Friday, April 6, 2012

Echinococcus granulosus: Hydatid worm

Introduction
Adult E. granulosus [7

Echinococcus granulosus is a cyclophylid cestode that parasitizes the small intestines of canids as adult and causes hydatid disease in humans and livestock in its intermediate hosts during its larvae stage. Effect of the parasite in canids is less than in its intermediate hosts.[3]   "Echinococcus granulosus  is found worldwide, mostly in rural and grazing areas." The most common treatment against hydatid disease in humans is to remove the hydatid disease through surgery. The treatment is notalways hundred percent successful because the cysts can burst resulting in spreading the cysts or cause allergic reactions.[1][2]  “Human infection of hydatid disease can be prevented if people are made aware of the risks and the proper safety precautions are taken.” [2] 

Symbiont Description

E. granulosus  is from the phylum Platyhelminthes, the class Cestoda, the order Cyclophyllidea, the family Taeniidea, and the genus Echinococcus. “This small tapeworm grows to about is 3- 6mm long, and lives in the small intestine of canines.”  It body includes a scolex with suckers and hooks that allows it to gain nutrients by attaching itself to the mucisal wall of the host. “The head and the three proglottids are connected by a short neck; proglottids are the body segments of the worm which contains the eggs that will be excreted in the feces. “[2]


Host Description

Bowflies, birds, arthropods serve as mechanical vectors of eggs of the parasite.  Then, the eggs are consumed by the intermediate hosts; livestock such as sheep, goat, swine, cattle, horses and camel serve as the intermediate hosts of the parasite.  E. granulosus  matures in its definite hosts after the definite hosts consume an infected livestock; dogs or other canids serve as definitive hosts. E. granulosus can also parasitize humans. [1][2] Humans serve as dead-end intermediate host.

Life Cycle

“The [matured] E. granulosus  live in the small bowel of the definitive hosts such as a dog.”  The hydatid worms releases eggs that are passed in dog feces. The intermediate host consumes the feces then, the eggs hatches in the bowel of the Intermediate host.  Afterwards, “the egg releases an oncosphere that penetrates the intestinal wall and migrates through the circulatory system of the intermediate host into various organs, especially the liver and lungs.  Within these organs, the oncosphere develops into a cyst.”  As the cysts develop, “protoscolices and daughter cysts fill the interior of the cysts.” The parasite is transmitted to the definite host by consumptionof organs of the intermediate host containing systs.  After trransmittance, “the protoscolices turns insode out, attach to the intestinal mucosa, and develop into adult stages within 32 to 80 days and the cycles continues.”  Humans are infected by E. granulosus  by consuming the eggs. The parasite develops and matures in the intestine and other organs but is not able to transfer to other hosts.[1][5]


Life cycle of E. granulosus [1]

Ecology

There are few available data on the clinical effects of the Cystic Hydatid Disease in the intermediate hosts since the cyst is slow in growing and animals are often slaughtered before it manages to create sufficient pressure on the tissue or organs.[3] The adult hydatid worm heavily affects its definite host when in large numbers which results in severe enteritis. Some dogs have developed immunity against the effects of E. granulosus.[4]  “Since E. granulosus mechanical vectors can increase the chances of eggs being ingested by the grazing animals  through mechanical dispersal, E. granulosus can be very epidemiological; a single dog can infect many sheep over a wide area.”[3]
Large daughter cysts [8]

“In Humans, Echinococcus granulosus infections remain silent for years before the enlarging cysts cause symptoms in the affected organs.  If the cyst(s) bursts, the resultant toxic (anaphylactic) shock would probably be fatal.” [5] “Infected humans cannot transmit E. granulosus” to other humans. [1]  “Hydatid disease is more prevalent in the northern hemisphere.  Human infection is most common in sheep-raising countries such as Australia and New Zealand, throughout England and Europe, the Middle East, Russia, Northern China, and Japan.  In the Americas the disease is especially prevalent in the Southern countries such as Argentina, Uruguay and Chile, and also occurs in Alaska and Canada. The incidence of human infection about 1 to 2 per 1000 population and may be higher in rural areas of affected regions.”[2] 


Global distribution of E. granulosa (black) [6]
Example of Host Resistance
           “The natural hosts of E. granulosus have developed a natural resistance to the hydatid worm.  Natural hosts have slowly developed some immunity against the parasite thus limiting the effects of the parasite; the definitive hosts needs to be heavily infected the parasite in other for the parasite to be harmful to the host.  E. granulosus don’t usually cause harm to its intermediate hosts except for humans.”[4] “The low virulence of E. granulosus in natural hosts reduces its potential as an important limiting factor on the population of the host.” [5] 

References


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