The fish tapeworm is a broad, long worm, often growing to lengths of 3-7 feet at maturity and capable of attaining 30 feet. It is the longest tapeworm invading humans with as many as 4,000 segments (proglottids). The main body of the worm is virtually filled with male and female reproductive organs allowing it to produce an incredible number of eggs, often more than 1,000,000 a day. The adult attaches to the wall of the intestine with the aid of two sucking grooves located in its head (scolex).

This tapeworm is sometimes called a broad fish tapeworm, because the reproductive segments are usually broader than they are long. The adult is ivory or grayish-yellow in color and can live in humans for 20 years. Humans are the final host of this worm, but first it must pass through a tiny freshwater crustacean, and then to a fish. The larva that infects people, a 'plerocercoid', is frequently found in the intestines of freshwater and marine fish. It is sometimes found in the flesh of freshwater fish or in fish that are migrating from salt waters to fresh water for breeding.

You can be infected by eating raw, lightly cooked, under-processed freshwater or certain migratory species of salmon, perch, pike, pickerel, and turbot. The popularity of eating raw fish dishes, such as Japanese sushi and sashimi, helps to spread this disease. Cooks who sample their fish dishes before they are properly cooked put themselves at risk of being infected. Fish tapeworms are found wherever humans, bears, and other fish-eating mammals defecate in the same lakes and streams from which this fish are obtained.

Most infected people do not produce any symptoms. During the acute stage of infection, which has its onset about 10 days after eating raw or insufficiently cooked fish, the symptoms may be similar to other tapeworm infections. This includes symptoms such as diarrhea, abdominal discomfort and pain, flatulence, vomiting, nausea, and weakness. Chronic infestations may produce some of the same symptoms or only vague discomforts including fullness in the upper abdomen, water retention, loss of weight, and malnutrition. Some people are constantly hungry because the tapeworms are eating most of the food. There are times when the worm gets so large that it will cause a colon blockage. In some people a severe anemia may develop, because of this tapeworm's ability to consume most of its host's vitamin B12. Folate may be reduced as well. With the anemia that results, neurological symptoms can manifest including numbness, loss of vibration sense, and even some eye symptoms.

Evidence of Diphyllobothrium spp. has been found in 4,000-10,000 year old human remains on the western coast of South America. There is no clear point in time when Diphyllobothrium latum and related species were 'discovered' in humans, but it is clear that diphyllobothriasis has been endemic in human populations for a very long time.

Life Cycle Diagram (Courtesy of the DPD)