Editing IPCC:AR6/WGII/Chapter-7 (section)

==== 7.3.1.3 Projected Impacts on Vector-Borne Diseases ====

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The distribution and abundance of disease vectors, and the transmission of the infections that they carry, are influenced both by changes in climate and by trends such as human population growth and migration, urbanisation, land use change, biodiversity loss and public health measures. Each of these may increase or decrease risk, interact with climate effects and may contribute to the emergence of infectious disease, although there are few studies assessing future risk of emergence ( [[#Gibb--2020|Gibb et al., 2020]] ). Unless stated otherwise, the assessments below are specifically for the effects of climate change on individual diseases, assuming other determinants remain constant.

''There is a high likelihood that climate change will contribute to increased distributional range and vectorial capacity of malaria vectors in parts of sub-Saharan Africa, Asia and South America'' ( ''high confidence'' ). In Nigeria, the range and abundance of ''Anopheles'' mosquitoes are projected to increase under both lower (RCP2.6) and especially under higher emissions scenarios (RCP8.5) due to increasing and fluctuating temperature, longer tropical rainfall seasons and rapid land use changes ( [[#Akpan--2018|Akpan et al., 2018]] ). Similarly, vegetation acclimation due to elevated atmospheric CO 2 under climate change will ''likely'' increase the abundance of ''Anopheles'' vectors in Kenya ( [[#Le--2019|Le et al., 2019]] ). Distribution of ''Anopheles'' may decrease in parts of India and Southeast Asia, but there is an expected increase in vectorial capacity in China ( [[#Khormi--2016|Khormi and Kumar, 2016]] ). In South America, climate change is projected to expand the distributions of malaria vectors to 35–46% of the continent by 2070, particularly species of the ''Albitarsis'' complex ( [[#Laporta--2015|Laporta et al., 2015]] ).

''Malaria infections have significant potential to increase in parts of sub-Saharan Africa and Asia, with risk varying according to the warming scenario'' ( ''medium confidence'' ). In Africa, where most malaria is due to the more deadly ''Plasmodium falciparum'' parasite, climate change is ''likely'' to increase the overall transmission risk due to the ''likely'' expansion of vector distribution and increase in biting rates ( [[#Bouma--2016|Bouma et al., 2016]] ; [[#M’Bra--2018|M’Bra et al., 2018]] ; [[#Nkumama--2017|Nkumama et al., 2017]] ; [[#Ryan--2015b|Ryan et al., 2015b]] ; [[#Tompkins--2016a|Tompkins and Caporaso, 2016a]] ). The projected effect of climate change varies markedly by region, with projections for west Africa tending to indicate a shortening of transmission seasons and neutral or small net reductions in overall risk, whereas studies consistently project increases in southern and eastern Africa, with potentially an additional 76 million people at risk of endemic exposure (10–12 months yr –1 ) by the 2080s ( [[#Nkumama--2017|Nkumama et al., 2017]] ; [[#Ryan--2015b|Ryan et al., 2015b]] ; [[#Semakula--2017|Semakula et al., 2017]] ; [[#Zaitchik--2019|Zaitchik, 2019]] ; [[#Leedale--2016|Leedale et al., 2016]] ; [[#Murdock--2016|Murdock et al., 2016]] ; [[#Yamana--2016|Yamana et al., 2016]] ; [[#Ryan--2020|Ryan et al., 2020]] ). In sub-Saharan Africa, malaria case incidence associated with dams in malaria-endemic regions will ''likely'' be exacerbated by climate change, with significantly higher rates projected under RCP8.5 in comparison to lower-emission scenarios ( [[#Kibret--2016|Kibret et al., 2016]] ). Incidence of malaria in Madagascar is projected to increase under RCP4.5 through RCP8.5 ( [[#Rakotoarison--2018|Rakotoarison et al., 2018]] ). Distribution of ''P. vivax'' and ''P. falciparum'' malaria in China is ''likely'' to increase under RCPs higher than 2.6, especially RCP8.5 ( [[#Hundessa--2018|Hundessa et al., 2018]] ). In India, projected scenarios for the 2030s under RCP4.5 indicate changes in the spatial distribution of malaria, with new foci and potential outbreaks in the Himalayan region, southern and eastern states, and an overall increase in months suitable for transmission overall, with some other areas experiencing a reduction in transmission months ( [[#Sarkar--2019|Sarkar et al., 2019]] ).

''Rising temperatures are'' likely ''to cause poleward shifts and overall expansion in the distribution of mosquitoes'' Aedes aegypti ''and'' Aedes albopictus '', the principal vectors of dengue, yellow fever, chikungunya and Zika'' ( ''high confidence'' ) ''.'' Globally, the population exposed to disease transmission by one of these vectors is expected to increase significantly due to the combination of climate change and non-climatic processes including urbanisation and socioeconomic inter-connectivity, with exposure rates rising under higher warming scenarios ( [[#Kamal--2018|Kamal et al., 2018]] ; [[#Kraemer--2019|Kraemer et al., 2019]] ). For example, approximately 50% of the global population is projected to be exposed to these vectors by 2050 under RCP6.0 ( [[#Kraemer--2019|Kraemer et al., 2019]] ). The effect of climate change alone is projected to increase the population exposed to ''Aedes aegypti'' by 8–12% by 2061–2080 ( [[#Monaghan--2018|Monaghan et al., 2018]] ), and its abundance is projected to increase by 20% under RCP2.6 and 30% under RCP8.5 by the end of the century ( [[#Liu-Helmersson--2019|Liu-Helmersson et al., 2019]] ; Figure 7.10). Exposure to transmission by ''Aedes albopictus'' specifically would be highest at intermediate climate change scenarios and would decrease in the warmest scenarios ( [[#Ryan--2019|Ryan et al., 2019]] ). Under scenarios other than RCP2.6, most of Europe would experience significant increases in exposure to viruses transmitted by both vectors ( [[#Liu-Helmersson--2019|Liu-Helmersson et al., 2019]] ).

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[[File:78378ba3c9c7cdbb4869928af561b408 IPCC_AR6_WGII_Figure_7_010.png]]

'''Figure 7.10 |''' '''Projected change in the potential abundance of''' '''Aedes aegypti''' '''over the 21st century (2090–2099 relative to 1987–2016) (Liu-Helmersson et al''' '''.''' ''', 2019).'''

''Climate change is expected to increase dengue risk and facilitate its global spread, with the risk being greatest under high emissions scenarios'' ( ''high confidence'' ) ''.'' Future exposure to risk will be influenced by the combined effects of climate change and non-climatic factors such as population density and economic development ( [[#Akter--2017|Akter et al., 2017]] ). Overall, risk levels are expected to rise on all continents ( [[#Akter--2017|Akter et al., 2017]] ; [[#Messina--2015|Messina et al., 2015]] ; [[#Rogers--2015|Rogers, 2015]] ; [[#Liu-Helmersson--2016|Liu-Helmersson et al., 2016]] ; [[#Messina--2019|Messina et al., 2019]] ). Compared to 2015, an additional 1 billion people are projected to be at risk of dengue exposure by 2080 under an SSP1-4.5 scenario, 2.25 billion under SSP2-6.0, and 5 billion under SSP3-8.5 ( [[#Messina--2019|Messina et al., 2019]] ). In North America, risk is projected to expand in north-central Mexico, with annual dengue incidence in Mexico increasing by up to 40% by 2080, and expand from US southern states to mid-western regions ( [[#Proestos--2015|Proestos et al., 2015]] ; [[#Colon-Gonzalez--2013|Colon-Gonzalez et al., 2013]] ). In China, under RCP8.5, dengue exposure would increase from 168 million people in 142 counties to 490 million people in 456 counties by the late 2100s ( [[#Fan--2019|Fan and Liu, 2019]] ). In Nepal, dengue fever is expected to expand throughout the 2050s and 2070s under all RCPs ( [[#Acharya--2018|Acharya et al., 2018]] ). In Tanzania, there is a projected shift in distribution towards central and northeastern areas and risk intensification in nearly all parts of the country by 2050 ( [[#Mweya--2016|Mweya et al., 2016]] ). Dengue vectorial capacity is projected to increase in Korea under higher RCP scenarios ( [[#Lee--2018a|Lee et al., 2018a]] ).

''There are insufficient studies for assessment of projected effects of climate change on other arboviral diseases, such as chikungunya and Zika.'' Zika virus transmits under different temperature optimums than does dengue, suggesting environmental suitability for Zika transmission could expand with future warming ( ''low confidence'' ) ( [[#Tesla--2018|Tesla et al., 2018]] ).

''Climate change can be expected to continue to contribute to the geographical spread of the Lyme disease vector'' Ixodes scapularis ( ''high confidence'' ) ''and the spread of tick-borne encephalitis and Lyme disease vector'' Ixodes ricinus ''in Europe'' ( ''medium confidence'' ) ''.'' In Canada, vector surveillance of the black-legged tick ''I. scapularis'' identified strong temperature effects on the limits of their occurrence, on recent geographic spread, temporal coincidence in emergence of tick populations and acceleration of the speed of spread ( [[#Clow--2017|Clow et al., 2017]] ; [[#Cheng--2017|Cheng et al., 2017]] ). In Europe, increasing temperatures over the 1950–2018 period significantly accelerated the life cycle of ''Ixodes ricinus'' and contributed to its spread ( [[#Estrada-Peña--2020|Estrada-Peña and Fernández-Ruiz, 2020]] ). Under RCP4.5 and RCP8.5 scenarios, projections indicate a northward and eastward shift of the distribution of ''I. persulcatus and I. ricinus,'' vectors of Lyme disease and tick-borne encephalitis in northern Europe and Russia, with an overall large increase in distribution in the second half of the current century ( [[#Popov--2014|Popov and Yasyukevich, 2014]] ; [[#Yasjukevich--2018|Yasjukevich et al., 2018]] ) and increases in intensity of tick-borne encephalitis transmission in central Europe ( [[#Nah--2020|Nah et al., 2020]] ).

''Climate change is projected to increase the incidence of Lyme disease and tick-borne encephalitis in the Northern Hemisphere'' ( ''high confidence'' ) (Figure 7.9). The basic reproduction number (R0) of ''I. scapularis'' in at least some regions of Canada is projected to increase under all RCP scenarios ( [[#McPherson--2017|McPherson et al., 2017]] ). In the USA, a 2°C warming could increase the number of Lyme disease cases by over 20% over the coming decades and lead to an earlier onset and longer length of the annual Lyme disease season ( [[#Dumic--2018|Dumic and Severnini, 2018]] ; [[#Monaghan--2015|Monaghan et al., 2015]] ).

''Climate change is projected to change the distribution of schistosomiasis in Africa and Asia'' ( ''high confidence'' ) '', with a possible increase in global land area suitable for transmission'' ( ''medium confidence'' ). A global increase in land area with temperatures suitable for transmission by the three main species of ''Schistosoma'' ( ''S. japonicum'' , ''S. mansoni'' and ''S. haematobium)'' is projected under the RCP4.5 scenario for the 2021–2050 and 2071–2100 periods ( [[#Yang--2018|Yang and Bergquist, 2018]] ), but regional outcomes are expected to vary. In Africa, shifting temperature regimes associated with climate change are expected to lead to reduced snail populations in areas with already high temperatures and higher populations in areas with currently low winter temperatures ( [[#Kalinda--2017|Kalinda et al., 2017]] ; [[#McCreesh--2014|McCreesh and Booth, 2014]] ). Infection risk with ''Schistosoma mansoni'' may increase by up to 20% over most of eastern Africa over the next 20–50 years but decrease by more than 50% in parts of north and east Kenya, southern South Sudan and eastern People’s Democratic Republic of Congo (PDRC) ( [[#McCreesh--2015|McCreesh et al., 2015]] ), with a possible overall net contraction ( [[#Stensgaard--2013|Stensgaard et al., 2013]] ). In China, currently endemic areas in Sichuan Province may become unsuitable for snail habitats, but currently non-endemic areas in Sichuan and Hunan/Hubei provinces may see a new emergence ( [[#Yang--2018|Yang and Bergquist, 2018]] ). In addition to the projected effects of temperature described above, distribution and transmission of schistosomiasis will also be affected positively or negatively by changes in the availability of freshwater bodies, which were not included in these models.

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