Echinococcus

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Echinococcus
Cotton rat infected with Echinococcus multilocularis 3MG0020 lores.jpg
Scientific classification e
Kingdom:Animalia
Phylum:Platyhelminthes
Class:Cestoda
Order:Cyclophyllidea
Family:Taeniidae
Genus:Echinococcus
 
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Echinococcus
Cotton rat infected with Echinococcus multilocularis 3MG0020 lores.jpg
Scientific classification e
Kingdom:Animalia
Phylum:Platyhelminthes
Class:Cestoda
Order:Cyclophyllidea
Family:Taeniidae
Genus:Echinococcus
schematic representation of the life cycle of Echinococcus

The genus Echinococcus includes six species of cyclophyllid tapeworms to date, of the family Taeniidae. Infection with Echinococcus results in hydatid disease, also known as echinococcosis.

Echinococcus is triploblastic, i.e. it has 3 layers- outermost ectoderm, middle mesoderm and inner endoderm. Anus is absent; and no digestive system. Body is covered by tegument and the worm is divided into scolex, short neck and 3-6 proglottids. Body is ribbon-like.

In humans, this causes a disease called echinococcosis. There are 3 types of echinococcosis i.e. cystic echinococcosis caused by Echinococcus granulosus, alveolar echinococcosis caused by E. multilocularis and polycystic echinococcosis caused by E. vogeli and/or E. oligarthrus.[1] Incubation period is usually long and can be up to 50 years. Cystic echinococcosis is mostly found in South and Central America, Africa, the Middle East, China, Italy, Spain, Greece, Russia and the West of the United States (e.g. Arizona, New Mexico and California).

Echinococcosis is a zoonosis; humans are dead-end hosts. The definitive hosts are carnivorous predators - dogs, wolves, foxes, lions. The adult tapeworm lives in their small intestine and delivers eggs that are excreted with the stool. The intermediate hosts are infected by ingesting eggs. Sheep, goat, cattle, camel, pig, wild herbivores and rodents are the usual intermediate hosts, but humans can also be infected.

The egg hatches in the digestive system of the intermediate host, producing planula larva. It penetrates the intestinal wall and is carried by bloodstream to liver, lung, brain or another organ. It settles there and turns into a bladder-like structure called hydatid cyst. From the inner lining of its wall, protoscoleces (i.e. scoleces with invaginated tissue layers) bud and protrude into the fluid that is filling the cyst.

After the death of the normal intermediate host, its body can be eaten by carnivores suitable as definitive hosts. In their small intestine, protoscoleces turn inside out, attach and give rise to adult tapeworms, completing the life cycle.

In humans, the cysts persist and grow for years. They are regularly found in the liver (and every possible organ: spleen, kidney, bone, brain, tongue and skin) and are asymptomatic until their growing size produces symptoms or are accidentally discovered. Disruption of the cysts (spontaneous or iatrogenic e.g. liver biopsy) can be life threatening due to anaphylactic shock.

Cysts are detected with ultrasound, CT or other imaging techniques. Anti-echinococcus antibodies can be detected with serodiagnostic tests e.g. Indirect Fluorescent Antibody (IFA) Test, CF (complement fixation), ELISA, Western Blot and other methods.[2]

Taxonomy[edit]

A phylogenetic tree has been created for several species in this genus - Echinococcus oligarthrus, Echinococcus vogeli, Echinococcus multilocularis, Echinococcus shiquicus, Echinococcus equinus, Echinococcus ortleppi and Echinococcus granulosus.[3] The first diverging species are the neotropical endemic species E. oligarthrus and E. vogeli. E. ortleppi and E. canadensis are sister species as are E. multilocularis and E. shiquicus. E. canadensis is related to E. granulosus.

The origin of these parasites based on host-parasite co-evolution comparisons was North America or Asia, depending on whether the ancestral definitive hosts were canids or felids.

A second study has shown that the species Echinococcus oligarthrus and Echinococcus vogeli are basal in this genus.[4] The genus is a sister to the genus Taenia from which it diverged more than 10 million years ago. The genus Echinococcus evolved in North America in canids and began to diversify 5.8 million years ago.

References[edit]

  1. ^ Thompson RCA, McManus DP. Aetiology: parasites and life cycles. In: Eckert J, Gemmell MA, Meslin F-X, Pawlowski ZS editor. WHO/OIE Manual on Echinococcosis in Humans and Animals: a Public Health Problem of Global Concern. Paris: Office Internationale des Epizooities; 2001;p. 1–19
  2. ^ Wenbao Zhang, Hao Wen, Jun Li, Renyong Lin, and Donald P. McManus, Immunology and Immunodiagnosis of Cystic Echinococcosis: An Update. Clinical and Developmental Immunology, 2012, 10 pages doi:10.1155/2012/101895
  3. ^ Nakao M, McManus DP, Schantz PM, Craig PS, Ito A (2007) A molecular phylogeny of the genus Echinococcus inferred from complete mitochondrial genomes. Parasitology 134(5):713-722
  4. ^ Knapp J, Nakao M, Yanagida T, Okamoto M, Saarma U, Lavikainen A, Ito A (2011) Phylogenetic relationships within Echinococcus and Taenia tapeworms (Cestoda: Taeniidae): an inference from nuclear protein-coding genes. Mol Phylogenet Evol 61(3):628-638. doi: 10.1016/j.ympev.2011.07.022

See also[edit]