Also like many other parasite infections, the course of Echinococcus infection is complex. The worm has a life cycle that requires definitive hosts and intermediate hosts. 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, unless eaten by dogs or wolves after death. Hosts Red fox with rodent 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 small rodents 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 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. Eggs Echinococcus egg in feces 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. Larval/hydatid cyst stage From the embryo released from an egg develops a hydatid cyst, which grows to about 5–10 cm within the first year and is able to survive within organs for years. 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 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. Adult worm Echinococcus adult worms develop from protoscolices and are typically 6 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). The scolex of the adult worm contains four suckers and a rostellum that has about 25–50 hooks. Morphological differences The major morphological difference among different species of Echinococcus is the length of the tapeworm. E. granulosus is approximately 2 to 7 mm while E. multilocularis is often smaller and is 4 mm or less. On the other hand, E. vogeli is found to be up to 5.6 mm long and E. oligarthrus is found to be up to 2.9 mm long. 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. 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. 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. 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.
Echinococcosis
Cholangiocarcinoma
Cholangiosarcoma
Cryptococcosis
Alveolar hydatid disease
Liver angiosarcoma
Amoebic liver abscess
Hepatocellular carcinoma