Life cycle of Spirometra tapeworms causing sparganosis in humansAdult tapeworms in the genus Spirometra live in the intestines of dogs and cats (7). Eggs are shed in feces (1) and embryonate in the environment (2). Eggs hatch in water and release
coracidia (3), which are ingested by copepods. The
coracidia develop into
procercoid larvae in the copepod intermediate host (4). 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 (5). The cycle is completed when a predator (dog or cat) eats an infected second intermediate host (6). Humans cannot serve as definitive hosts for Spirometra spp., but serve as paratenic hosts ("transport hosts") or second intermediate hosts (8) 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.From
Centers for Disease Control Parasites and Health website.
