Schistocephalus solidus is a tapeworm with a three-host life cycle. Free-swimming coracidia are eaten by copepods, the first host. After about 2 weeks of development in copepods, the worm is ready to be transmitted to the second intermediate host, three-spined sticklebacks. In the fish host, the worm grows to gargantuan sizes; in exceptional cases it can even weigh more than the host. Fish are impacted in various ways by infection, showing altered behaviours (risk-taking, flight response, etc.), brain chemistry, and immune responses. These modifications are thought to lead to a higher transmission rate of the parasite to its final host, fish-eating birds. Within 48 hrs of reaching the intestine of the final host, the parasite is reproductively mature and producing eggs. After about a week, the worm has produced all its eggs and dies. This short and explosive reproductive period is presumably the reason why S. solidus is one of the few helminths that can be bred in vitro.
Contributed by Daniel Benesh.