Editing Yellow fever (section)

===Transmission===
[[File:Aedes aegypti bloodfeeding CDC Gathany.jpg|thumb|''Aedes aegypti'' feeding]]
[[File:Aedes aegypti E-A-Goeldi 1905.jpg|thumb|Adults of the yellow fever mosquito ''A. aegypti'': The male is on the left, females are on the right. Only the female mosquito bites humans to transmit the disease.]]

Yellow fever virus is mainly transmitted through the bite of the yellow fever mosquito ''Aedes aegypti'', but other mostly ''[[Aedes]]'' mosquitoes such as the tiger mosquito (''[[Aedes albopictus]]'') can also serve as a [[vector (epidemiology)|vector]] for this virus.<ref name="Yellow fever">{{Cite web |title=Yellow fever |url=https://www.who.int/news-room/fact-sheets/detail/yellow-fever |access-date=2022-04-24 |website=World Health Organization |language=en}}</ref> Like other [[arbovirus]]es, which are transmitted by mosquitoes, Yellow fever virus is taken up by a female mosquito when it ingests the blood of an infected human or another primate.<ref name="who.int">{{Cite web |title=Yellow fever |url=https://www.who.int/health-topics/yellow-fever |access-date=2022-04-24 |website=World Health Organization |language=en}}</ref> Viruses reach the stomach of the mosquito, and if the virus concentration is high enough, the virions can infect [[epithelial cell]]s and replicate there. From there, they reach the [[haemocoel]] (the blood system of mosquitoes) and from there the [[salivary gland]]s.<ref>{{cite journal | vauthors = Kumar A, Srivastava P, Sirisena P, Dubey SK, Kumar R, Shrinet J, Sunil S | title = Mosquito Innate Immunity | journal = Insects | volume = 9 | issue = 3 | page = 95 | date = August 2018 | pmid = 30096752 | pmc = 6165528 | doi = 10.3390/insects9030095 | doi-access = free }}</ref> When the mosquito next sucks blood, it injects its saliva into the wound, and the virus reaches the bloodstream of the bitten person.<ref name="Monath Encyclopedia of Insects Yellow Fever">{{cite book |doi=10.1016/B978-0-12-374144-8.00279-4 |chapter=Yellow Fever |title=Encyclopedia of Insects |date=2009 |last1=Monath |first1=Thomas P. |pages=1064–1065 |isbn=978-0-12-374144-8 }}</ref> Transovarial transmissionial and [[transstadial transmission]] of yellow fever virus within ''A. aegypti'', that is, the transmission from a female mosquito to its eggs and then larvae, are indicated.<ref>{{cite journal | vauthors = Nag DK, Payne AF, Dieme C, Ciota AT, Kramer LD | title = Zika virus infects Aedes aegypti ovaries | journal = Virology | volume = 561 | pages = 58–64 | date = September 2021 | pmid = 34147955 | doi = 10.1016/j.virol.2021.06.002 | pmc = 10117528 | doi-access = free }}</ref> This infection of vectors without a previous blood meal seems to play a role in single, sudden breakouts of the disease.<ref>{{cite journal | vauthors = Fontenille D, Diallo M, Mondo M, Ndiaye M, Thonnon J | title = First evidence of natural vertical transmission of yellow fever virus in Aedes aegypti, its epidemic vector | journal = Transactions of the Royal Society of Tropical Medicine and Hygiene | volume = 91 | issue = 5 | pages = 533–535 | year = 1997 | pmid = 9463659 | doi = 10.1016/S0035-9203(97)90013-4 }}</ref>

Three epidemiologically different infectious cycles occur<ref name=Barr2007/> in which the virus is transmitted from mosquitoes to humans or other primates.<ref>{{cite web |title=Infectious Diseases Related to Travel |url=http://wwwnc.cdc.gov/travel/yellowbook/2016/infectious-diseases-related-to-travel/yellow-fever |website=Yellow Book |publisher=Centers for Disease Control and Prevention |access-date=20 March 2016 |url-status=live |archive-url=https://web.archive.org/web/20160320215700/http://wwwnc.cdc.gov/travel/yellowbook/2016/infectious-diseases-related-to-travel/yellow-fever |archive-date=20 March 2016}}</ref> In the "urban cycle", only the yellow fever mosquito ''A. aegypti'' is involved. It is well adapted to urban areas, and can also transmit other diseases, including [[Zika fever]], [[dengue fever]], and [[chikungunya]].<ref>{{cite journal |last1=Zerbo |first1=Alexandre |last2=Castro Delgado |first2=Rafael |last3=Arcos González |first3=Pedro |title=Aedes-borne viral infections and risk of emergence/resurgence in Sub-Saharan African urban areas |journal=Journal of Biosafety and Biosecurity |date=December 2020 |volume=2 |issue=2 |pages=58–63 |doi=10.1016/j.jobb.2020.10.002 |doi-access=free }}</ref> The urban cycle is responsible for the major outbreaks of yellow fever that occur in Africa. Except for an outbreak in Bolivia in 1999, this urban cycle no longer exists in South America.<ref>{{Cite book | vauthors = Kotar SL, Gessler JE |url= https://books.google.com/books?id=odYBDgAAQBAJ&pg=PA7 |title=Yellow Fever: A Worldwide History |date=2017-02-03 |publisher=McFarland |isbn=978-1-4766-2628-4 |language=en}}</ref>

Besides the urban cycle, both in Africa and South America, a [[sylvatic cycle]] (forest or jungle cycle) is present, where ''[[Aedes africanus]]'' (in Africa) or mosquitoes of the genus ''[[Haemagogus]]'' and ''[[Sabethes]]'' (in South America) serve as vectors.<ref>{{cite book |doi=10.1016/B978-012373960-5.00385-3 |chapter=Yellow Fever, Historical |title=International Encyclopedia of Public Health |date=2008 |last1=Chastel |first1=C. |pages=665–675 |isbn=978-0-12-373960-5 }}</ref> In the jungle, the mosquitoes infect mainly nonhuman primates; the disease is mostly asymptomatic in African primates.<ref name="Monath Encyclopedia of Insects Yellow Fever"/> In South America, the sylvatic cycle is currently the only way unvaccinated humans can become infected, which explains the low incidence of yellow fever cases on the continent.<ref name="Yellow fever"/> People who become infected in the jungle can carry the virus to urban areas, where ''A. aegypti'' acts as a vector. Because of this sylvatic cycle, yellow fever cannot be eradicated except by completely eradicating the mosquitoes that serve as vectors.<ref name=Barr2007/>

In Africa, a third infectious cycle known as "savannah cycle" or intermediate cycle, occurs between the jungle and urban cycles.<ref>{{EMedicine|article|232244|Yellow Fever}}</ref> Different mosquitoes of the genus ''Aedes'' are involved. In recent years, this has been the most common form of transmission of yellow fever in Africa.<ref name="WHO_2001">{{cite web |title=Yellow fever fact sheet |work=WHO—Yellow fever |url=https://www.who.int/mediacentre/factsheets/fs100/en/ |access-date=2006-04-18 |url-status=live |archive-url=https://web.archive.org/web/20060418004055/http://www.who.int/mediacentre/factsheets/fs100/en/ |archive-date=2006-04-18 }}</ref>

Concern exists about yellow fever spreading to southeast Asia, where its vector ''A. aegypti'' already occurs.<ref>{{cite web |title=Ebola outbreak Alert and response operations Diseases Biorisk reduction Yellow fever: a current threat |url=https://www.who.int/csr/disease/yellowfev/impact1/en/ |website=WHO |access-date=4 August 2016 |archive-url=https://web.archive.org/web/20160808215707/http://www.who.int/csr/disease/yellowfev/impact1/en/ |archive-date=8 August 2016}}</ref>