Echinococcus granulosus

Also known as:

Dwarf dog tapeworm Hydatidosis

Scientific Classification

Kingdom	Animalia
Phylum	Platyhelminthes
Class	Cestoda
Order	Cyclophyllidea
Family	Taeniidae
Genus	Echinococcus
Species	E. granulosus

Echinococcus granulosus is an important zoonosis as its metacestode, the hydatid cyst, can develop in humans, as well as in many other animals. The hydatid cyst can grow to the size of a ping-pong ball in sheep, a tennis ball in horses, and a football in man. The metacestode of the related Echinococcus multilocularis (the alveolar cyst) is even more dangerous, but fortunately does not occur in the UK.

Hosts

Intermediate hosts: Ruminants and man.

Definitive hosts: Dogs and wild canids.

Identification

The adults are 6 mm in length. They have a scolex and three or four individual segments. The brood capsules float in the vesicular fluid, forming 'hydatid sand.'

Life-Cycle

There may be several thousand adult tapeworms in an infected animal. The scolex is deeply buried in an intestinal crypt, so microscopic inspection of a mucosal scraping is necessary to detect infection.

The terminal proglottid forms approximately half the total length. The proglottids are shed in the faeces. The oncosphere are capable of surviving in the environment, for up to two years. They are then ingested by the intermediate host e.g. the sheep, where they penetrate the gut lining.

The oncosphere then travels to the liver and the lungs, where the hydatid then begins growth. This is a slow process taking around 6-12 months, and the cysts are capable of growing up to 20cm in length.

The definitive host e.g. the dog, then ingests the offal of the intermediate host, and the life cycle continues.

The prepatent period of E. granulosus is 6-7weeks. No more than one gravid segment is passed by each tapeworm each week. This species therefore has a low biotic potential.