Echinococcosis is the general term used to describe four disease forms caused by the larval stage of Echinococcus tapeworms, the smallest tapeworms in the Taeniidae family. Human echinococcosis is a zoonotic disease that occurs in four clinically well-described forms:
Cystic echinococcosis, also called hydatid disease or hydatidosis, is caused by infection with Echinococcus granulosus (E. granulosus).
Alveolar echinococcosis, also called alveolar hydatid echinococcosis is caused by infection with E. multilocularis.
Polycystic echinococcosis is caused by infection with E. vogeli.
Unicystic echinococcosis is caused by infection with E. oligarthrus.
Cystic echinococcosis (CE) and alveolar echinococcosis (AE) are the key forms of echinococcosis that are of significant medical and public health concerns for humans.  These disease processes will be the focus of this article.
Cystic echinococcosis (CE) is caused by infection with the larvae of E granulosus. CE is found worldwide and on every continent except Antarctica. Areas of the world with noted prevalence are rural regions of Africa, southern Europe, Asia, the Middle East, Central and South America (especially the countries of Argentina and Uruguay). Cases are rare in North America. The parasite is transmitted to dogs when they eat organs of other animals that contain hydatid cysts. Once ingested by canines, these cysts develop into adult tapeworms. Infected dogs shed tapeworm eggs in their feces, which contaminate the ground. Sheep, cattle, goats, and pigs ingest the E granulosus eggs in the contaminated soil. Once eaten, the eggs hatch and develop into cysts in the animal’s internal organs.  CE is principally maintained in a dog–sheep–dog cycle, however other domestic animals may be involved, including goats, swine, horses, cattle, camels and yaks. 
The most common mode of transmission of E granulosus to humans is via the accidental ingestion of soil, water, or food contaminated by fecal matter from an infected dog. Echinococcus eggs deposited in soil can remain viable for a year. The disease is most commonly found in people who raise sheep, due to the sheep's role as the parasite’s intermediate host and the lives of working dogs that are allowed to eat the offal of infected sheep. 
Alveolar echinococcosis (AE) is caused by infection with E multilocularis larvae. AE is found worldwide and is especially prevalent in the northern latitudes of Europe, Asia (China and Russia), and North America.  The adult tapeworm is found in foxes, coyotes, and dogs. Infection with the larval stages is transmitted to people through ingestion of food or water contaminated with tapeworm eggs.  AE is typically maintained within a wildlife cycle between foxes, other carnivores and small mammals (mostly rodents); however domesticated dogs and cats can also be infected. 
The Echinococcus tapeworm's life cycle (see image below) involves a definitive host, usually a dog (although foxes and coyotes have also been implicated), and intermediate hosts such as sheep, goats, and swine. The 3-mm to 6-mm adult tapeworm is found in the definitive host's intestines. Eggs are excreted via the feces into the environment. Dogs typically become infected by eating the remains of infected sheep and other livestock.
Humans are accidental intermediate hosts and are not typically involved in the life cycle of the organism. Infected humans are not contagious to others. Humans become infected through accidental exposure to canine feces. Humans can also become infected by eating food contaminated with tapeworm eggs, which hatch in human small intestine under the influence of gastric and intestinal secretions. Larvae hatched from these eggs can penetrate the small intestine and become widely distributed in the human body. These larvae, then, develop into hydatid cysts. Human disease is usually caused by the mass effect of slow-growing hydatid cysts, although bacterial secondary infection and/or anaphylaxis due to cyst rupture have also been reported.
Hydatid cysts may contain many protoscolices, each with the ability to develop into an adult tapeworm in a definitive host. The definitive host usually becomes infected by ingesting the protoscolices in hydatid cysts from the intermediate host; adult tapeworms only develop in the intestines of the definitive host.
Cystic echinococcosis (CE), also known as hydatid disease, is caused by infection with the larval stage of E granulosus, a ~2-7 millimeter long tapeworm found in dogs (definitive host) and sheep, cattle, goats, and pigs (intermediate hosts). Most infections in humans are asymptomatic; infected humans may remain symptom-free for 20 years.
CE is characterized by harmful, slowly enlarging cysts that grow in the liver, lungs, and other organs, often for years going unnoticed and neglected.  Less frequently, cysts develop in the bones, kidneys, spleen, muscles, CNS and eyes.  In CE, the liver is involved in two-thirds of cases; in the remaining one-third of cases, other sites of involvement, in descending order of frequency, are the lung, spleen, skin, muscle, kidney, retroperitoneum, bone, heart, and brain.
Alveolar echinococcosis (AE) disease is caused by infection with the larval stage of E multilocularis, a ~1-4 millimeter long tapeworm found in foxes, coyotes, and dogs (definitive hosts). Small rodents are intermediate hosts for E. multilocularis. Although cases of AE in animals in endemic areas are relatively common, human cases are rare. Parasitic tumors can form in the liver (primary location), lungs, brain, and other organs. If left untreated, AE can be fatal. [1, 2]
Echinococcosis is rare. Indigenous cases have been reported in Minnesota and among Native Americans in western Alaska. Imported cases are uncommon. 
Human infection rates are generally highest in areas that practice poor sanitation methods for sheep raising, particularly in common areas with dogs. Dogs in these areas may become infected by eating the entrails of infected butchered sheep or other herded animals. Humans then become accidentally infected when they ingest Echinococcus eggs via soil, water, or food contaminated by fecal matter from an infected dog. Human infection rates are also high in areas where intestines are ingested as part of the diet.
Echinococcosis has been reported in Manitoba and Saskatchewan, Canada.  Outside North America, incidence rates vary considerably, from less than 1 case per 100,000 people in many parts of the world, to 13 cases per 100,000 people in Greece, 143 cases per 100,000 people in Argentina, 197 cases per 100,000 people in the Xinjiang province of China, and 220 cases per 100,000 people in Kenya's Turkana District. Echinococcosis is endemic in sub-Saharan Africa.  Of those cases occurring in Europe, one-half of the cases occur in France. 
The high incidence rate reported in the Turkana District of Kenya is attributed to cultural practices. For tribe members who eat canine intestine, disease transmission likely occurs when infected intestine is not thoroughly cooked. In tribes communities who do not bury their dead; carnivores may become the direct intermediate host after eating human remains.
Most human infection is asymptomatic. Death may occur from hydatid rupture, which can cause anaphylactic shock and/or lead to bacterial infection. The mass effect of slow-growing hydatid cysts into the tissue space of vital organs can also be fatal.
Males and females are equally affected. 
Most patients infected by E granulosus (CE) are 21-50 years of age. Patients infected by E multilocularis (AE) are typically older, 31-80 years of age.
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