Editing IPCC:AR6/WGII/Chapter-2 (section)

===== Case Study 3: Arctic and sub-Arctic disease expansion and intensification =====

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High Arctic regions have warmed by more than double the global average, &gt;2°C in most areas (Sections 2.3.1.1.2, Figure 2.11, and Atlas 11.2.1.2 in ( [[#IPCC--2021a|IPCC, 2021a]] )). Experimental field ecology studies and computational models of Arctic and sub-Arctic regions indicate that milder winters have reduced the mortality of vectors and reservoir hosts and increased their habitat as forested taiga expands into previously treeless tundra (Table SM2.1) ( [[#Parkinson--2014|Parkinson et al., 2014]] ). Warmer temperatures and longer seasonal windows have allowed faster reproduction/replication, accelerated development and increased the number of generations per year of pathogens, vectors and some host animals, which, in turn, increases the populations of disease organisms and disease transmission (Sections 2.4.2.4, 2.4.4.3.3). Higher numbers of ticks, mosquitoes, ''Culicoides'' biting midges, deer flies, horseflies and Simuliidae black flies, that transmit a variety of pathogens, are being documented in high-latitude regions and where they have been historically absent ( ''robust evidence'' , ''high agreement'' ) ( [[#Waits--2018|Waits et al., 2018]] ; [[#Caminade--2019|Caminade et al., 2019]] ; [[#Gilbert--2021|Gilbert, 2021]] ). In concert with these poleward shifts of hosts and vectors, pathogens, particularly tick-borne pathogens and helminth infections, have increased dramatically in incidence and severity from once-rare occurrences and have appeared in new regions ( ''very high confidence'' ) ( [[#Caminade--2019|Caminade et al., 2019]] ; [[#Gilbert--2021|Gilbert, 2021]] ).

Zoonoses and VBDs that have been historically rare or never documented in the Arctic and sub-Arctic regions of Europe, Asia, and North America, such as anthrax, cryptosporidiosis, elaphostrongylosis, filariasis ( [[#Huber--2020|Huber et al., 2020]] ), tick-borne encephalitis and tularemia ( [[#Evander--2009|Evander and Ahlm, 2009]] ; [[#Parkinson--2014|Parkinson et al., 2014]] ; [[#Pauchard--2016|Pauchard et al., 2016]] ), are spreading poleward and increasing in incidence, associated with warming temperatures ( ''robust evidence'' , ''high agreement'' , ''very high confidence'' ) (Table SM2.1) ( [[#Omazic--2019|Omazic et al., 2019]] ). Recent anthrax outbreaks and mass mortality events of humans and reindeer, respectively, have been linked to abnormally hot summer temperatures that caused the permafrost to melt and exposed diseased animal carcasses, releasing thawed, highly infectious ''Bacillus anthracis'' spores ( ''medium evidence'' , ''medium agreement'' ) (Ezhova et al., 2019; [[#Hueffer--2020|Hueffer et al., 2020]] ; [[#Ezhova--2021|Ezhova et al., 2021]] ). Multiple contributing factors conspired over different timescales to compound a 2016 anthrax outbreak occurring on the Yamal peninsula: (i) rapid permafrost thawing for 5 years preceding the outbreak, (ii) thick snow cover the year before the outbreak insulated the warmed permafrost and kept it from re-freezing, and (iii) anthrax vaccination rates had decreased or ceased in the region (Ezhova et al., 2019; [[#Ezhova--2021|Ezhova et al., 2021]] ). These precursors converged with an unusually dry and hot summer that: (i) melted permafrost, creating an anthrax exposure hazard; (ii) increased the vector insect population; and (iii) weakened the immune systems of reindeer, thereby increasing their susceptibility ( [[#Waits--2018|Waits et al., 2018]] ; [[#Hueffer--2020|Hueffer et al., 2020]] ).

Warmer temperatures have increased blood-feeding insect harassment of reindeer with compounding consequences: (1) increased insect-bite rates lead to higher parasite loads, (2) time spent by reindeer in trying to escape biting flies reduces foraging while simultaneously increasing their energy expenditure, (3) the combination of (1) and (2) leads to poor body condition which subsequently leads to (4) reduced winter survival and fecundity ( [[#Mallory--2017|Mallory and Boyce, 2017]] ). As temperatures warm and connectivity increases between the Arctic and the rest of the world, tourism, resource extraction and increased commercial transport will create additional risks of biological invasion by infectious agents and their hosts ( [[#Pauchard--2016|Pauchard et al., 2016]] ). These increases in introduction risk compounded with climate change have already begun to harm Indigenous Peoples dependent on hunting and herding livestock (horses and reindeer) that are suffering increased pathogen infection ''(high confidence)'' ( [[#Deksne--2020|Deksne et al., 2020]] ; [[#Stammler--2020|Stammler and Ivanova, 2020]] ).

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