Answer to Case 817

 Answer to the Parasite Case of the Week 817: Fish tape worm, Dibothriocephalus, Diphyllobothrium, or Adenocephalus species. 

There are several characteristic morphologic features present in this case. First, we have a long portion of a tapeworm body (strobila) with proglottids that are broader than they are long. As noted by Florida Fan, the strobila is craspedote, meaning that the posterior margin of each proglottid overlaps the anterior portion of the following segment. Additionally, a dark, centrally located uterus is present in each mature proglottid. In well-cleared specimens, the uterus has a characteristic rosette-like appearance. Together, these features are characteristic of the fish tapeworms in the family Diphyllobothriidae.

Several readers wondered whether the very narrow portion of the worm represented the scolex (arrow below).

Unfortunately, we don't actually have the scolex in this specimen. On closer examination, the narrow portion indicated above consists of very small proglottids and likely represents the anterior strobila near the neck.

Had we been lucky enough to recover the scolex, we would expect it to have two longitudinal grooves called bothria, rather than the four muscular suckers seen in the more familiar cyclophyllidean tapeworms such as Taenia species.

Fish tapeworm infection is acquired by eating raw or inadequately cooked fish containing the infective plerocercoid larva. The life cycle involves a small aquatic crustacean (usually a copepod) as the first intermediate host and a fish as the second. Larger predatory fish may acquire plerocercoids by eating smaller infected fish, allowing the parasite to work its way up the food chain until, eventually, somebody decides that fish would be delicious raw or undercooked. I enjoyed HLCM fan's reminder that sushi and sashimi aren't the only potential exposures. Ceviche and inadequately processed fish are also possibilities, and smoking, salting, or pickling shouldn't automatically be assumed to kill the plerocercoids. Adequate cooking or appropriate freezing is needed to reliably dispatch these unwanted dinner guests.

So, could someone acquire a fish tapeworm from a fish caught in Michigan? Potentially, yes! Diphyllobothriid larvae have actually been documented in Michigan fish. A Michigan Department of Natural Resources review describes historical human infections with Dibothriocephalus latus (then Diphyllobothrium latum) associated with fish from Portage Lake in Michigan's Upper Peninsula. Unidentified Diphyllobothrium plerocercoids have also been reported from salmon collected from Michigan tributaries of Lakes Michigan, Superior, and Huron. Fish-eating mammals and birds can serve as definitive hosts and keep the cycle going quite nicely without us. Having said that, the extent of today's globally transported seafood means that the place where someone lives tells us surprisingly little about where their tapeworm had been living previously. A good dietary and travel history would be very helpful here.

Regarding the taxonomy, since several of you asked: the classic "broad fish tapeworm," Diphyllobothrium latum, is now Dibothriocephalus latus. Other important human pathogens include Dibothriocephalus nihonkaiense (formerly Diphyllobothrium nihonkaiense), particularly associated with Pacific salmon; Dibothriocephalus dendriticus; and Adenocephalus pacificus (formerly Diphyllobothrium pacificum), predominantly associated with the Pacific coast of South America. Several true Diphyllobothrium species can also infect humans.

Unfortunately, the proglottids and eggs generally don't allow us to reliably distinguish these species. Molecular testing is usually needed if species-level identification is desired. Thus, without molecular identification, "Dibothriocephalus, Diphyllobothrium, or Adenocephalus species" is a reasonable identification for this impressive specimen. 

Many thanks again to Dr. Ben Von Bredow for sharing this spectacular specimen!