Diphyllobothriasis is caused by fish tapeworms of the genus Diphyllobothrium. Humans become infected mainly by eating freshwater (perch, pike, and salmonids), but also marine, fish that are raw, partially cooked, or smoked.
PATHOGENESIS AND EPIDEMIOLOGY
The Diphyllobothrium tapeworm lives in the small intestine, where it may reach a length greater than 10 m. The gravid proglottids are wider than they are long, hence the name broad fish tapeworm. More than 1 million eggs may be passed in the feces every day with some Diphyllobothrium species. The eggs are operculated and measure approximately 60 μm by 40 μm. When the eggs reach water, a ciliated embryo or coracidia develops and is released through the operculum in about 2 weeks. The coracidia is ingested by copepod species (water fleas), and within this first intermediate host, it turns into a procercoid larva in 2 or 3 weeks. When a fish eats the infected copepod (second intermediate host), the procercoid penetrates the fish’s intestinal wall and migrates to the muscles, where it grows into a ribbon-like plerocercoid larva (also called a sparganum) in approximately 1 month. Predatory fish such as salmon, pike, perch, and trout may eat the infected fish, and the plerocercoid will again invade the muscle of the new host. When a suitable definitive host eats the infected fish, the plerocercoid larva attaches to the wall of the small intestine and matures into a tapeworm after approximately 5 weeks (Fig. 331-1). Specific characteristics of the life cycle may vary depending on the Diphyllobothrium species. While Diphyllobothrium latum requires freshwater in ponds or lakes and infects freshwater fish, Diphyllobothrium pacificum requires sea water and fish to complete its life cycle. As a consequence, their definitive hosts may vary considerably, including humans, foxes, dogs, bears, seals, and sea lions.
1-6: The scolex (1) of an adult worm attaches by bothria (sucking grooves) to the wall of the small intestine. Mature segments (2) deposit eggs in the gut lumen that are passed in stool (3). Eggs that reach a freshwater pond hatch after a period of development, releasing the ciliated coracidium (4), which develops in the first intermediate (copepod) host into the procercoid (5). Fish—often minnows—feed on the copepods and digest the procercoids free. The procercoids penetrate the gut, pass to the fish musculature, and mature into a nonencysted plerocercoid (6) capable of passing from the gut of one transport fish host to the flesh of a larger piscivorous host. The final transfer occurs when a human or other piscivorous mammal feeds on the infected fish and digests the plerocercoid free. The young worm attaches by its scolex and grows into an adult tapeworm, often 8 m or more in length and up to 2 cm in breadth. (Reproduced with permission from Goldsmith R, Heyneman D: Tropical Medicine and Parasitology. New York: The McGraw-Hill Companies, Inc; 1989.)