Transmission of echinococcosis
A number of herbivorous and omnivorous animals act as intermediate hosts of Echinococcus, which means they get infected by ingesting the parasite eggs in the contaminated ground and develop parasitic larval stages in their viscera. Carnivores are definitive hosts for the parasite, and are infected through the consumption of viscera of intermediate hosts that harbour the parasite, also through scavenging infected carcases. Humans are accidental intermediate hosts and are not able to transmit the disease.
Cystic echinococcosis is principally maintained in a dog–sheep–dog cycle, yet several other domestic animals may be involved including goats, swine, horses, cattle, camels and yaks. Alveolar echinococcosis usually occur in a wildlife cycle between foxes, other carnivores and small mammals (mostly rodents). Domesticated dogs and cats can also be infected.
For both diseases, humans become infected through the ingestion of soil, water or food (e.g. green vegetables, berries) contaminated with the parasites’ eggs shed in the faeces of the carnivores, and also by hand-to-mouth transfer of eggs after contact with the contaminated fur of a carnivore, most commonly a dog.
Cystic echinococcosis is globally distributed, with highly endemic areas mostly found in the eastern part of the Mediterranean region, northern Africa, southern and eastern Europe, at the southern tip of South America, in Central Asia, Siberia and western China. Conversely, alveolar echinococcosis is restricted to the northern hemisphere, in particular to regions of China, the Russian Federation and countries in continental Europe and North America.