Diphyllobothrium latum - Causes of a tapeworm infection

In the UK, a tapeworm infection usually occurs when you eat raw contaminated pork, beef or freshwater fish.

Not all tapeworms are acquired in the same way.

Types of tapeworm

The different types of tapeworm which can infect humans are:

Pork and beef tapeworms

Infection with adult pork or beef tapeworms can be caused by eating raw or undercooked pork or beef that contain tapeworm larvae (newly hatched worms). The larvae grow into adult worms in your intestines (bowel).

In the case of the pork tapeworm, you can:

• swallow the eggs in food or water contaminated with human faeces (stools)
• transfer the eggs to your mouth after contact with an infected person or with contaminated clothing

The eggs then develop into larvae inside your body and invade other areas, such as your muscles and brain. This is why symptoms of a tapeworm larvae infection are different to those of an adult tapeworm infection, which is confined to your intestines.

Pork and beef tapeworms are more commonly found in developing areas of the world such as Africa, the Middle East, Eastern Europe, Southeast Asia, and Central and South America.

Fish tapeworm

Infection with the fish tapeworm can be caused by eating raw or undercooked freshwater fish, such as salmon.

The fish tapeworm is more common in countries where people commonly eat raw fish, such as Eastern Europe, Scandinavian countries and Japan.

Dwarf tapeworm

The eggs of a dwarf tapeworm can pass from one person to another through poor hygiene. You can also re-infect yourself through poor hygiene.

The eggs can hatch, develop into adults and reproduce in your intestines, without leaving your digestive system.

Insects, such as fleas or grain beetles, can also pick up the eggs by eating droppings from infected rats or mice, and pass the eggs onto humans if they are accidentally eaten.

Infection with the dwarf tapeworm usually affects children more than adults. It is also more common where people live in unhygienic conditions, particularly where there are fleas.

Dog tapeworm

People can occasionally be infected with the dog tapeworm. This infection is called hydatid disease.

Read more about the complications of tapeworm infections.

Children can accidentally swallow the eggs of the dog tapeworm after touching dog faeces or through close contact with dogs.

The dog tapeworm is common in Asia, eastern Australia, Africa, Greece, southern Spain, South and North America and Turkey. It can be more common in rural areas, particularly sheep-farming areas.

In the UK, hydatid disease is found mainly in sheep-farming areas such as Herefordshire, mid-Wales and Scotland.

Tapeworm lifecycle

Stage 1 – animal or fish swallows the eggs

Tapeworm eggs are found in the stools (poo) of infected humans. The eggs are swallowed by an animal (usually a pig or cattle) when it:

• eats food or drinks water containing traces of the contaminated faeces
• grazes on soil that contains traces of the contaminated faeces

Sometimes, the eggs can be swallowed by a crustacean that is then eaten by a freshwater fish.

Stage 2 – larvae develop

Once inside the animal or fish, the tapeworm eggs hatch into larvae, which invade the wall of the intestines and are carried in the bloodstream to the muscles, where they form cysts (tiny sacs of larvae).

Stage 3 – cysts are eaten by humans

A human swallows tapeworm cysts when they eat the undercooked meat of the contaminated animal or the raw contaminated fish. The cysts hatch inside the human and develop into adult worms, which attach themselves to the wall of the intestines, grow in length and produce eggs.

In the case of the pork tapeworm, the human may have swallowed the eggs directly, so the cysts would form inside the human body before hatching and growing into adult worms.

The eggs of the adult tapeworms are passed out of the human body in faeces, and the cycle starts again.