Echinococcus

Echinococcus Background

Echinococcus granulosus sensu lato occurs worldwide, and most frequently in rural, grazing areas where dogs ingest organs from infected animals. Genotypes associated with sheep (G1 and G3) are the most commonly reported and broadly distributed. E. multilocularis occurs in the northern hemisphere, including central and northern Europe, Central Asia, northern Russia, northern Japan, north-central United States, northwestern Alaska, and northwestern Canada. E. vogeli and E. oligarthrus occur in Central and South America.

The adult E. granulosus (sensu lato) is 2-7 mm in length and resides in the small intestine of its definitive host, wild and domestic canids. Gravid proglottids release eggs that are passed in the feces, and are immediately infectious. After ingestion by a suitable intermediate host, eggs hatch in the small intestine and release six-hooked oncospheres that penetrate the intestinal wall and migrate through the circulatory system into various organs, especially the liver and lungs. In these organs, the oncosphere develops into a thick-walled hydatid cyst that enlarges gradually, producing protoscolices and daughter cysts that fill the cyst interior. The definitive host becomes infected by ingesting the cyst-containing organs of the infected intermediate host. After ingestion, the protoscolices evaginate, attach to the intestinal mucosa, and develop into adult stages image in 32 to 80 days. Humans are aberrant intermediate hosts, and become infected by ingesting eggs. Oncospheres are released in the intestine, and hydatid cysts develop in a variety of organs. If cysts rupture, the liberated protoscolices may create secondary cysts in other sites within the body (secondary echinococcosis).

Echinococcus granulosus definitive hosts are wild and domestic canids. Natural intermediate hosts depend on genotype. Intermediate hosts for zoonotic species/genotypes are usually ungulates, including sheep and goats (E. granulosus sensu stricto), cattle (E. ortleppi/G5), camels (E. canadensis/G6), and cervids (E. canadensis/G8, G10). For E. multilocularis, foxes, particularly red foxes (Vulpes vulpes), are the primary definitive host species. Other canids including domestic dogs, wolves, and raccoon dogs (Nyctereutes procyonoides) are also competent definitive hosts. Many rodents can serve as intermediate hosts, but members of the subfamily Arvicolinae (voles, lemmings, and related rodents) are common.

Echinococcus granulosus infections often remain asymptomatic for years before the cysts grow large enough to cause symptoms, often in the liver and lungs. Rupture of the cysts can produce a host reaction manifesting as fever, urticaria, eosinophilia, and potentially anaphylactic shock; rupture of the cyst may also lead to cyst dissemination. Echinococcus multilocularis affects the liver as a slow growing, destructive tumor, which may be misdiagnosed as liver cancer. Metastatic lesions into the lungs, spleen, and brain may occur and untreated infections have a high fatality rate, between 50% and 75%. It is estimated that more than 1M people are infected with Echinococcosis which is often expensive and complicated to treat and may require extensive surgery and/or prolonged drug therapy (CDC, 2019; WHO, 2020).

References

• Parasites – Echinococcosis. Centers for Disease Control and Prevention (CDC), 2019.
• Echinococcosis fact sheet. World Health organisation (WHO), 2020.