Tapeworms are flat parasitic worms that infect human intestines. People are most commonly infected by ingesting the immature form of the worms (called the larvae) in raw or undercooked meat or fish. Once ingested, the larvae travel to the intestine and mature into adult tapeworms. They lay eggs, which are passed in the feces. Common human tapeworms include beef, pork and fish tapeworms. The rat tapeworm, known as the dwarf tapeworm, also sometimes infects humans.
Raw or Undercooked Beef
Beef tapeworms (Taenia saginata) infect humans when raw or undercooked beef that contains tapeworm larvae is ingested. Adult beef tapeworms can grow to a length of 33 feet in the intestine of an infected person. Moving segments of the worm may be seen in the stool, sometimes several feet long. Remarkably, most people infected with beef tapeworms are asymptomatic. Mild abdominal cramping occurs in some patients.
Raw and Undercooked Pork
Pork tapeworm (Taenia solium) infection is caused by the ingestion of raw or undercooked pork infected with larvae. Adult pork tapeworms grow in the intestine, typically reaching a length of 6 to 23 feet. Pork tapeworm infection usually produces no symptoms.
Food Contaminated with Pork Tapeworm Eggs
Ingestion of pork tapeworm eggs--as opposed to the larvae--causes a condition called cysticercosis. Typically, the eggs are accidentally ingested when food is contaminated with the feces of people who have pork tapeworm infection. Cysts containing the larval form of the pork tapeworm can form in any organ. The cysts typically do not cause symptoms unless they form in the brain or spinal cord. Symptoms of central nervous system cysts include seizures, confusion, delirium, localized muscle weakness and meningitis.
Raw and Undercooked Freshwater Fish
Fish tapeworm (Diphyllobothrium latum) is the largest human tapeworm. It is contracted by eating raw or undercooked freshwater fish contaminated with the larval form of the parasite. Notably, viable larvae may be present in dried or smoked fish because it has not been thoroughly cooked. Fish tapeworms can reach 33 feet or more in length. Although many people with fish tapeworm are asymptomatic, weakness and vague abdominal discomfort may occur. Longstanding infection can cause vitamin B12 and folate deficiencies, leading to anemia and peripheral neuropathy (numbness, tingling or weakness in the extremities). Uncommonly, these tapeworms cause intestinal blockage due to their size and the fact that most infected people harbor several worms. Fish tapeworms can live for up to 30 years.
Food Contaminated with Dwarf Tapeworm Eggs
As their name implies, dwarf tapeworms (Hymenolepsis nana) are small tapeworms that range from roughly 1/2 to 1 1/2 inches in length. Humans are infected when dwarf tapeworm eggs are ingested with food contaminated by the feces of infected rodents or people. The infestation is usually heavy with many worms. Common symptoms include abdominal cramps, diarrhea, loss of appetite and dizziness.
References
- "Principles and Practice of Infectious Diseases, Sixth Edition"; Gerald L. Mandell, M.D., et.al., Editors; 2004
- DPDx: Taeniasis
- DPDx: Diphyllobothriasis
- DPDx: Hymenolepiasis
- Merck: Diphyllobothriasis
