Echinococcus multilocularis

It is the causative agent of highly pathogenic Alveolar Hydatid disease in man and other mammals. The fox is the most important definitive host, although dogs, and occasionally cats, may also be infected with the adult parasite. Again there appears to be a number of sub-species of this organism, E. m. multilocularis in Europe and E. m. sibiricencis in North America.

 This is very similar to that of E. granulosus, but with more adaptations for colder climates. For example the eggs are highly resistant to cold temperatures, being able to survive at -20° for more than 2 weeks. In addition the pre patent period in the definitive host is much shorter, usually between 4 to 5 weeks.

 Morphology

Larvae - The larval E. multilocularis is very different from that of E. granulosus. In this case the 'cyst' grows invasively by external budding, forming a diffuse growth through the infected organ, replacing that organ’s tissues. The growth itself, (it cannot truly be called a cyst as there is no real cyst wall), is composed of numerous cavities containing a gelatinous matrix within which protoscolices and numerous brood capsules are produced, and which in its behaviour, most closely
resembles a malignant neoplasm. In contrast to E. granulosus this growth is also very rapid, infective protoscolesces being present after only 2 to 3 months, as compared to the 1 to 2 years in the related metacestode.

Adults - The adult parasite is very similar to E. granulosus, being slightly smaller,
with a maximum length of approximately 4mm, and consisting of 4 to 5
proglottids.

 Pathology of Infection

Larvae - The multilocular cyst is highly pathogenic due to its fast growth rate and
invasive nature, in extreme cases completely replacing liver tissue. As the cyst lacks the tough laminated layer seen in E. granulosus, and by its nature grows by budding, metastases of growth may also be seen, colonising other body organs. Due to this aspect of the parasite, it may also be transferred by transplantation. This parasite must be considered one of the most pathogenic of the parasitic helminths.

Adults - As with E. granulosa the adult tapeworm is usually non-pathogenic to its canine hosts