Tapeworm infection of the intestine occurs mainly when people eat raw or undercooked contaminated pork, beef, or freshwater fish or, for the dwarf tapeworm, contaminated food or water.
A Beef Tapeworm
Adult tapeworms, which live in people's intestine, usually cause no symptoms, but they may cause abdominal discomfort, diarrhea, and weight loss.
The pork tapeworm can also form cysts in the brain and elsewhere in the body (called cysticercosis).
Cysts in the brain can cause various symptoms, such as headaches, seizures, confusion, and sometimes severe, life-threatening disease.
Doctors diagnose intestinal tapeworm infection by finding worm segments or eggs in a stool sample, and they diagnose cysticercosis by identifying cysts elsewhere in the body by doing imaging or blood tests.
Thoroughly cooking pork, beef, and freshwater fish can help prevent the infection.
(See also Overview of Parasitic Infections.)
Several species of tapeworms can cause infection in people. They include
Taenia saginata (beef tapeworm)
Taenia solium (pork tapeworm)
Taenia asiatica, which is acquired by eating pork in Asia (Asian tapeworm)
Diphyllobothrium latum (fish tapeworm)
Hymenolepis nana (dwarf tapeworm)
Image courtesy of Dr. Mae Melvin via the Centers for Disease Control and Prevention Public Health Image Library.
Adult tapeworms of other species—Echinococcus granulosus and Echinococcus multilocularis (dog tapeworms)—live in the intestine of dogs or other canines. These tapeworms sometimes infect people, causing cysts in the liver or other organs.
Tapeworms have three main parts:
The head, which is the part that attaches to intestine
The neck, which can regenerate
The rest of the worm, which consists of many segments (called proglottids) and which may contain eggs
If treatment does not eliminate the head and neck, the entire worm may re-form.
(From GC Cook, Manson's Tropical Diseases, 20 ed, 1996, WB Saunders, Fig. 74.11; with permission.)
Pork, beef, and fish tapeworm life cycle
The pork, beef, and fish tapeworms are large, flat, ribbon-like worms that live in the intestine of people and can grow 15 to 30 feet (4.5 to 9 meters) in length. People are considered the definitive host because adult tapeworms live in their intestine. Egg-bearing sections of the worm (proglottids) are passed in the stool.
If untreated human waste is released into the environment, the eggs may be ingested by intermediate hosts, such as pigs, cattle, or, in the case of fish tapeworms, small freshwater crustaceans, which are in turn ingested by fish. The eggs hatch into larvae in the intermediate host. The larvae invade the intestinal wall and are carried through the bloodstream to skeletal muscle and other tissues, where they form cysts.
People acquire the parasite by eating the cysts in raw or undercooked meat or certain types of freshwater fish. The cysts hatch and develop into adult worms, which latch onto the wall of the intestine. The worms then grow in length and begin producing eggs.
Life Cycle of the Pork Tapeworm
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Dwarf tapeworm life cycle
Dwarf tapeworms are small, only up to 2 inches (40 millimeters) long. People become infected with dwarf tapeworms by consuming tapeworm eggs, rather than cysts, as occurs in pork and beef tapeworm infection. The eggs may be consumed in food or water contaminated by human stool or may be transferred to the mouth after contact with infected people. Sometimes people accidentally consume infected insects, such as fleas and beetles, in grains.
The eggs develop into adults that live in the intestine. The adult tapeworms lay eggs that are passed in stool. The eggs may be consumed by another person or by the same person, resulting in infection with a new generation of adult tapeworms.
Sometimes the eggs hatch in the intestine and develop into adult dwarf tapeworms, causing infection without ever leaving the body. This type of infection is called autoinfection (infection of self). A large number of dwarf tapeworms can accumulate and cause symptoms.
Cysticercosis
People who ingest the eggs of the pork tapeworm can become an intermediate host for the tapeworm (people cannot be intermediate hosts for the beef and fish tapeworms). People may become an intermediate host when one of the following occurs:
They swallow the pork tapeworm eggs in food or water contaminated with human stool.
They transfer the eggs to their mouth after contact with an infected person or with contaminated clothing and furniture.
People with an adult worm in their intestine may reinfect themselves when they swallow eggs from their stool (for example, in contaminated food or water) or possibly when egg-bearing segments of the worm (proglottids) move from their intestine into the stomach and release eggs (autoinfection).
As in animal intermediate hosts, the eggs develop into spheres containing larvae (oncospheres) when they reach the intestine. The spheres penetrate the intestinal wall and travel to the brain, muscles, other organs, or tissue under the skin, where they form cysts. In people, this form of the disease is called cysticercosis.
Symptoms of Tapeworm Infection
Although adult tapeworms in the intestine usually cause no symptoms, some people experience upper abdominal discomfort, diarrhea, and other symptoms. Occasionally, people with a tapeworm can feel a piece of the worm move out through the anus or see part of the ribbon-like tapeworm in stool. The dwarf tapeworm is more likely than other tapeworms to cause abdominal symptoms such as nausea, vomiting, diarrhea, abdominal discomfort, loss of appetite, and weight loss.
The fish tapeworm can cause anemia because it absorbs vitamin B12, which is necessary for red blood cells to be produced.
Cysticercosis symptoms are caused by cysts that form in the brain and other organs (such as the spinal cord, liver, lungs, and eyes). These cysts can cause severe, sometimes life-threatening symptoms years after the initial infection, when the cysts start to deteriorate and cause inflammation. Cysts in the brain and the tissues covering the brain (meninges) can result in headaches, seizures, confusion, or other neurologic symptoms. Rarely, cysts develop in the eyes, sometimes causing blindness, or in the spinal cord, sometimes causing muscle weakness or paralysis.
Diagnosis of Tapeworm Infection
For intestinal tapeworm infection, examination of a stool sample
To check for cysticercosis, computed tomography or magnetic resonance imaging and sometimes blood tests
A doctor diagnoses intestinal tapeworm infection by finding worm segments or eggs in a stool sample.
In people with cysticercosis, cysts in the brain or other tissues can be seen using computed tomography (CT) or magnetic resonance imaging (MRI). Blood tests for antibodies to the pork tapeworm may also be helpful. (Antibodies are proteins produced by the immune system to help defend the body against attack, including that by parasites.) Sometimes hardened cysts can be felt under the skin.
Treatment of Tapeworm Infection
For symptoms due to cysticercosis, sometimes corticosteroids and an antiparasitic medication with or without surgery
Cysticercosis treatment
Cysticercosis treatment depends on various factors, such as symptoms and the number and location of cysts in the brain.
Antiparasitic medications are not used to treat cysts in the eye or spinal cord because they can trigger severe inflammation that can damage nearby tissues.
Occasionally, surgery is necessary—for example, when cysts are blocking the flow of fluid that surrounds the brain and spinal cord (cerebrospinal fluid) or when cysts are causing problems in the spinal cord or eye.
Prevention of Tapeworm Infection
The first line of defense against tapeworms is
Thoroughly cooking whole cuts of meat and freshwater fish to a temperature of more than 145° F (63° C) for 3 minutes or more.
If the meat is ground, a cooking temperature of at least 160° F (71° C) is recommended.
Whole cuts of meat should rest for 3 minutes after being cooked before carving or consuming. Ground meats do not require a rest period.
Freshwater fish should not be served raw (as sushi) and should be eaten only after it has been cooked to a temperature of 145° F (63° C) or properly frozen at temperatures below that of typical home freezers. These measures kill the fish tapeworm.
The following are recommended freezing methods for freshwater fish:
At -4° F (-20° C) or below for 7 days
At -31° F (-35° C) or below until solid, then stored at -31° F (-35° C) or below for 15 hours
At -31°F (-35° C) or below until solid, then stored at -4° F (-20° C) or below for 24 hours
Smoking and drying do not kill cysts.
Another line of defense is
Careful evaluation of meat and fish by trained inspectors
Cysts are visible in infected meat.
Adequate treatment of human waste interrupts the life cycle and thus helps prevent infections with beef tapeworms or pork tapeworms, including cysticercosis.
Dwarf tapeworm infection can be prevented by avoiding the following:
Food and water likely to be contaminated with stool
Grains contaminated by infected insects
For example, when traveling in countries where food is likely to be contaminated with human stool, people should wash, peel, and/or cook all raw vegetables and fruits using safe water before these foods are eaten.
Washing hands with soap and warm water after using the toilet, after changing diapers, and before preparing foods can also help.
