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Intestinal Parasites

What is a parasite?

A parasite is an organism, or small animal, that lives in or on and takes its nourishment from another organism. A parasite cannot live by itself. Parasites include fleas, lice, and worms. Parasitic diseases include infections by protozoa (single-cell organisms such as malaria), helminthes (worms), and arthropods (such as scabies).

What is an intestinal parasite?

An intestinal parasite lives in the intestines (guts). Intestinal parasites are usually protozoa (such as Giardia) or worms (such as pinworms or tapeworms) that get into your child’s body and uses the intestine as shelter. The parasite will live in the intestine or other parts of the body and often reproduce. The parasite may or may not cause symptoms or infection.

Parasites are very common around the world. The infections are transmitted in crowded places like day cares. Also, children in developing countries are usually carrying some type of parasite. Poor sanitation and unsafe water increase the risk of catching parasites.

The most common intestinal parasites in children include giardiasis (say: jee-ahr-DIE-uh-sis) and pinworm. Other parasites are common in children born outside of Canada or who spent time living in developing countries where sanitation and hygiene are poor. These include ascariasis, amebiasis, and tapeworm.


Giardiasis is a waterborne disease. Your child can catch the infection if she drinks from a contaminated water source. Children are more likely to catch giardiasis in areas with inadequate water and sanitation facilities or places where many people are in close contact, such as child care centres.

Giardiasis is also known as beaver fever because it may be acquired from drinking untreated lake water. 

Signs and symptoms of giardiasis

While some children may never develop signs or symptoms, they can still carry the parasite and spread it through their stool.

For children who do get sick, symptoms may include:

  • watery diarrhea​

  • soft, greasy stools

  • tiredness

  • stomach cramps

  • bloating

  • nausea

  • severe weight loss

Blood and mucus in stool does not occur in giardiasis. Symptoms usually show about 2 weeks after infection. With treatment, the symptoms can improve in 6 weeks. Most children will never require treatment. 

Causes of giardiasis

Its parasites can be found in rivers, streams, and lakes, or municipal water supplies, swimming pools, and spas. The infection can also be passed on through infected food or person-to-person contact. The parasites can also be found in animal feces.

Potential complications of giardiasis

Severe diarrhea, which is quite rare, can lead to dehydration.

How giardiasis is diagnosed

To find out if your child has giardiasis, your doctor will likely test your child’s stool. Your child may have to repeat this test over a few days to ensure accuracy.

Treating giardiasis

There are some medicines used to treat the parasite. Your child’s doctor will determine which treatments best suit your child’s needs.


Pinworms are tiny, white, thread-like worms that live in the rectum. At night, the worm crawls out of the anus and lays eggs in nearby skin. This can cause a terrible itch. Pinworm can be unpleasant but it does not cause disease.

Pinworm eggs are very persistent. Pinworm eggs can live for up to 2 weeks outside the body, on clothing, bedding, or other objects.

Signs and symptoms of pinworm

In general, children with pinworm do not usually have symptoms. They may get very itchy around the anus, especially at night. They may lose sleep. They may also be cranky from the irritation.

You may or may not see the adult worm around your child’s anus, in underwear or diapers, or in the toilet after going to the bathroom.

Causes of pinworm

Pinworm can spread easily between children. When a child scratches an infected area, she gets eggs on her fingers or under the fingernails. Transmission of the infection happens when the infected child passes the eggs to another child, and the child unknowingly swallows the eggs.

Pinworm can also be transmitted indirectly. Eggs from an infected person can stick to toys, toilet seats, clothes, or bedding. A child will catch the infection when they touch the eggs and then put them in their mouth. Children are more likely to catch pinworms in milder climates and in child care centres.

How pinworms are diagnosed

To diagnose pinworms, the doctor may recommend a tape test to look for pinworm eggs. A tape test can be performed by you or your child’s doctor. Cellophane tape is pressed against the skin around the anus. The tape is then examined under a microscope for pinworm eggs. If doing the test at home, it is best to do it in the morning before your child bathes or goes to the bathroom. Take the tape sample to your child’s doctor.

Another way to test for pinworm is if you see the adult worm around your child's anus, in underwear or diapers, or in the toilet after going to the bathroom. Remember, they are white and only about a centimetre long.

Treating pinworms

If diagnosed, your child’s doctor may prescribe an oral medication. The medicine usually takes about 2 weeks to work. The itching may continue for 1 week.

If one family member is diagnosed with pinworm, other family members should be examined and treated.

Other common parasites in children

Blastocystis hominis

Blastocystis hominis is a tiny parasite that is found in the stools of healthy or children with diarrhea or stomach pain. This infection usually clears up on its own.


Dientamoeba lives in the intestines. It is spread through infected food or water. Dientamoeba does not cause disease.


Ascariasis (say: as-kuh-RI-uh-sis) is a type of roundworm infection. The worms can grow as long as 41 centimetres. This parasite is only serious when the body becomes infested with hundreds of worms. Your child will develop symptoms.


Amebiasis occurs when ameba get into your child’s system and multiply. This infection may not show symptoms. Severe infection, which is rare, will lead to symptoms like fever, diarrhea, jaundice, or weight loss.


A tapeworm infection is caused by contaminated food or water. The swallowed tapeworm eggs move from your child’s intestines to form cysts in body tissues and organs. Eaten tapeworm larvae can grow into adult tapeworms, which live in the intestine.

When to seek medical assistance

See your child’s regular doctor if:

  • your child’s diarrhea or nausea lasts longer than 2 weeks

  • your child becomes dehydrated

Be sure to tell the doctor if your child is in child care, has recently travelled to another country, or may have drunk contaminated water.


The best defence against parasites is good hygiene.

Your child, your family, and your child's day care should adopt these healthy habits:

  • Wash hands frequently, especially after changing diapers, going to the bathroom, and playing outside.

  • Potentially contaminated water should be avoided, or boiled or filtered.

  • Wash bedding regularly.

  • Wash children’s toys with disinfectant.

  • Change underwear regularly.

  • Try not to swallow water while swimming in pools, lakes, or streams.

  • Since pinworm eggs are sensitive to light, open blinds or curtains in bedrooms during the day.

Key points

  • An intestinal parasite will live in the intestine or other parts of the body and often reproduce.

  • Poor sanitation and unsafe water increase the risk of catching intestinal parasites.

  • For children who do get sick, symptoms may include watery diarrhea, tiredness, and nausea.

  • Treatment may include oral medication.

  • The best defence against parasites is good hygiene.

Mark Feldman, MD, FRCPC