccination,water and sanitation services, and climate-resilient healthcare infrastructure is seen as essential to reducing these impacts.[8]
TheWorld Health Organization considers climate change as one of the greatest threats to human health.[9] Global warming, increased rainfall, flooding and drought are some of the consequences of climate change that are leading to the escalation of vector, food and water-borne diseases.[10] Climate change can make it easier for infectious diseases to spread to new regions and surge in areas where they were previously under control. As a result, diseases that have never previously infected humans (Disease X) may 'spill over' from animals and become a threat to humans too.[10][11] More than half (218 out of 375) of infectious diseases that affect humans worldwide have already been worsened by climate change.[12][13]
The successful emergence or reemergence of infectious diseases depend on people coming into contact with thepathogen (for example a virus) but also on the extent to which peoples' resistance is weakened, or the pathogen is strengthened, by changes in the environment.[13]
Climate change can increase the spread of infectious diseases through multiple possible pathways, including:[13]
Infectious diseases that are sensitive to climate can be grouped into:
Climate change is affecting the distribution of these diseases due to the expanding geographic range and seasonality of these diseases and their vectors.[16]: 9
Though many infectious diseases are affected by changes in climate, vector-borne diseases, such as malaria, dengue fever and leishmaniasis, present the strongest causal relationship. One reason for that is that temperature and rainfall play a key role in the distribution, magnitude, and viral capacity of mosquitoes, who are primary vectors for many vectors borne diseases. Observation and research detect a shift of pests and pathogens in the distribution away from the equator and towards Earth's poles.[17]
Climate change affects vector-borne diseases by affecting the survival, distribution and behavior of vectors such as mosquitoes, ticks and rodents.[18]: 29 The viruses, bacteria and protozoa are carried by these vectors transferring them from one carrier to another.[19] Vectors and pathogens can adapt to the climate fluctuations by shifting and expanding their geographic ranges, which alter the rate of new cases of disease depending on vector-host interaction, host immunity and pathogen evolution.[20] This means that climate change affects infectious diseases by changing the length of the transmission season and their geographical range.[9]
Climate change is leading to latitudinal and altitudinal temperature increases. Global warming projections indicate that surface air warming for a "high scenario" is 4 C, with a likely range of 2.4–6.4 C by 2100.[21] A temperature increase of this size would alter the biology and the ecology of many mosquito vectors and the dynamics of the diseases they transmit such as malaria.
Changes in climate and global warming have significant influences on the biology and distribution ofvector-borne diseases,parasites,fungi, and their associated illnesses. Regional changes resulting from changing weather conditions and patterns within temperate climates will stimulate the reproduction of certain insect species that are vectors for disease.
The vectors of transmission are the major reason for the increased ranges and infection of these diseases. If the vector has a range shift, so do the associated diseases; if the vector increases in activity due to changes in climate, then there is an effect on the transmission of disease.[22] However it will be hard to classify exactly why the range shifts or an increase in infection rates occurs as there are many other factors to consider besides climate change, such ashuman migration,poverty,infrastructure quality, andland usage; but climate change is still potentially a key factor.[23]
Mosquitoes
One major disease-spreading insect is themosquito, which can transmit diseases likemalaria,West Nile virus, anddengue fever. With regional temperatures changing due to climate change, the range of mosquitos will change as well.[24] The range of mosquitoes and disease transmission will move farther north and south, and places will have a longer period of mosquito habitability than at present, leading to an increase in the mosquito population in these areas.[25] This range shift has already been seen in highland Africa. Since 1970, the incidence of malaria in high-elevation areas in East Africa has increased greatly. This has been proven to be caused by the warming of regional climates.[22][26]
Many major cities in Africa (e.g.:Nairobi,Harare) were sited above the mosquito breeding-transmission altitude, but due to increasing temperatures mosquitos are now more likely to thrive in these areas as well. In Europe, Dengue cases were transmitted by populations of mosquitoes established in France and Italy.[27]
Environmental changes,climate variability, and climate change are factors that could affect biology anddisease ecology ofAnopheles mosquitoes and theirdisease transmission potential.[28] When the temperature rises, thelarvae take a shorter time to mature[29] and, consequently, a greater capacity to produce more offspring. In turn this leads to an increase in malaria transmission to humans.
Environmental changes such asdeforestation could also increase local temperatures in the highlands thus could enhance the vector capacity of the anopheles.[28] Anopheles mosquitoes are responsible for the transmission of a number of diseases in the world, such as, malaria,lymphatic filariasis and viral fevers such as theO'nyong'nyong virus.[28]
High temperatures can alter the survival, replication, andvirulence of a pathogen.[30] Higher temperatures can also increase the pathogen yields in animal reservoirs. An increase in yield of bacteria from drinking water delivery systems has been recorded in warmer summer months. During periods of warmer temperatures water consumption rates are also typically higher. Together these increase the probability of pathogen ingestion and infection.[31]
With an increase in both temperature as well as higher nutrient concentrations due torunoff there will be an increase incyanobacterial blooms.[32]
The warming oceans and lakes are leading to more frequentharmful algal blooms.[33][34][35] Also, during droughts, surface waters are even more susceptible to harmful algal blooms and microorganisms.[36]Algal blooms increase waterturbidity, suffocating aquatic plants, and can deplete oxygen, killing fish. Some kinds ofblue-green algae (cyanobacteria) createneurotoxins, hepatoxins, cytotoxins or endotoxins that can cause serious and sometimes fatal neurological, liver and digestive diseases in humans. Cyanobacteria grow best in warmer temperatures (especially above 25 degrees Celsius), and so climate change makes harmful algal blooms to be more frequent and last longer.[37]
One of these toxin producing algae isPseudo-nitzschia fraudulenta. This species produces a substance calleddomoic acid which is responsible foramnesic shellfish poisoning.[38][39] The toxicity of this species has been shown to increase with greater CO2 concentrations associated withocean acidification.[38] Some of the more common illnesses reported from harmful algal blooms include;Ciguatera fish poisoning,paralytic shellfish poisoning, azaspiracid shellfish poisoning,diarrhetic shellfish poisoning,neurotoxic shellfish poisoning and the above-mentioned amnesic shellfish poisoning.[38]
Climate change is forecast to have substantial effects on thewater cycle, with an increase in both frequency and intensity of droughts and heavy precipitation events.[30]
There is generally an increase indiarrheal disease (except for viral diarrheal disease) during or after elevated ambient temperature, heavy rainfall, and flooding.[40] These three weather conditions are predicted to increase or intensify with climate change in future. A high current baseline rate of the diarrheal diseases is already present in developing countries. Climate change therefore poses a real risk of an increase of these diseases in those regions.[40]
The climate crisis can causepsychological stress for populations threatened by changing weather patterns.[41] The climate crisis can cause stress especially in people living in areas where flooding is increasing, severe storms are destroying homes and they are being forced to move away.[41] Climate change can alsoimpact people's mental health through affect their workplaces and livelihoods. For example, farmers are particularly under stress because of the reducing capability ofclimate-affected land to produce crops and animal feed.[42] Stress increases the level of stress-hormonecortisol in blood, a suppressor of theimmune mechanisms responsible for protection against infections and cancers.[43] This leads to humans becoming inherently less able to resist new infections because of the climate crisis.[44]
One proven mitigation against psychological stress is interaction with the natural environment[45] but the climate crisis is also a nature andbiodiversity crisis[46] and the human race is becoming less able to find natural spaces to relax and to release the stresses of modern day living.[47]
Malaria is amosquito-borne disease that infects humans and other animals caused by microorganisms in thePlasmodium family. It begins with a bite from an infected female mosquito, which introduces the parasite through its saliva and into the infected host's circulatory system. It then travels through the bloodstream into the liver, where it can mature and reproduce.[48]
Climate is an influential driving force of diseases such as malaria, which kills about 300,000 children annually. Malaria is especially susceptible to the effects of climate change because mosquitoes lack the mechanisms to regulate their internal temperature.[49] This implies that there is a limited range of climatic conditions within which the pathogen (malaria) and vector (a mosquito) can survive, reproduce, and infect hosts.[50] Increased rainfall and temperatures could increase the number ofmosquitos indirectly by creating better environments for survival and reproduction, expand larval habitat and food supply.[49][50] There will also be greater risks of outbreaks of malaria due to increased flooding events, especially affecting the drier areas of thetropics, such as theSahel andEast Africa.[51][52][53]
Conservative estimates suggest that the risk of malaria will increase 5–15% by 2100 due to climate change.[54] In Africa alone, there is a projected increase of 16–28% in person-month exposures to malaria by 2100.[55][56]
Dengue fever is an infectious disease caused bydengue viruses found in tropical and sub-tropical regions.[57] It is transmitted through bites from female mosquitos in thegenusAedes, primarilyA. aegypti.[58] Dengue fever can be fatal.[59][57]
The cases of dengue fever have been increasing dramatically and is projected to continue to do so with changing climate conditions.[60] In just the past 50 years, transmission has increased drastically with new cases of the disease (incidence) increasing 30-fold.[57][61] The number of reported cases of dengue has increased from 505 430 cases in 2000 to 14.6 million in 2024. However, dengue cases are under-reported and it is estimated that there might be 100–400 million infections occurring each year.[61]
High temperatures, increased precipitation, and humidity all increase the risk of dengue infection.[62][63] The presence and number ofAedes aegypti mosquitoes is strongly influenced by the amount of water-bearing containers in the home and rubbish such as tin cans and tires contributing to pockets of stagnant water in an area. The species is also influenced by daily temperature and variation in temperature, moisture, andsolar radiation.[56]
Climate change is altering the geographic range and seasonality of the mosquito that can carry dengue.[61][64] While dengue fever is primarily considered atropical and subtropical disease, the geographic ranges of the Aedes aegypti are expanding with dengue cases reported in European andEastern Mediterranean regions.[27][57] Climate change also contributed to the spread of distinct variants of the disease to new areas.
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Tick-borne disease, which affect humans and other animals, are caused by infectious agentstransmitted bytick bites. Diseases spread by ticks includetick-borne encephalitis,Lyme disease,anaplasmosis,babesiosis,ehrlichiosis and spotted fever rickettsiosis.[65]
Climate, especially temperature and precipitation, affects the life-cycle, population size and geographic range of ticks.[66] A high humidity of greater than 85% is ideal for a tick to start and finish its life cycle.[67] Higher maximum temperatures play the most influential role in sustaining tick populations.[68] Tick life cycles span multiple seasons as they mature from larva to nymph to adult, and infection and spread of diseases such asLyme disease can happen across the multiple stages and different species of animal hosts.[69]
The expansion of tick populations is concurrent with global climate change. Species distribution models of recent years indicate that thedeer tick (Ixodes scapularis) is pushing its distribution to higher latitudes in Europe, Canada and Northeastern United States, and pushing and maintaining populations in the South Central and Northern Midwest regions of the United States.[70][71] Climate models project further expansion of tick habit north into Canada as progressing Northwest from the Northeastern United States. Additionally, however, tick populations are expected to retreat from the Southeastern coast of the U.S., but this has not yet been observed.[72] It's estimated that coinciding with this expansion, increased average temperatures may double tick populations by 2020 as well as bring an earlier start to the tick exposure season.[73][71] In Europe, expansion of tick-borne diseases to the north due to climate change has already been observed, especially for Lyme disease and tick-borne encephalitis.[74]
Leishmaniasis is aneglected tropical disease, caused by parasites of the genusLeishmania and primarily transmitted byPhelbotomus sand flies. It is distributed mostly in tropical and subtropical regions around the world, wherever thesand flies and suitable animal hosts are present.[75] Around 12 million people around the world are living with leishmaniasis.[75]Risk factors that increase the spread of this disease include poverty,urbanization,deforestation, and climate change.[76][77]
As in othervector-borne diseases, one of the reasons climate changes can affect the incidence ofleishmaniasis is the susceptibility of the sandfly vectors to changes in temperature, rainfall and humidity. These conditions alter their range of distribution and seasonality.[76] For example, climate change could increase the suitable conditions forPhlebotomus sandfly species inCentral Europe[78][79] andLutzomyia longipalpis in theAmazon Basin.[80] Parasite development inside the sand for some species of Leishmania can also be affected by temperature changes.[81]
TheEbola virus has caused periodic outbreaks across several African countries. With an average fatality rate of about 40%, the disease has led to more than 28,600 reported cases and 11,310 deaths.[82] Areas undergoingdeforestation are among the most likely places for outbreaks due to changes in the landscape bringing wildlife into closer contact with humans.[83][84]
Climate change may indirectly contribute to the rise in Ebola cases.Extreme weather events such as droughts, strong winds, thunderstorms, heat waves, floods, landslides, and shifting rainfall patterns can disrupt wildlife migration, pushing animals out of their natural habitats and nearer to human settlements.[85] For instance, a severe drought in Central Africa intensifiedfood insecurity, leading some West African communities to hunt and consume infected animals such as bats, which likely fueled an Ebola outbreak.[83]
Zika virus, a vector-borne virus, was historically presented in cluster outbreaks in the tropical regions of Africa and Asia.[86] Zika fever is mainly spread via the bite of mosquitoes of the Aedes type, which is expanding its range.[87]Zika fever epidemics have affected larger populations including Micronesia and South Pacific Islands in 2007, and the Americas in 2013.[88] Brazil experienced one of the largest outbreaks of Zika virus with approximately 1.5 million cases reported in 2015.[89] Pregnant women infected with Zika virus are at a higher risk of giving birth to children withbirth defects, includingmicrocephaly.[90]
It is predicted that Zika virus will expose more than 1.3 billion new people by 2050 due to climate change. This increase is largely due to the expansion of habitats conducive to vector growth and life cycles, particularly areas with temperatures ranging from 23.9 °C to 34 °C.[91] Rising temperatures also influence mosquito behavior, leading to higher breeding and biting rates.[92] Extreme climate patterns, such as drought, floods and heatwaves further enhance mosquito breeding conditions and as a result escalate the rate of virus-borne diseases.[93]
There is no direct evidence that the spread ofCOVID-19 is worsened or caused by climate change, although investigations continue. As of 2020[update], theWorld Health Organization summarized the current knowledge about the issue as follows: "There is no evidence of a direct connection between climate change and the emergence or transmission of COVID-19 disease. [...] However, climate change may indirectly affect the COVID-19 response, as it undermines environmental determinants of health, and places additional stress on health systems."[94]
A 2021 study found possible links between climate change and transmission of COVID-19 by bats.[95] The authors found that climate-driven changes in the distribution and richness of bat species increased the likelihood of bat-borne coronaviruses in the Yunnan province, Myanmar, and Laos.[95] This region was also the habitat of Sunda pangolins and masked palm civits which were suspected as intermediate hosts of COVID-19 between bats and humans.[95] The authors suggest, therefore, that climate change possibly contributed to some extent to the emergence of the pandemic.[95][96]
Climate change might induce changes to bat habitats which may have driven them closer to populated areas.[97] Increased aridity and drought periods are predicted to push bats out of their endemic areas and into populated areas.[97] This creates a knock-on effect of increasing their interactions with humans and hence the likelihood of zoonotic disease transfer.[97]
There is a growing impact of the climate crisis in drivingHIV risk which is affecting the availability of and access to services for HIV treatment, prevention and care.[98] 3.6 billion people live in regions highly susceptible to climate change, and many of those populations are disproportionately affected by HIV.[99][100] Increased migration and population displacement, food insecurity, economic stress, conflict, communicable diseases, and the erosion of health infrastructure can increase the rates of HIV infection and worsen the health and wellbeing of people with HIV.[98]
The climate crisis can impact HIV programmes and people with HIV in various ways, for example reducing access to prevention, testing, and treatment services, contributing to poor nutrition, impaired immunity, and pooradherence to treatment.[100] It can also increase the risk of contracting HIV, for example through food insecurity forcing people to engage intransactional sex, and can worsen the discrimination of populations most affected by HIV.[100][101] Reductions in international aid reductions, the discontinuation of the USPresident's Emergency Plan for AIDS Relief (PEPFAR) could cause around 4–11 million new HIV infections and 1–3 million HIV-related deaths between 2025 and 2030.[102]
Based on mathematical models, if there is no reduction incarbon emissions it is expected that there will be an increase of between 11 million and 16 million HIV cases in countries in sub-Saharan Africa by 2050 as a result of increasing temperatures due toclimate change.[103]
HIV outcomes may be affected by the increased spread of vector-borne pathogens to whichimmunocompromised individuals are more susceptible, notably malaria and fungal diseases such ascryptococcal disease,histoplasmosis, andtalaromycosis.[100] TheJoint United Nations Programme on HIV/AIDS emphasized that it is essential that HIV programmes consider and monitor the potential effects of climate change.[98]
Diarrheal diseases are caused by bacteria, viruses and protozoans. These diseases are transmitted mainly by the feces of an infected individual passing to the mouth of another individual (fecal–oral route), due to limited access to water, sanitation and hygiene.[104] They are the most common waterborne diseases,[30] transmitted through unsafe drinking water or recreational water contact.[32]
Diarrhoeal diseases are the third greatest cause of death in children under 5 years killing around 444,000 children every year.[104] They are also the second leading cause of death inlow and middle income countries. Diarrhea diseases account for an estimated 1.4–1.9 million deaths in all ages worldwide.[31]
Diarrhoeal diseases due to bacteria increase with ambient temperature, and increase following heavy rainfall and flooding events. Heavy rainfall and flooding can also affect pathogen transmission via impacts on sanitation and/or drinking water treatment infrastructure, contaminating drinking water sources or food products. Floods can overwhelm water systems, causingbackflows that lead to contamination ofgroundwater and other drinking water sources. Investing in the prevention of diarrheal diseases through improvements inWater, Sanitation and Hygiene (WaSH) systems and other proven diarrhea prevention strategies should be a global priority. A key diarrhoeal disease ischolera.[31]
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Disease outbreaks caused byvibrio (in particular the bacterium that causescholera, calledVibrio cholerae) are increasing in occurrence and intensity.[4]: 1107 Vibrio illnesses arewaterborne disease that were prevalent especially in Asia and Africa, but they are increasing worldwide as well as, being reported where historically they did not occur. Vibrio infections are caused by consuming contaminated water, raw or undercooked seafood, or by exposing an open wound to contaminated sea water. Vibrio infections are most likely to occur during the warm seasons.[105]
The warming climate plays a substantial role in the increase in cases and area of occurrence.[106][107] Even modest temperature increases substantially amplify disease transmission. For example, a 1°C warming in Zanzibar was associated with a doubling of confirmed cholera cases.[107] Furthermore, prolonged water scarcity concentrates Vibrio cholerae in limited freshwater supplies, while intense precipitation events compromise sanitation infrastructure and create conditions favorable for the spread of the disease.[107][4]: 1045 The area of coastline with suitable conditions for vibrio bacteria has increased due to changes insea surface temperature and sea surface salinity caused by climate change.[16]: 12 These pathogens can causegastroenteritis,cholera, wound infections, andsepsis. It has been observed that in the period of 2011–21, the "area of coastline suitable for Vibrio bacterial transmission has increased by 35% in theBaltics, 25% in the Atlantic Northeast, and 4% in the Pacific Northwest.[16]: 12
Rising temperatures and humidity increase the growth of skin-associated bacteria and alter the geographical distribution of other organisms that infect humans. The various microorganisms that make up the skinmicroflora each have different optimal temperature ranges for survival and growth.Staphylococcus aureus andCorynebacterium sp. amongst others are more tolerant to rising temperatures and higher salt conditions compared to other, non-commensal bacteria.[108]
Fungal infections will also see an increase due to the warming of certain climates.[22] For example, the fungusCryptococcus gattii is normally found in warmer climates such as inAustralia, but has now been found in Canada. There are now twostrains of this fungus in the northwestern part of North America, affecting many terrestrial animals. The spread of this fungus is hypothesized to be linked to climate change.[23]
Further Information:Naegleri Fowleri
Naegleri Fowleri, apercolozoa, thrives as free-living, thermophilicamoeba in freshwaters such as lakes, rivers, ponds, hot springs.[109] Although infection is still considered rare, cases are becoming more prevalent each year as freshwater temperatures continue to rise. Exposure of thispathogen is more commonly caused by recreational water activities, such as swimming or diving, in freshwaters where the bacteria is present.[109] Humans can become infected when water enters the nasal canal. From there the amoeba penetrate nasal mucous and travel to the brain, creating inflammation and eventually causingprimary amoebic meningoenephilitis (PAM);[109] which if not treated immediately can be fatal. Naegleri Fowleri favor warm, moist conditions, feeding off othermicroorganisms. It is also known to be found in tap water, well water and water distribution tanks as it favors both naturally heated and artificially heated waters, though it has also been known to survive in various temperatures.
There is concern about theemergence of new diseases from thefungal kingdom. Mammals haveendothermy andhomeothermy, which allows them to maintain elevated body temperature through life; but it can be defeated if the fungi were to adapt to higher temperatures and survive in the body.[110] Fungi are able to adapt rapidly to higher temperatures. The emergence ofCandida auris on three continents is proposed to be as a result of global warming and has raised the danger that increased warmth by itself will trigger adaptations on certain microbes to make them pathogenic for humans.[111]
It is projected thatinterspecies viral sharing, that can lead tonovel viral spillovers, will increase due to ongoing climate change-caused geographic range-shifts of mammals (most importantlybats).Risk hotspots would mainly be located at "high elevations, in biodiversity hotspots, and in areas of high human population density in Asia and Africa".[112]
Climate change may also lead to new infectious diseases due to changes in microbial and vector geographic range. Microbes that are harmful to humans can adapt to higher temperatures, which will allow them to build better tolerance against human endothermy defences.[110]
Climate change and increasing temperatures will also impact the health of wildlife animals. Climate change will impact wildlife disease, specifically affecting "geographic range and distribution of wildlife diseases, plant and animal phenology, wildlife host-pathogen interactions, and disease patterns in wildlife".[113]
The health of wild animals, particularly birds, is assumed to be an indicator of early climate change effects because very little or no control measures are undertaken to protect them.[9]
Geographic shifts of disease vectors and parasitic disease in the Northern Hemisphere have likely been due to global warming. The range ofParelaphostrongylus odocoilei, a lung parasite that impacts ungulates like caribou and mountain goats, has been shifting northward since 1995, as has a tick vector for Lyme disease and other tick-bornezoonotic diseases known asIxodes scapularis.
It is predicted that climate warming will also lead to changes in disease distribution at certain altitudes. At high elevation in the Hawaiian Islands, for example, it is expected that climate warming will allow for year-round transmission ofavian malaria. This increased opportunity for transmission will likely be devastating to endangered native Hawaiian birds that have little or no resistance to the disease.[113]
Predicting the impact of climate change on disease patterns in different geographic regions can be difficult, because its effects likely have high variability. This has been more evident in marine ecosystems than terrestrial environments, where massive decline incoral reefs has been observed due to disease spread.[113]
Phenology is the study of seasonal cycles, and with climate change the seasonal biologic cycles of many animals have already been affected. For example, the transmission of tick-borne encephalitis (TBE) is higher to humans when early spring temperatures are warmer. The warmer temperatures result in an overlap in feeding activity of ticks who are infected with the virus (nymphal) with ticks who aren't (larval). This overlapped feeding leads to more uninfected larval ticks acquiring the infection and therefore increases the risk of humans being infected with TBE. Conversely, cooler spring temperatures would result in less overlapped feeding activity and would therefore decrease the risk of zoonotic transmission of TBE.[113]
The transmission of pathogens happens through either direct contact from a diseased animal to another, or indirectly through a host like infected prey or a vector. Higher temperatures as a result of climate change results in an increased presence of disease producing agents in hosts and vectors, and also increases the "survival of animals that harbor disease".[113] Survival ofParelaphostrongylus tenuis, a brain worm ofwhite-tailed deer that affects moose, could be increased due to the higher temperatures and milder winters caused by climate change. In moose this worm causes fatal neurological disease. Moose are already facing heat stress due to climate change, and may have increased susceptibility to parasitic and infectious diseases like the brain worm.[113]
Vector-borne diseases seriously affect the health of domestic animals andlivestock (e.g.,trypanosomiasis,Rift Valley Fever, andbluetongue). Climate change will also indirectly affect the health of humans through its multifaceted impacts on food security, including livestock and plant crops.[9]
While climate-induced heat stress can directly reduce domestic animals' immunity against all diseases,[114] climatic factors also impact the distribution of many livestock pathogens themselves. For instance,Rift Valley fever outbreaks in East Africa are known to be more intense during the times of drought or when there is anEl Nino.[115] Another example is that ofhelminths in Europe which have now spread further towards the poles, with higher survival rate and higher reproductive capacity (fecundity).[116]: 231 Detailed long-term records of both livestock diseases and various agricultural interventions in Europe mean that demonstrating the role of climate change in the increased helminth burden in livestock is actually easier than attributing the impact of climate change on diseases which affect humans.[116]: 231
Temperature increases are also likely to benefitCulicoides imicola, a species ofmidge which spreadsbluetongue virus.[115] Without a significant improvement in epidemiological control measures, what is currently considered an once-in-20-years outbreak of bluetongue would occur as frequently as once in five or seven years by midcentury under all but the most optimistic warming scenario.Rift Valley Fever outbreaks in East African livestock are also expected to increase.[117]: 747 Ixodes ricinus, atick which spreads pathogens likeLyme disease andtick-borne encephalitis, is predicted to become 5–7% more prevalent on livestock farms in Great Britain, depending on the extent of future climate change.[118]
The impacts of climate change onleptospirosis are more complicated: its outbreaks are likely to worsen wherever flood risk increases,[115] yet the increasing temperatures are projected to reduce its overall incidence in the Southeast Asia, particularly under the high-warming scenarios.[119]Tsetse flies, the hosts oftrypanosoma parasites, already appear to be losing habitat and thus affect a smaller area than before.[117]: 747
Tropical diseases will likely migrate and becomeendemic in many other ecosystems due to an increase in mosquito range.[120] Other diseases carried by mosquitoes, such asDirofilaria immitis (heartworm) which affects dogs, will be able to spread to new areas.[121] Heartworm larvae also need temperatures above 14°C for maturation within mosquitos. As climate change increases the temperature, heartworm will be able to expand the geographical range where it is able to mature and transmit.[122]
Global efforts focus on disease surveillance and research, adaptation strategies, and interdisciplinary collaboration to counter climate change's role in expanding vector-borne diseases like malaria and dengue. Organisations like theIPCC andWHO emphasise developing resilient health systems, early warning mechanisms, and vector control to mitigate risks from shifting pathogen ranges and extreme weather.[123][124][125] Enacting policy that will reduce greenhouse gas emissions will mitigate climate change.[126]
Traditional vector-control measures like bed nets, indoor residual spraying, and improved water and sanitation remain central disease prevention strategies. Investment in vaccines and therapeutics for climate-sensitive diseases has increased, including malaria and dengue vaccine development and deployment. Climate considerations are increasingly incorporated into pandemic preparedness and research agendas, with emphasis on anticipating shifts in disease burden driven by warming temperatures and changing ecosystems.[123]
Significant progress has been achieved in surveillance systems, disease and vector control measures, vaccine development, diagnostic tests, and mathematical risk modeling/mapping in recent decades.[9] However, a 2023 review found that existing climate change and infectious diseases research does not adequately represent different populations or regions of the world, and tends to focus on a narrow set of diseases. Due to these gaps, our understanding of how climate change impacts human health is incomplete.[127]
Scientists conduct attribution studies to determine the degree to which climate change affects the spread of infectious diseases. There is also a need for scenario modeling which can help further our understanding of future climate change consequences on infectious disease rates.[128] Disease surveillance systems are used to forecast outbreaks of climate-sensitive infectious diseases, enabling earlier public-health responses and targeted interventions.[126][129][130] Governments should accurately model changes in vector populations as well as the burden of disease, educate the public on ways to mitigate infection and prepare health systems for the increasing disease load.
A tool that has been used to predict disease distribution trends is the Dynamic Mosquito Simulation Process (DyMSiM). DyMSiM usesepidemiological andentomological data and practices to model future mosquito distributions based upon climate conditions and mosquitos living in the area.[131] This modeling technique helps identify the distribution of specific species of mosquito, some of which are more susceptible to viral infection than others.[citation needed]
The policy implications of climate change and infectious diseases fall into three categories:[126]
Addressing all three of these areas is critical, and those in the poorest countries face the greatest burden. Additionally, when countries are forced to contend with a disease like malaria (for example), their prospects forself-sufficiency andprosperity are slowed. This contributes to continued and worsening global inequality.[128]
Policies and implementation are required that will significantly increase investments in public health in developing countries. This achieves two goals, the first being better outcomes related to diseases like malaria in the affected area, and the second being an overall better health environment for populations.[126]
As many climate-sensitive infectious diseases are zoonotic or vector-borne there has been increased adoption of theOne Health approach which integrates human, animal, andenvironmental health.[126] TheUnited Nations Environment Programme states that: "The most fundamental way to protect ourselves fromzoonotic diseases is to prevent destruction of nature. Where ecosystems are healthy and biodiverse, they are resilient, adaptable and help to regulate diseases."[132] Governance responses need to incorporate more holistic and intersectoral understandings of the relationship between infectious diseases, climate change and environmental degradation, as in the concept ofplanetary health.[133] These responses particularly require pro-active participation, leadership and ownership from indigenous communities and other local groups whose voices are often marginalised and sidelined in decision-making processes at the national, regional, international and global levels.[134][135]
The challenges posed by infectious diseases require increased cross-border and transnational cooperation involving a wide range of relevant stakeholders, such as universities, pharmaceutical laboratories, non-governmental organisations and national governments. This cooperation should coordinate initiatives and policy approaches across different governance levels (from local to global) and develop transnational responses that reflect the transnational reality of climate-sensitive infectious diseases.[136]
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