The fox-tapeworm Echinococcus multilocularis
The fox-tapeworm is endemic in the Northern Hemisphere and lives as an adult in the intestine of foxes, dogs or raccoon dogs. Infective eggs which contain the oncosphere larval stage are shed into the environment via feces of the definitive hosts and are taken up by intermediate hosts such as small rodents or humans. In the liver of the intermediate host (sometimes in other organs such as the lung), the oncosphere transforms into the metacestode larva which grows infiltrative, like a malignant tumor, into the surrounding host tissue (causing the disease alveolar echinococcosis). During the infection of natural intermediate hosts (e.g. rodents – but usually not in human infections), numerous protoscoleces (i.e. head regions of the adult worm) are produced by the metacestode after several weeks of infection which are passed on to the definitive host when it takes its prey. Within the intestine of the definitive host, the protoscolex can then grow out to the adult worm thus closing the life cycle.
In humans, the asexual reproduction of the metacestode can be a process of several years to decades during which the host does not even notice that he/she is infected (i.e. the parasite almost behaves like a perfectly transplanted organ). Space-demanding processes that are associated with the unrestricted growth of the parasite can, eventually, cause functional disorders or a total loss of function of the affected organ. Lethality rates of more than 90% are observed in untreated patients within 10 years after diagnosis, rendering alveolar echinococcosis one of the most dangerous parasitoses in the world. Therapeutic measure are, at present, very limited and comprise surgery (only possible after early diagnosis) or prolonged chemotherapy with benzimidazoles (directed against the parasite’s beta-tubulins).