Adult tapeworms in the genus Spirometra live in the intestines of dogs and cats. Eggs are shed in feces and embryonate in the environment. Eggs hatch in water and release coracidia, which are ingested by copepods. The coracidia develop into procercoid larvae in the copepod intermediate host. Second intermediate hosts, including fish, reptiles and amphibians, ingest infected copepods and acquire procercoid larvae. The procercoid larvae develop into plerocercoid larvae in the second intermediate host. The cycle is completed when a predator (dog or cat) eats an infected second intermediate host. Humans cannot serve as definitive hosts for Spirometra spp., but serve as paratenic hosts ("transport hosts") or second intermediate hosts and develop sparganosis. Humans acquire sparganosis by either drinking water contaminated with infected copepods or consuming the flesh of an under-cooked second intermediate or paratenic host. Larvae of these tapeworms (spargana) can live up to 20 years in the human host.
The genus Spirometra has a worldwide distribution, although most human cases of sparganosis are recorded from Southeast Asian countries. Sparganosis occurs in animals throughout North America, but human cases in this region are rare.
Spirometra tapeworms have a complex multi-host life cycle in which the definitive host (i.e., the host in which the parasite matures and reproduces) is a dog or cat. However, although humans cannot serve as definitive hosts for Spirometra tapeworms, they can serve as paratenic hosts ("transport hosts") or second intermediate hosts and develop sparganosis. Humans acquire sparganosis by either drinking water contaminated with infected copepods or consuming the flesh of an under-cooked second intermediate or paratenic host. Larvae of these tapeworms (spargana) can live up to 20 years in the human host.
Spirometra species known to infect humans and cause sparganosis include S. mansoni, S. ranarum, S. mansonoides, S. erinacei, and the related Sparganum proliferum (with ambiguous taxonomic affinities).
Infection of humans and some other mammals by the plerocercoid larvae (spargana) of various diphyllobothroid tapeworms belonging to the genus Spirometra is the cause of the parasitic disease knowm as sparganosis. Sparganosis has been reported sporadically around the world, but especially in several Asian countries, including South Korea, Japan, Thailand, and China.
(Li et al. 2011)
