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

===== 2.4.2.7.1 Direct effects of climate and climate change on reproduction, seasonality, the length of the growing season and the transmission of pathogens, vectors and hosts =====

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VBDs require arthropod vector hosts (e.g., insects or ticks), while other infectious diseases (e.g., fungi, bacteria and helminths) have free-living life stages and/or complex life cycles that require intermediate hosts (e.g., snails), all of which have temperature-driven rates of development and replication/reproduction ( ''robust evidence'' , ''high agreement'' ) ( [[#Mordecai--2013|Mordecai et al., 2013]] ; [[#Liu-Helmersson--2014|Liu-Helmersson et al., 2014]] ; [[#Moran--2014|Moran and Alexander, 2014]] ; [[#Bernstein--2015|Bernstein, 2015]] ; [[#Marcogliese--2016|Marcogliese, 2016]] ; [[#Ogden--2016|Ogden and Lindsay, 2016]] ; [[#Mordecai--2017|Mordecai et al., 2017]] ; [[#Short--2017|Short et al., 2017]] ; [[#Caminade--2019|Caminade et al., 2019]] ; [[#Cavicchioli--2019|Cavicchioli et al., 2019]] ; [[#Mordecai--2019|Mordecai et al., 2019]] ; [[#Liu--2020|Liu et al., 2020]] ; [[#Rocklöv--2020|Rocklöv and Dubrow, 2020]] ). Additionally, microbes such as bacteria thermally adapt to temperature changes through multiple mechanisms, indicating that warming will not reduce antibiotic resistance ( [[#MacFadden--2018|MacFadden et al., 2018]] ; [[#Pärnänen--2019|Pärnänen et al., 2019]] ; [[#Shukla--2019|Shukla, 2019]] ; [[#McGough--2020|McGough et al., 2020]] ; [[#Rodriguez-Verdugo--2020|Rodriguez-Verdugo et al., 2020]] ).

There is increasing evidence of the role of extreme events in disease outbreaks ''(very high confidence)'' ( [[#Tjaden--2018|Tjaden et al., 2018]] ; [[#Bryson--2020|Bryson et al., 2020]] ). Heat waves have been associated with outbreaks of helminth pathogens, especially in sub-Arctic and Arctic areas. For example, a severe outbreak of microfilaremia, a VBD spread by mosquitoes and flies, plagued reindeer in northern Europe following extreme high temperatures ( [[#Laaksonen--2010|Laaksonen et al., 2010]] ). More frequent and severe extreme events such as floods, droughts, heat waves and storms can either increase or decrease outbreaks, depending upon the region and disease ( ''robust evidence'' , ''high agreement'' ) ( [[#Anyamba--2001|Anyamba et al., 2001]] ; [[#Marcheggiani--2010|Marcheggiani et al., 2010]] ; [[#Brown--2013|Brown and Murray, 2013]] ; [[#Paz--2015|Paz, 2015]] ; [[#Boyce--2016|Boyce et al., 2016]] ; [[#Wu--2016b|Wu et al., 2016b]] ; [[#Wilcox--2019|Wilcox et al., 2019]] ; [[#Nosrat--2021|Nosrat et al., 2021]] ). Heavy precipitation events have been shown to increase some infectious diseases with aquatic life-cycle components such as mosquito-borne, helminth, and rodent-borne diseases ( ''robust evidence'' , ''high agreement'' ) ( [[#Anyamba--2001|Anyamba et al., 2001]] ; [[#Zhou--2005|Zhou et al., 2005]] ; [[#Wu--2008|Wu et al., 2008]] ; [[#Brown--2013|Brown and Murray, 2013]] ; [[#Anyamba--2014|Anyamba et al., 2014]] ; [[#Boyce--2016|Boyce et al., 2016]] ). Conversely, flooding also increases flow rate and decreases parasite load and diversity in other aquatic wildlife ( [[#Hallett--2008|Hallett and Bartholomew, 2008]] ; [[#Bjork--2009|Bjork and Bartholomew, 2009]] ; [[#Marcogliese--2016|Marcogliese, 2016]] ; [[#Marcogliese--2016|Marcogliese et al., 2016]] ) and can reduce mosquito abundance by flushing them out of the system ( [[#Paaijmans--2007|Paaijmans et al., 2007]] ; [[#Paz--2015|Paz, 2015]] ).

Droughts reduce the aquatic habitat of some mosquito species while simultaneously increasing the availability of stagnant standing pools of water that are ideal breeding habitats for other species, such as dengue-vector ''Aedes'' mosquitoes ( ''medium evidence'' , ''medium agreement'' ) ( [[#Chareonviriyaphap--2003|Chareonviriyaphap et al., 2003]] ; [[#Chretien--2007|Chretien et al., 2007]] ; [[#Padmanabha--2010|Padmanabha et al., 2010]] ; [[#Trewin--2013|Trewin et al., 2013]] ; [[#Paz--2015|Paz, 2015]] ). Extreme drought has been associated with an increase in bluetongue virus haemorrhagic disease in wildlife in eastern North America, although the mechanisms involved were not identified ( [[#Christensen--2020|Christensen et al., 2020]] ). Heat waves in some regions, especially coastal regions, have increased parasitism and decreased host richness and abundance, leading to population crashes ( [[#Larsen--2014|Larsen and Mouritsen, 2014]] ; [[#Mouritsen--2018|Mouritsen et al., 2018]] ). Changes in temperature and precipitation, especially extreme events, can alter community structure ( [[#Larsen--2011|Larsen et al., 2011]] ) by increasing or decreasing parasites and their host organisms, and even altering host behaviour in ways that are advantageous to parasites ( [[#Macnab--2012|Macnab and Barber, 2012]] ).

Climate change not only affects the occurrence of pathogens and their hosts in terms of geographic space but also impacts the temporal patterns of disease transmission. Warmer winters allow greater over-winter survival of arthropod vectors, which, coupled with lengthened transmission seasons, drive increases in vector population sizes, pathogen prevalence, and thus the proportion of vectors infected ( ''robust evidence'' , ''high agreement'' ) ( [[#Laaksonen--2009|Laaksonen et al., 2009]] ; [[#Molnár--2013|Molnár et al., 2013]] ; [[#Waits--2018|Waits et al., 2018]] ). For example, a parasitic nematode lung worm ( ''Umingmakstrongylus pallikuukensis'' ) has shortened its larval development time by half (from two years to one year), which has increased infection rates in North American musk oxen ( [[#Norwegian%20Polar%20Institute--2009|Norwegian Polar Institute, 2009]] ).

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