Editing Bluetongue disease (section)

==Epidemiology==
[[File:Map of molecular epidemiology of bluetongue virus in Europe.gif|thumb|280px|The molecular epidemiology of Bluetongue virus in Europe since 1998: routes of introduction of different serotypes and individual virus strains]]
The presence of the insect vectors determines the bluetongue disease's global distribution, with regions in Africa, Asia, Australia, Europe, North America, and other tropical/subtropical area being most affected.<ref name="WOAH" />

The virus persists in areas where climatic conditions support the survival of ''Culicoides'' midges during winter. This adaptability allows the disease to establish itself in new regions when conditions become favorable.<ref name="WOAH" />

An outline of the transmission cycle of BTV is illustrated in article [[Parasitic flies of domestic animals]].

Its occurrence is seasonal in the affected [[Mediterranean]] countries, subsiding when temperatures drop and hard frosts kill the adult midge [[vector (epidemiology)|vectors]].<ref>{{cite journal | vauthors = Purse BV, Mellor PS, Rogers DJ, Samuel AR, Mertens PP, Baylis M | title = Climate change and the recent emergence of bluetongue in Europe | journal = Nature Reviews. Microbiology | volume = 3 | issue = 2 | pages = 171–181 | date = February 2005 | pmid = 15685226 | doi = 10.1038/nrmicro1090 | s2cid = 62802662 }}</ref>
Viral survival and vector longevity is seen during milder winters.<ref>{{cite web   | title = Bluetongue – Europe (51)   | publisher = International Society for Infectious Diseases   | date = 2007-10-30   | url = http://www.promedmail.org/pls/promed/f?p=2400:1001:6789444728823816325::NO::F2400_P1001_BACK_PAGE,F2400_P1001_PUB_MAIL_ID:1000,39875   | access-date = 2007-10-31   | archive-url = https://web.archive.org/web/20071226015924/http://www.promedmail.org/pls/promed/f?p=2400%3A1001%3A6789444728823816325%3A%3ANO%3A%3AF2400_P1001_BACK_PAGE%2CF2400_P1001_PUB_MAIL_ID%3A1000%2C39875   | archive-date = December 26, 2007   | url-status = dead   }}</ref>
A significant contribution to the northward spread of bluetongue disease has been the ability of ''C. obsoletus'' and ''C.pulicaris'' to acquire and transmit the pathogen, both of which are spread widely throughout Europe. This is in contrast to the original ''C.imicola'' vector, which is limited to North Africa and the Mediterranean. The relatively recent novel vector has facilitated a far more rapid spread than the simple expansion of habitats north through global warming.<ref>{{cite journal | vauthors = Baylis M, Caminade C, Turner J, Jones AE | title = The role of climate change in a developing threat: the case of bluetongue in Europe | journal = Revue Scientifique et Technique | volume = 36 | issue = 2 | pages = 467–478 | date = August 2017 | pmid = 30152470 | doi = 10.20506/rst.36.2.2667 }}</ref>

In August 2006, cases of bluetongue were found in the Netherlands, then Belgium, Germany, and Luxembourg.<ref>{{cite press release
  | title = Blue Tongue confirmed in Belgium and Germany   | publisher = [[European Commission]]   | date = 2006-08-21
  | url = http://europa.eu/rapid/pressReleasesAction.do?reference=IP/06/1113&format=HTML&aged=0&language=EN&guiLanguage=en|access-date =  2006-08-21}}</ref><ref name = "jdfxyd">{{cite press release   | title = Lethal horse disease knocking on Europe's door   | publisher = Horsetalk.co.nz  | date = 2007-03-28  | url = http://www.horsetalk.co.nz/archives/2007/03/165.shtml  | access-date =  2007-03-27}}</ref>
In 2007, the first case of bluetongue in the Czech Republic was detected in one bull near [[Cheb]] at the Czech-German border.<ref>{{cite web   | title = Bluetongue dobývá Evropskou unii   | publisher = Agroweb   | date = 2008-02-19   | url = http://www.krmivarstvi.cz/zivocisna-vyroba/Bluetongue-dobyva-Evropskou-unii__s45x29979.html   | archive-url = https://web.archive.org/web/20090306005903/http://www.krmivarstvi.cz/zivocisna-vyroba/Bluetongue-dobyva-Evropskou-unii__s45x29979.html   | url-status = dead   | archive-date = 2009-03-06   | access-date = 2008-05-29   }}</ref>
In September 2007, the UK reported its first ever suspected case of the disease, in a Highland cow on a rare-breeds farm near [[Ipswich, Suffolk]].<ref>{{cite news
  | title = Bluetongue disease detected in UK   | work = [[BBC News Online]]   | date = 2007-09-22   | url = http://news.bbc.co.uk/1/hi/uk/7008788.stm
  | access-date =  2007-09-22 }}</ref>
Since then, the virus has spread from cattle to sheep in Britain.<ref>{{cite news
  | title = Bluetongue spreads from cattle to sheep   | publisher = [[Telegraph.co.uk]]   | date = 2007-10-14   | url = https://www.telegraph.co.uk/news/uknews/1566090/Bluetongue-spreads-from-cattle-to-sheep.html   | access-date = 2007-10-15 | location=London | vauthors = Gray R }}</ref>
By October 2007, bluetongue had become a serious threat in Scandinavia and Switzerland<ref>{{cite web
  | title = Bluetongue – Europe (50)
  | publisher = International Society for Infectious Diseases
  | date = 2007-10-30
  | url = http://www.promedmail.org/pls/promed/f?p=2400:1001:6789444728823816325::::F2400_P1001_BACK_PAGE,F2400_P1001_ARCHIVE_NUMBER,F2400_P1001_USE_ARCHIVE:1001,20071030.3522,Y
  | access-date = 2007-10-31
  | archive-url = https://web.archive.org/web/20071226015919/http://www.promedmail.org/pls/promed/f?p=2400%3A1001%3A6789444728823816325%3A%3A%3A%3AF2400_P1001_BACK_PAGE%2CF2400_P1001_ARCHIVE_NUMBER%2CF2400_P1001_USE_ARCHIVE%3A1001%2C20071030.3522%2CY
  | archive-date = December 26, 2007
  | url-status = dead
  }}</ref>
and the first outbreak in Denmark was reported.<ref>{{cite web | url = https://www.reuters.com/article/idUSL13660817 | title = Bluetongue outbreak detected in Denmark – EU | work = [[Reuters]] | date = 13 October 2007 }}</ref> In autumn 2008, several cases were reported in the southern Swedish provinces of Småland, Halland, and Skåne,<ref>{{cite web
  | title = Blue tongue outbreak spreads south   | publisher = [[Thelocal.se]]   | date = 2008-09-24   | url = http://www.thelocal.se/14538/20080924/
  | access-date = 2008-09-24 }}</ref>
as well as in areas of the Netherlands bordering Germany, prompting veterinary authorities in Germany to intensify controls.<ref>{{cite journal | vauthors = Mehlhorn H, Walldorf V, Klimpel S, Schaub G, Kiel E, Focke R, Liebisch G, Liebisch A, Werner D, Bauer C, Clausen H, Bauer B, Geier M, Hörbrand T, Bätza HJ, Conraths FJ, Hoffmann B, Beer M | title = Bluetongue disease in Germany (2007-2008): monitoring of entomological aspects | journal = Parasitology Research | volume = 105 | issue = 2 | pages = 313–319 | date = August 2009 | pmid = 19322587 | doi = 10.1007/s00436-009-1416-y | s2cid = 35616241 }}</ref>
Norway had its first finding in February 2009, when cows at two farms in [[Vest-Agder]] in the south of Norway showed an immune response to bluetongue.<ref>{{cite web | title = Blåtunge påvist i Norge | trans-title = Bluetongue detected in Norway | language = Norwegian | url = http://www.vetinst.no/nor/Nyheter/Blaatunge-paavist-i-Norge | work = Veterinærinstituttet | trans-work = Norway National Veterinary Institute | archive-url=https://web.archive.org/web/20090306004518/http://www.vetinst.no/nor/Nyheter/Blaatunge-paavist-i-Norge | archive-date=2009-03-06 | date = 24 February 2009 }}</ref> A number of countries, including Norway and Finland, were certified as free of the disease in 2011 and 2021, respectively.<ref>{{Cite web |title=Bluetongue serotype 8 outbreak in Norway |url=https://www.vetinst.no/rapporter-og-publikasjoner/rapporter/2011/bluetongue-serotype-8-outbreak-in-norway |access-date=2023-12-12 |website=www.vetinst.no}}</ref><ref>{{Cite web |publisher=European Commission |date=2021-04-15 |title=Commission Implementing Regulation, EU 2021/620. |url=https://eur-lex.europa.eu/legal-content/EN/TXT/HTML/?uri=CELEX:32021R0620 |access-date=2024-12-12 |website=eur-lex.europa.eu}}</ref>

In 2023, Europe witnessed a series of notable epizootic occurrences at higher latitudes, partially attributable to the emergence of a novel serotype, BTV-3. The serotype was first identified in the Netherlands in September 2023 and has since been documented in numerous European countries, including Belgium, Germany, the Netherlands, France, Spain, the UK, Norway, and Sweden.<ref name="WOAH_2">{{Cite web |title=Bluetongue in Europe: How climate change is shifting disease patterns |url=https://www.woah.org/en/article/bluetongue-in-europe-how-climate-change-is-shifting-disease-patterns/ |access-date=2024-12-12 |website=WOAH - World Organisation for Animal Health |language=en-GB}}</ref><ref>{{Cite web |title=29 November 2023: updated outbreak assessment for Bluetongue virus in Europe |url=https://www.gov.uk/government/publications/bluetongue-virus-in-europe/29-november-2023-updated-outbreak-assessment-for-bluetongue-virus-in-europe |access-date=2023-12-12 |website=GOV.UK |language=en}}</ref>

Although the disease is not a threat to humans, the most vulnerable common domestic ruminants are cattle, goats, and especially, sheep.

===Overwintering===
A puzzling aspect of BTV is its survival between midge seasons in temperate regions. Adults of ''Culicoides'' are killed by cold winter temperatures, and BTV infections typically do not last for more than 60 days, which is not long enough for BTV to survive until the next spring. It is believed that the virus somehow survives in overwintering midges or animals. Multiple mechanisms have been proposed.  A few adult ''Culicoides'' midges infected with BTV may survive the mild winters of the temperate zone.  Some midges may even move indoors to avoid the cold temperature of the winter.  Additionally, BTV could cause a chronic or latent infection in some animals, providing another means for BTV to survive the winter.  BTV can also be transmitted from mother to fetus.  The outcome is abortion or stillbirth if fetal infection occurs early in gestation and survival if infection occurs late.  However infection at an intermediate stage, before the fetal immune system is fully developed, may result in a chronic infection that lingers until the first months after birth of the lamb. Midges then spread the pathogen from the calves to other animals, starting a new season of infection.<ref name=Wilson2008>{{cite journal | vauthors = Wilson A, Darpel K, Mellor PS | title = Where does bluetongue virus sleep in the winter? | journal = PLOS Biology | volume = 6 | issue = 8 | pages = e210 | date = August 2008 | pmid = 18752350 | pmc = 2525685 | doi = 10.1371/journal.pbio.0060210 | doi-access = free }}</ref>

===Climate change===
{{See also|Climate change and infectious diseases|Effects of climate change on livestock}}The spread of bluetongue to Southern, Central, and Northern Europe provides an illustrative example of the complex interactions between climate change, vector habitat suitability, animal population density, distribution, and movement, which collectively influence the patterns of disease emergence and transmission.<ref name="WOAH_2" />{{Excerpt|Culicoides imicola#Role of climate change}}