Editing Echinococcosis (section)

==Cause==
Also like many other parasite infections, the course of ''Echinococcus'' infection is complex. The worm has a life cycle that requires [[intermediate host|definitive hosts]] and [[intermediate host]]s. Definitive hosts are normally [[carnivores]] such as dogs, while intermediate hosts are usually [[herbivores]] such as sheep and cattle. Humans function as accidental
hosts, because they are usually a dead end for the parasitic infection cycle.{{cn|date=May 2021}}

===Hosts===
[[File:Renardrouxcampagnol.jpg|thumb|Red fox with rodent]]
{| class="wikitable"
|-
! Organism !! Definitive Hosts!! Intermediate Hosts
|-
|''E. granulosus''|| dogs and other canidae || sheep, goats, cattle, camel, buffalo, swine, kangaroos, and other wild herbivores
|-
|''E. multilocularis''|| foxes, dogs, other canidae and cats || small rodents
|-
|''E. vogeli''|| bush dogs and dogs || rodents
|-
|''E. oligarthrus''||wild felids<ref name="dpd" /><ref name="Sréter03"/> || small rodents<ref name="dpd" /><ref name="Sréter03" />
|}

===Life cycle===
An adult worm resides in the small intestine of a definitive host. A single gravid proglottid releases eggs that are passed in the feces of the definitive host.  The egg is then ingested by an intermediate host. The egg then hatches in the small intestine of the intermediate host and releases an oncosphere that penetrates the intestinal wall and moves through the circulatory system into different organs, in particular the liver and lungs. Once it has invaded these organs, the oncosphere develops into a cyst. The cyst then slowly enlarges, creating protoscolices (juvenile [[Cestoda#Scolex|scolices]]), and daughter cysts within the cyst. The definitive host then becomes infected after ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices attach to the intestine. They then develop into adult worms and the cycle starts all over again.{{citation needed|date=July 2020}}

===Eggs===
[[Image:Echinococcus Egg.jpg|thumb|''Echinococcus'' egg in feces]]
[[Image:Echinococcus Proscolices.jpg|thumb|Protoscolices being released from a hydatid cyst]]
''Echinococcus'' eggs contain an embryo that is called an [[oncosphere]] or hexcanth. The name of this embryo stems from the fact that these embryos have six hooklets. The eggs are passed through the feces of the definitive host and it is the ingestion of these eggs that lead to infection in the intermediate host.<ref name="JohnDavid" />

===Larval/hydatid cyst stage===
From the embryo released from an egg develops a ''hydatid cyst'', which grows to about 5–10&nbsp;cm within the first year and is able to survive within organs for years.<ref>{{cite book |chapter=Ch. 290 |editor1-first=Gerald L. |editor1-last=Mandell |editor2-first=John Eugene |editor2-last=Bennett |editor3-first=Raphael |editor3-last=Dolin |title=Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases |publisher=Elsevier |year=2010 |isbn=978-0443068393 |edition=7th}}</ref> Cysts sometimes grow to be so large that by the end of several years or even decades, they can contain several liters of fluid. Once a cyst has reached a diameter of 1&nbsp;cm, its wall differentiates into a thick outer, non-cellular membrane, which covers the thin germinal epithelium. From this epithelium, cells begin to grow within the cyst. These cells then become vacuolated, and are known as brood capsules, which are the parts of the parasite from which protoscolices bud. Often, daughter cysts also form within cysts.<ref name="JohnDavid" />

===Adult worm===
''Echinococcus'' adult worms develop from protoscolices and are typically 6&nbsp;mm or less in length and have a scolex, neck and typically three proglottids, one of which is immature, another of which is mature and the third of which is gravid (or containing eggs).<ref name="JohnDavid" /> The scolex of the adult worm contains four suckers and a [[Rostellum (helminth)|rostellum]] that has about 25–50 hooks.<ref>{{cite web |title=Echinococcosis |date= 17 April 2019|work=DPDx. Parasite Image Library |publisher=CDC |url=http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Echinococcosis_il.htm |url-status=live |archive-url=https://web.archive.org/web/20100304141540/http://www.dpd.cdc.gov/dpdx/HTML/ImageLibrary/Echinococcosis_il.htm |archive-date=4 March 2010  }}</ref>

===Morphological differences===
The major morphological difference among different species of ''Echinococcus'' is the length of the tapeworm. ''E. granulosus'' is approximately 2 to 7&nbsp;mm while ''E. multilocularis'' is often smaller and is 4&nbsp;mm or less.<ref name=Eckert04>{{cite journal |vauthors=Eckert J, Deplazes P |title=Biological, epidemiological, and clinical aspects of echinococcosis, a zoonosis of increasing concern |journal=Clin. Microbiol. Rev. |volume=17 |issue=1 |pages=107–35 |date=January 2004 |pmid=14726458 |pmc=321468 |doi=10.1128/cmr.17.1.107-135.2004  }}</ref> On the other hand, ''E. vogeli'' is found to be up to 5.6&nbsp;mm long and ''E. oligarthrus'' is found to be up to 2.9&nbsp;mm long.<ref name="dpd" /> In addition to the difference in length, there are also differences in the hydatid cysts of the different species. For instance, in ''E. multilocularis'', the cysts have an ultra thin limiting membrane and the germinal epithelium may bud externally. Furthermore, ''E. granulosus'' cysts are unilocular and full of fluid while ''E. multilocularis'' cysts contain little fluid and are multilocular. For ''E. vogeli'', its hydatid cysts are large and are actually polycystic since the germinal membrane of the hydatid cyst actually proliferates both inward, to create septa that divide the hydatid into sections, and outward, to create new cysts. Like ''E. granulosus'' cysts, ''E. vogeli'' cysts are filled with fluid.<ref name="JohnDavid" />

===Transmission===
As one can see from the life cycles illustrated above, all disease-causing species of ''Echinococcus'' are transmitted to intermediate hosts via the ingestion of eggs and are transmitted to definitive hosts by means of eating infected, cyst-containing organs. Humans are accidental intermediate hosts that become infected by handling soil, dirt or animal hair that contains eggs.<ref name=Eckert04 />

While there are no biological or mechanical vectors for the adult or larval form of any ''Echinococcus'' species, coprophagic flies, carrion birds and arthropods can act as mechanical vectors for the eggs.<ref>{{cite web |title=''Echinococcus granulosus'' |date=2001 |work=Material Safety Data Sheets (MSDS) |publisher=Public Healthy Agency of Canada |url=http://www.phac-aspc.gc.ca/msds-ftss/msds54e-eng.php |url-status=live |archive-url=https://web.archive.org/web/20100305181917/http://www.phac-aspc.gc.ca/msds-ftss/msds54e-eng.php |archive-date=5 March 2010  }}</ref>

===Aberrant cases===
There are a few aberrant cases in which carnivores play the role of the intermediate hosts. Examples are domestic cats with hydatid cysts of ''E. granulosus.''<ref name="BonelliMasu2018">{{cite journal|last1=Bonelli|first1=Piero|last2=Masu|first2=Gabriella|last3=Giudici|first3=Silvia Dei|last4=Pintus|first4=Davide|last5=Peruzzu|first5=Angela|last6=Piseddu|first6=Toni|last7=Santucciu|first7=Cinzia|last8=Cossu|first8=Assunta|last9=Demurtas|first9=Nicola|last10=Masala|first10=Giovanna|title=Cystic echinococcosis in a domestic cat (''Felis catus'') in Italy|journal=Parasite|volume=25|year=2018|pages=25|issn=1776-1042|doi=10.1051/parasite/2018027|pmid=29727269|pmc=5935470}} {{open access}}</ref>