Tapeworms are parasites that humans can pick up from undercooked pork, beef, or fish containing the worms' eggs or larvae. The condition can also follow ingesting food or water contaminated with feces from an animal – or human – with a tapeworm infection. The worms fasten on to intestinal walls where they grow and produce more eggs. A tapeworm can live for as long as 20 years in the human intestinal tract, and pork and beef tapeworms can grow 15 to 30 feet long. You can have a tapeworm without knowing it, but when symptoms do occur, they include nausea, weakness, loss of appetite, abdominal pain, diarrhea, and weight loss. Infected individuals may not be able to absorb adequate nutrients from food; the fish tapeworm often causes anemia by using up vitamin B12, which is needed for red blood cells to mature. If tapeworm larvae travel beyond the intestines to invade tissues elsewhere in the body, they can cause more serious problems: seizures, fever, and allergic reactions, depending on where they lodge.
In the United States, tapeworm infections are rare these days. (On the other hand, because so many cases are symptomless, they're seldom reported, so incidence may be higher than we think.) The U.S. Centers for Disease Control and Prevention reported in 2009 that tapeworm incidence has been rising in Japan and in Europe due to infected wild Pacific salmon eaten as sushi or sashimi. (In the United States, the FDA requires that fish to be served raw remain frozen for seven days at a temperature of minus four degrees Fahrenheit or for 15 hours at a temperature of minus 31 degrees Fahrenheit in a blast freezer. Freezing kills parasitic worms and their larvae.)
You can pick up tapeworm infection by eating raw or undercooked beef or pork, by traveling to developing countries with poor sanitation, by exposure to livestock (or human) feces that aren't disposed of properly, or as a result of poor personal hygiene (not washing your hands carefully after handling food, using the toilet, or before eating).
Tapeworm infections are diagnosed by lab tests of a stool sample. (You might see tapeworm segments in your stool, but this is unlikely.) Invasive tapeworm infections can also be diagnosed via blood tests for antibodies.
In many cases there are no symptoms and the tapeworm will leave the body of its own accord sooner or later. Otherwise, treatment usually is with a single dose of an oral medication known as praziquantel that destroys the tapeworm. If the infection has traveled beyond the intestines, treatment will depend on where it is and what kind of trouble it is causing. After treatment, you'll have to have your stool rechecked to make sure that the worm, its eggs, and larvae are gone.
In addition to handwashing, be sure to wash and cook all fruits and vegetables in safe water if you're traveling or living in an area where tapeworms are common, be sure to cook meat at 125 degrees Fahrenheit (52 C) to kill tapeworm eggs and larvae, and avoid undercooked pork, beef, and fish. You can kill tapeworm eggs and larvae if you freeze meat and fish for at least 24 hours before cooking and eating. Make sure that pet dogs with tapeworm get prompt, proper treatment.
Andrew Weil, M.D.