Intestinal Parasites

Medically Reviewed By William C. Lloyd III, MD, FACS
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What is an intestinal parasite?

Parasites are organisms that rely on a host for food and nutrients. They live in or on the host at the host’s expense. Intestinal parasites live in the digestive tract in the intestines. Most of the time humans are accidental hosts in the parasite life cycle. In humans, there are two intestinal parasite types:

  • Helminths, which are large organisms you can see with the naked eye. There are nearly 50,000 different helminth species including flatworms and roundworms. Flatworms include flukes and tapeworms. Roundworms include pinworms, which are the most common helminth intestinal parasite in the United States.
  • Protozoa, which are one-celled, microscopic organisms. There are over 10,000 different protozoa species. In the United States, the most common ones include Giardia lamblia, Entamoeba histolytica, Cryptosporidium, and Cyclospora cayetanensis.

Intestinal parasite infection spreads via the oral-fecal route. This includes infection through contaminated water, food, or surfaces such as toilet handles. These parasites can also spread through person-to-person contact, such as changing diapers or other anal or fecal contact.

Intestinal parasites are very common in developing countries. Lack of adequate sanitation in these countries is the main reason for this. In industrialized countries, including the United States, they aren’t as common. However, they can still be problematic in certain populations. For example, experts estimate that about 13% of the American population has pinworms. Children are most likely to have this parasite and pass it to household members and caregivers.

Intestinal parasite symptoms depend on the type of parasite. For example, pinworm infection usually causes very mild symptoms, primarily anal itching. However, protozoa infection can cause severe symptoms, including watery diarrhea, abdominal pain, and nausea.

Intestinal parasite treatment may or may not be necessary. Protozoal infections may go away on their own if symptoms are mild and you are healthy. When symptoms are severe or persistent, medications can help resolve the infection. For helminth infections, anti-worm medications can clear the infection. With all intestinal infections, it’s important to prevent dehydration by drinking plenty of fluids. Preventing reinfection is another vital part of treatment by laundering clothes, sheets and towels.

See your doctor if you have diarrhea or other symptoms that last for more than 2 to 3 days. Seek immediate medical care if have potentially serious symptoms including:

  • Bloody vomitus or blood or pus in your stool
  • Dehydration
  • Diarrhea that does not improve after 24 hours in a child
  • Inability to keep liquids down for 24 hours or vomiting that lasts for more than two days
  • Sudden, severe abdominal pain

What are the symptoms of intestinal parasites?

Intestinal parasites can live in the body and not cause any noticeable problems or very mild symptoms. On the other hand, some intestinal parasite infections can cause symptoms that are quite severe.

Common symptoms of intestinal parasites

The most common helminth infection is pinworm. Pinworm infection is usually mild or it causes no symptoms at all. When symptoms occur, the most common one is anal itching, especially at night. You may also be able to see the worms in the anal area at night. This is when adult worms travel to the anus to lay eggs. Heavy pinworm infection can lead to abdominal pain and nausea.

Protozoal infections more commonly cause intestinal symptoms and other symptoms including:

  • Diarrhea, which may be very watery, foul-smelling, or greasy
  • Fever

See your doctor if these symptoms persist for more than 2 to 3 days without improvement.

Serious symptoms that might indicate a life-threatening condition

When intestinal parasites cause diarrhea, the big danger is dehydration. Dehydration can quickly become serious and even life threatening. Symptoms of dehydration include dark-colored urine, urinating less than normal, excessive thirst, headache, dizziness, fatigue, and skin that remains raised after pinching it. In infants and toddlers, look for dry mouth and tongue, listlessness, irritability, no tears with crying, no wet diapers for three hours, and sunken eyes, cheeks, or soft spots.

Seek immediate medical care (call 911) if you, or someone you are with, have signs of dehydration or any of these serious symptoms including:

  • Diarrhea that persists for more than 2 to 3 days in an adult or 24 hours in a child
  • Fever over 102 degrees Fahrenheit
  • Inability to keep liquids down for 24 hours
  • Severe or sudden abdominal pain

What causes intestinal parasites?

The other type of intestinal parasites in humans is protozoa. In the United States, the most common protozoa infections are Giardia lamblia, Entamoeba histolytica, Cryptosporidium, and Cyclospora cayetanensis. This group can spread through food, water, soil and surfaces contaminated with fecal matter from humans and animals. For example, these organisms can reside in contaminated water of poorly-maintained spas and hot tubs. Some of these parasites form cysts that can live for months outside a host. Ingesting the microscopic cysts or the parasites themselves causes infection. You can also get the giardia parasite through anal sex.

There are two main types of intestinal parasites that infect humans. The first is helminths. In the United States, the most common helminth infection is a roundworm infection with Enterobius vermicularis. The common name for this infection is pinworm. Pinworms only live in humans and spread very easily from person to person. You can get this infection by transferring microscopic eggs to your mouth after touching something contaminated with fecal material. This is oral-fecal transmission. Other types of helminth infections include tapeworms, flukes and hookworms. Helminth infections can be transmitted by contact with certain animals.

What are the risk factors for intestinal parasites?

For pinworm infection, children are the highest risk group. Since it spreads so easily, family members, caretakers, friends and schoolmates of an infected child are also at risk. Their transmission also makes pinworms common in crowded or institutional living situations.

A number of factors increase the risk of contracting a protozoal intestinal infection including:

  • Having oral-anal sexual contact
  • Ingesting contaminated water
  • Sharing utensils, food, or other contaminated objects with an infected person
  • Traveling to areas with poor sanitation

Reducing your risk of intestinal parasites

You may be able to lower your risk of getting a pinworm intestinal parasite by:

  • Changing underwear, pajamas, towels, washcloths and bedding daily, washing them in hot water, and drying them with high heat
  • Keeping fingernails short and clean
  • Not scratching the anal area
  • Showering, instead of bathing, in the morning to wash away eggs laid during the night

You may be able to lower your risk of getting a protozoa intestinal parasite by:

  • Avoiding close contact with an infected person and disinfecting hard surfaces, such as door knobs and faucets
  • Boiling water from unknown or questionable sources before drinking it or drinking only sealed bottled water while traveling, hiking or camping. Brushing your teeth with bottled water is also good prevention.
  • Cooking food thoroughly, keeping hot foods hot and cold foods cold, and avoiding contamination with raw poultry, eggs and meats
  • Ensuring you don’t have fecal contact during sexual activity
  • Using clean utensils to eat food and not sharing food or utensils with an infected person
  • Washing your hands regularly, including after using the bathroom and before preparing or eating food. A good rule of thumb is to sing Happy Birthday twice to ensure enough washing time.

If you will be traveling or possibly coming into contact with sources of intestinal parasites, talk with your doctor beforehand. Find out what specific precautions you need to take.

How are intestinal parasites treated?

For a pinworm infection, there are over-the-counter and prescription antiworm medicines. Treatment requires two doses spaced two weeks apart. In most cases, household members and caretakers will need treatment at the same time as the infected child. This helps prevent reinfection and spreading of the infection. Strict hygiene is necessary until all doses are complete.

Protozoal intestinal infections may or may not require treatment. When the infection is mild, it may go away on its own within a few weeks in healthy people. Severe or persistent infections often require medications, as do infections in people with weakened immune systems. There are several anti-parasitic medicines doctors can use, depending on the specific parasite. If diarrhea is present, it’s also important to maintain adequate hydration to avoid problems.

What are the potential complications of intestinal parasites?

Healthy people are often able to fight off protozoa intestinal parasites. The infection and diarrhea usually resolves within a few weeks when the immune system is strong. However, people with weakened immune systems can develop life-threatening complications from intestinal parasites. This includes severe dehydration, malnutrition, dangerous weight loss, and spread to other areas of the body, such as the respiratory tract.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2020 Dec 1
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THIS TOOL DOES NOT PROVIDE MEDICAL ADVICE. It is intended for informational purposes only. It is not a substitute for professional medical advice, diagnosis or treatment. Never ignore professional medical advice in seeking treatment because of something you have read on the site. If you think you may have a medical emergency, immediately call your doctor or dial 911.
  1. Pinworm Infections. Nemours Foundation.
  2. About Parasites. Centers for Disease Control and Prevention.
  3. Cryptosporidium. Centers for Disease Control and Prevention.
  4. Cryptosporidium Infection. Mayo Foundation for Medical Education and Research.
  5. Giardia. Centers for Disease Control and Prevention.
  6. Giardia Infection (Giardiasis). Mayo Foundation for Medical Education and Research.
  7. Haque R. Human intestinal parasites. J Health Popul Nutr. 2007;25(4):387-391.
  8. Kucik CJ, Martin GL, Sortor BV. Common intestinal parasites. Am Fam Physician. 2004 Mar 1;69(5):1161-1169.
  9. Ova and Parasite Test. U.S. National Library of Medicine.
  10. Overview of Parasitic Infections. Merck Manual Consumer Version.
  11. Pinworm Infection. Mayo Foundation for Medical Education and Research.
  12. Pinworm Infection FAQs. Centers for Disease Control and Prevention.