Diphyllobothriasis is an infectious disease that is caused by the largest tapeworm that impacts humans. The broad tapeworm is called Diphyllobothrium latum. There are other species of Diphyllobothrium that cause disease in humans, albeit less commonly. Examples of such species are: D. yonagoensis, D. ursi, D. pacificum, D. lanceolatum, D. cordatum, D. dalliae, and D. dendriticum.

The life cycle of the Diphyllobothrium begins when eggs are expelled in the feces from the carrier. In less than a month, the eggs develop and oncospheres are formed which mature into the coracidia form. The coracidia is then consumed by a freshwater crustacean. In the first intermediate host, the coracidia mature into procercoid larvae. When the freshwater crustacean is consumed by a second intermediate host, usually a small fish found in freshwater, the procercoid larvae travel into the fish flesh and develop into a plerocercoid larvae, also called sparganum.

It is at the stage of the plerocercoid larvae that the Diphyllobothrium are infectious to humans. When small freshwater fish carrying the plerocercoid larvae are consumed by larger fish, humans consume the larger fish such as trout, pike, and perch. Inside the human host, the plerocercoid larvae mature into young adults, and then reside in the small intestine of the human body as fully matured tapeworms.

The D. latum in particular attaches itself to the mucosa of the intestine using bilateral groves on their scolex. The adult form may have over three thousand proglottids. The proglottids discharge immature eggs which are passed through the feces, beginning the cycle again. Each worm can produce up to a million eggs a day.

Diphyllobothriasis has been recorded all over the Northern Hemisphere, including North America, parts of Asia, Europe, Chile, and Uganda. Import of fresh water fish from these areas is a cause for concern.

While most cases of Diphyllobothriasis do not have symptoms, the disease can last for several decades. Symptoms may include weight loss, diarrhea, abdominal pain, and vomiting. Less common symptoms include pernicious anaemia and a deficiency of the vitamin B12. In severe cases, there may be intestinal obstruction and cholangitis or cholecystitis.

Diagnosis of Diphyllobothriasis requires stool samples. Eggs can be identified via microscopy and comparison may be necessary with various intestinal parasites. Proglottids found in the stool may also help with diagnosis of the disease. Treatment requires a prescription drug called Praziquantel. Niclosamide is another option.

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