Intestinal Parasites

Intestinal parasites * are organisms that live in the gastrointestinal * tract of animals, including humans. They can cause diarrhea (dye-uh-REE-uh) and other symptoms.


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Amebic dysentery







What Are Intestinal Parasites?

In humans, three types of intestinal parasites may live in the small and large intestines: tapeworms, roundworms (or nematodes, NEE-muh-todes), and protozoa (pro-tuh-ZOH-uh). Certain types remain in the intestines; others travel outside the intestines to invade other organs. Some are so small they can only be seen under a microscope; others can be many feet long. Most tapeworms and roundworms develop in the human body and lay their eggs there. The eggs then pass out of the body through feces (FEE-seez, or bowel movements) and can infest others.

Intestinal parasites exist throughout the world. The World Health Organization (WHO) estimates that 3.5 billion people worldwide are infested with some type of intestinal parasite, and as many as 450 million of them are sick as a result. Children are most frequently infected with these parasites.

Intestinal parasites spread in areas with poor sanitation and are most common in tropical developing countries on the African, Asian, and South American continents. They are not a large problem in the United States, and Americans are most likely to get intestinal parasites when they travel to remote areas.

* parasites (PAIR-uh-sites) are organisms such as protozoa (one-celled animals), worms, or insects that must live on or inside a human or other organism to survive. An animal or plant harboring a parasite is called its host. Parasites live at the expense of the host and may cause illness.

* gastrointestinal (gas-tro-in-TES-tih-nuhl) means having to do with the organs of the digestive system, the system that processes food. It includes the mouth, esophagus, stomach, intestines, colon, and rectum and other organs involved in digestion, including the liver and pancreas.

How Are Intestinal Parasites Spread?

Intestinal parasites can be acquired in many ways. Some parasites can live in the soil for extended periods. They may penetrate the body through the skin or if contaminated soil is ingested accidentally. Other parasites live in animals, such as pigs and cows. People can become infested with these by eating undercooked meat or drinking unpasteurized milk (milk that has not been processed with heat to kill parasites and bacteria).

Both public water supplies and natural water sources can become contaminated with human or animal waste (mainly from dogs and beavers) harboring the parasite (shown here) that causes giardiasis. The disease causes stomach upset and diarrhea when the parasite attaches itself to the lining of the digestive system, where it interferes with the body's ability to absorb fats and carbohydrates. Custom Medical Stock Photo, Inc.
Both public water supplies and natural water sources can become contaminated with human or animal waste (mainly from dogs and beavers) harboring the parasite (shown here) that causes giardiasis. The disease causes stomach upset and diarrhea when the parasite attaches itself to the lining of the digestive system, where it interferes with the body's ability to absorb fats and carbohydrates.
Custom Medical Stock Photo, Inc.

The eggs of some intestinal parasites pass through an infested person's gastrointestinal tract and into feces. The parasite then can spread to other people through unintentional contact with the feces. Depending on the type of parasite, a person may become infested by touching his or her mouth after contact with feces that contain the organism (when changing a diaper or doing laundry, for example) or a contaminated area. Parasites can spread when a person eats contaminated food (such as unwashed raw fruits or vegetables, which can carry parasites from the soil or from people who have handled them) or drinks water contaminated by feces. Swimming in contaminated water also may result in infestation by certain parasites.

Parasitic intestinal infestations * often occur in outbreaks, when several people have symptoms at the same time. This is especially likely if many people come into contact with the same supply of contaminated food or water.

What Are Some Common Intestinal Parasites?


Ascariasis (as-kuh-RYE-uh-sis) is caused by Ascaris lumbricoides, an intestinal roundworm. It is one of the most common intestinal parasites, affecting people in all parts of the world, especially in areas with poor sanitation. In the United States, ascariasis is rare, but it occurs most frequently in the rural parts of the Southeast. The worm also can infest pigs.

The life cycle of Ascaris lumbricoides begins when an adult worm lays its eggs in the intestines of an infected person. The eggs leave the body through the feces and can live in soil for up to 2 years. When people eat raw food containing this contaminated soil, they may swallow the worm's eggs, which hatch in the stomach as larvae (LAR-vee, or immature worms). The larvae migrate through the blood to the lungs and then to the throat, where they are swallowed. Eventually, they pass into the intestines, where they begin the cycle again. The adult worms, which can grow to be more than 12 inches long, can live 1 to 2 years in the small intestine * . Ascariasis is not contagious, and a person can become infested only by ingesting the worm's eggs.

Ascariasis usually causes no symptoms or only mild stomachaches or bloating. If a person is heavily infested, he or she may experience more severe pain. Some people also may have a cough or breathing problems when the larvae move through their lungs.

* infestations refer to illnesses caused by multi-celled parasitic organisms, such as tapeworms, roundworms, or protozoa.

* small intestine is the part of the intestine between the stomach and large intestine.

People often discover they have ascariasis when a worm passes in their bowel movements, or when they cough up a worm or it crawls out through the nose. This can be frightening, but the ascaris worm usually does not cause permanent damage to the body. Because of the relatively large size of adult ascaris worms, they can partially block the intestinal tract as well as the ducts leading from the biliary tract * and pancreas * . In rare cases, surgery may be needed to remove them.


Strongyloidiasis (stron-juh-loy-DYE-uh-sis) is caused by another roundworm, Strongyloides stercoralis. This common infestation can be especially dangerous in people with weakened immune systems. If a person comes into contact with contaminated soil, the larva of the parasite can burrow through the skin. It travels to the lungs and then, in a manner similar to ascaris, is swallowed and ends up in the intestines, where the worm grows to adulthood and begins laying eggs. What is special about this parasite is that the eggs can hatch inside the intestines and the worms can continue to cycle through many generations (called the autoinfective cycle), causing an infestation that can last for decades.

In people with weakened immune systems, particularly those taking drugs such as corticosteroids * , strongyloidiasis can become overwhelming, and huge numbers of larvae can invade the lungs and other organs. This problem is called the hyperinfection syndrome and, although rare, it can be fatal.


Giardiasis (jee-ar-DYE-uh-sis) is the most common waterborne parasitic infection in the United States. Caused by Giardia in testinalis, a single-cell protozoan (also known as Giardia lamblia), this infection can lead to diarrhea, cramping, and an upset stomach.

Giardia intestinalis lives in humans and animals. People become infected by drinking or swimming in contaminated water or by touching the feces of an infected person, or a contaminated surface, and then their mouths. People can spread the parasite if they do not wash their hands properly. Giardiasis occurs most frequently in settings where contaminated feces can be spread easily, such as in children in diapers, especially those in daycare centers, and in people who live in institutional settings such as nursing homes. Some people who are infected do not become sick but still can pass the infection on to others.

In people who do develop symptoms, stomach pain and watery diarrhea usually start 1 to 2 weeks after infection. About half the people who are infected also lose weight. The illness lasts 2 to 6 weeks, or longer in people who are sick with another disease.


Hookworms (a type of roundworm) are another common intestinal parasite. The U.S. Centers for Disease Control and Prevention (CDC) estimates that 1 billion people worldwide have hookworm infestations, although improved sanitation has reduced the number of cases in the United States.

* biliary (BIH-lee-ah-ree) tract refers to the organs and ducts, including the liver and gallbladder, that produce, store, and transport bile, a substance which aids in digestion.

* pancreas (PAN-kree-us) is a gland located behind the stomach that produces enzymes and hormones necessary for digestion and metabolism.

* corticosteroids (kor-tih-ko-STIR-oyds) are chemical substances made by the adrenal glands that have several functions in the body, including maintaining blood pressure during stress and controlling inflammation. They can also be given to people as medication to treat certain illnesses. People being treated with corticosteroid medication, particularly with high doses, may have a reduced ability to fight certain infections.

Two species, Ancylostoma duodenale and Necator americanus, infest humans. The worms' eggs hatch into larvae in warm, moist soil. Hookworms can penetrate human skin, so many people become infested when they walk barefoot in or touch contaminated soil. They also can become infested when they eat such soil (on unwashed raw fruit or vegetables, for example). The hookworm larvae travel to the lungs via the bloodstream; the larvae then travel to the throat and are swallowed, in a similar fashion to the ascaris worm. When they reach the small intestine, the larvae latch onto the intestinal walls and suck blood. There they mature and eventually lay eggs, which pass out of the body in feces. Hookworms can live for 1 to 2 years in the body.

A rash or itching at the site where the larvae entered the skin may signal hookworm infestation, followed by mild cramping and diarrhea. Heavily infested people may lose their appetite, lose weight, and have abdominal * pain. Hookworms can cause serious problems, including malnutrition and anemia * from intestinal bleeding. Newborns, young children, pregnant women, and malnourished people are most susceptible to these complications.

Dogs and cats sometimes carry their own types of hookworms (Ancylostoma ceylanicum and Ancylostoma braziliense) , and these occasionally infest humans who come into contact with soil contaminated with cat or dog feces. In this type of infestation, called cutaneous larva migrans (kyoo-TAY-nee-us LAR-vuh MY-granz) or creeping eruption, the worm larvae burrow into the skin and cause severe itching but do not invade deeper into the body. The condition resolves without treatment after several weeks or months.


Amebiasis (ah-mih-BYE-uh-sis) is caused by a single-cell parasite called Entamoeba histolytica. It occurs mainly in areas with poor sanitary conditions. Cases in the United States usually are seen in people who have recently arrived from or traveled in remote areas.

Amebiasis spreads when people touch infected feces or contaminated surfaces and then touch their mouths, or when they eat or drink contaminated food or water. It also can spread through certain types of sexual contact. Symptoms such as mild diarrhea and stomach pain may occur 1 to 4 weeks after infection, but only 1 infected person in 10 becomes sick and develops symptoms.

Amebic dysentery (uh-ME-bik DIH-sen-ter-e), a more severe form of the illness, causes bloody diarrhea, extreme stomach pain, and fever. Rarely, the infection spreads to other body organs, particularly the liver * , where the parasite can form large abscesses * . Because of the risk of amebic dysentery, Entamoeba histolytica is one of the most dangerous intestinal parasites, and infection with it can be fatal.

Other forms of amebas, including Entamoeba coli, Entamoeba dispar, and Entamoeba hartmanni, infect humans but cause no illness. These amebas can live in the human body for months or years without causing problems.

* abdominal (ab-DAH-mih-nul) refers to the area of the body below the ribs and above the hips that contains the stomach, intestines, and other organs.

* anemia (uh-NEE-me-uh) is a blood condition in which there is a decreased amount of oxygen-carrying hemoglobin in the blood and, usually, fewer than normal numbers of red blood cells.

* liver is a large organ located beneath the ribs on the right side of the body. The liver performs numerous digestive and chemical functions essential for health.

* abscesses (AB-seh-sez) are localized or walled off accumulations of pus caused by infection that can occur anywhere within the body.

Cyclosporiasis and Cryptosporidiosis

Scientists identified cyclosporiasis (sy-klo-spoh-RYE-uh-sis), caused by the protozoan Cyclospora cayetanensis, in 1979. The infection is found worldwide, most

Two species of hookworm, Ancylostoma duodenale (left) and Necator americanus (right), can infect humans. Custom Medical Stock Photo, Inc.
Two species of hookworm, Ancylostoma duodenale (left) and Necator americanus (right), can infect humans.
Custom Medical Stock Photo, Inc.
frequently in developing countries, although there have been outbreaks in the United States and Canada.

Because Cyclospora cayetanensis must spend some time outside the human body to become infectious, it is not contagious from person to person. Infection usually results from ingesting contaminated water or soil or fresh produce grown in them.

Symptoms, which appear 1 week after infection and last from 1 to several weeks, may include diarrhea with frequent, watery, and sometimes explosive bowel movements; loss of appetite; stomach cramps; bloating; nausea (NAW-zee-uh); fever; vomiting; and weight loss. The diagnosis can be made by examining a sample of the patient's bowel movements under a microscope to view the organism. If the illness is not treated, its symptoms may return.

Cryptosporidiosis (krip-toh-spor-id-e-O-sis) is an intestinal infection with symptoms similar to cyclosporiasis caused by the protozoan Cryptosporidium parvum that can live in people and animals. People can pick up the parasite through person-to-person contact or through water contaminated by the feces of infected animals.

Initially, it was thought that only people with weak immune systems, such as those with AIDS, contracted the infection. It is now known that the organism can infect people with normal immune systems and that cryptosporidiosis is one of the most common causes of protozoal diarrhea in the world. The infection goes away on its own in most people, but antibiotics and other treatments may be necessary for people with weak immune systems who contract cryptosporidiosis.


Enterobiasis (en-tuh-roh-BY-uh-sis), also known as pinworm infestation, is caused by a staple-size worm known as Enterobius vermicularis. It is the most common worm infestation in the United States and is found primarily in children. Outbreaks of pinworm often occur in schools and daycare centers. From there, infested children may spread the worms to their family members.

Enterobius vermicularis lives in the rectum, the last part of the large intestine * , and comes out at night to lay eggs on the perineum (per-ih-NEE-um), the area around the anus and genitals. These eggs become contagious in a few hours and can spread to sheets and clothing, where they can remain contagious for about 2 weeks. Infestation occurs when people touch a contaminated area and then their mouths.

Itching of the perineum is the most common symptom of pinworm. This can lead to sleeplessness and irritability. Frequently, however, people show no signs of infestation.

Human tapeworm

Human tapeworm infestations usually are caused by eating meat or fish contaminated with worm larvae. Like other intestinal parasites, these worms frequently cause infestations in areas with poor sanitation, where livestock animals are exposed to contaminated soil or fish to contaminated water.

There are three common species of tapeworms: Taenia solium (pork tapeworm), Taenia saginata (beef tapeworm), and Diphyllobothrium latum (fish tapeworm). After someone eats contaminated meat or fish, tapeworm larvae travel to the intestines, where they latch onto the lining of the intestines and gradually grow into adults. The largest tapeworms can reach amazing sizes, measuring more than 20 feet long in some cases. The worms shed their eggs into the feces, from which they find their way into soil and water and are ingested by animals or fish. Humans ingest the larvae when they eat the contaminated meat or fish. Symptoms of a tapeworm infestation are often mild or nonexistent but can include abdominal pain, diarrhea, and malnutrition.

Two other diseases in humans can be traced to tapeworms that usually infest animals. In echinococcosis (ih-kye-nih-kah-KO-sis), large cysts * can develop in the liver, lungs, and other organs; in cysticercosis (sis-tuhsir-KO-sis), the parasites can invade the muscles, brain, and eyes. Both echinococcosis and cysticercosis can occur when people eat food contaminated with the eggs of tapeworms that are found in the droppings of certain animals.


Trichinosis (trih-kih-NO-sis) arises from several varieties of Trichinella roundworms. Although once very common, it is now relatively rare in the United States, with the CDC reporting an average of just 38 cases per year. Trichinosis is more common in developing countries, however.

* large intestine is the part of the intestine that contains the colon and rectum.

* cysts (SISTS) are shell-like enclosures that contain small organisms in a resting stage.

Trichinella larvae live in cysts in pigs and wild animals. When people eat their meat raw or undercooked, the cysts travel to the stomach, where acid dissolves the walls of the cysts and releases the immature worms. They move to the small intestine, mature, and lay eggs. Once the eggs hatch, the worms travel through the bloodstream to muscles, where they burrow in, forming cysts. This ends the cycle in humans.

The first symptoms of trichinosis, which include stomach pain, extreme tiredness, nausea, diarrhea, vomiting, and fever, appear 1 or 2 days after people eat infested meat. Headaches, chills, swelling of the eyes, cough, muscle aches and pains, and constipation (infrequent bowel movements) may follow. People with severe infestations also may have heart problems or trouble breathing.

How Are Intestinal Parasitic Diseases Diagnosed?

Doctors use samples of feces, sometimes taken a day or two apart, to diagnose intestinal parasitic diseases. The feces are examined for evidence of parasites, such as eggs, larvae, or adults. Blood samples can be taken to check for antibodies * to specific parasites, and doctors may use a medical instrument called an endoscope * to examine the intestines for infection.

To detect pinworms, doctors often request that patients take a "tape test." For this test, patients briefly apply a piece of transparent tape to the skin around the anus in the early morning, after the worm has laid its eggs. The tape is removed and examined at the doctor's office for any eggs that might be sticking to it.

How Are Intestinal Parasitic Diseases Treated?

Some cases require little or no treatment, and the parasites eventually disappear on their own. People with diarrhea and other signs of intestinal parasitic disease should talk to a doctor if their symptoms last more than a few days.

Medication used to treat the illnesses varies with the type of infection. Doctors may use antibiotics or antiparasitic medicines. In most cases, patients can remain at home and maintain a normal schedule. Children must stay out of daycare until they have been treated adequately and can no longer spread the infection. While they recover, patients are advised to drink plenty of fluids to avoid dehydration * . Antidiarrhea medicine is not recommended because it may keep the parasites in the body longer. More severe cases may require treatment in the hospital.

In most cases, patients who have symptoms should feel better within 1 to 2 weeks, although it may be several more weeks before their bowel movements are completely back to normal.

* antibodies (AN-tih-bah-deez) are protein molecules produced by the body's immune system to help fight specific infections caused by microorganisms, such as bacteria and viruses.

* endoscope (EN-doh-skope) is a tool for looking inside parts of the body. It consists of a lighted tube and optical fibers and/or lenses.

* dehydration (dee-hi-DRAY-shun) is a condition in which the body is depleted of water, usually caused by excessive and unreplaced loss of body fluids, such as through sweating, vomiting, or diarrhea.

Can Intestinal Parasitic Diseases Cause
Medical Complications?

Dehydration is the most common general complication of intestinal parasite infections. Infants and young children are particularly vulnerable to dehydration and nutrition problems when they become infected. In

Intestinal Parasites
people with weak immune systems (such as people undergoing chemotherapy), infants, and the elderly, these infections can be fatal.

Symptoms of late-stage hookworm infestation are an enlarged abdomen and diarrhea. Worms can live up to 15 years in the human body, and females can lay 10,000 to 25,000 eggs every day. In severe cases the number of parasites may grow so large that the intestines become blocked. Photo Researchers, Inc.
Symptoms of late-stage hookworm infestation are an enlarged abdomen and diarrhea. Worms can live up to 15 years in the human body, and females can lay 10,000 to 25,000 eggs every day. In severe cases the number of parasites may grow so large that the intestines become blocked.
Photo Researchers, Inc.

Some infections cause specific complications: amebiasis can affect the liver, lungs, and brain; parasites migrating through the lungs may cause difficulty breathing; and hookworm infestation can cause anemia and malnutrition, which can affect growth and development in children.

How Can Infection with Intestinal Parasites
Be Prevented?

Good hygiene is the best defense against intestinal parasites. This includes frequent and thorough hand washing, especially after changing diapers, after going to the bathroom, and before handling food.

Doctors advise that travelers to undeveloped countries drink and brush their teeth with bottled water and avoid eating raw fruits and vegetables, food from street vendors, and unpasteurized dairy products. In addition, cooking all food until it is steaming hot kills parasites. Always wearing shoes and avoiding swimming in bodies of fresh water such as ponds, rivers, and lakes can minimize the risk of contact with contaminated soil and water.

See also
Intestinal Infections
Pinworm Infestation



U.S. Centers for Disease Control and Prevention (CDC), 1600 Clifton Road, Atlanta, GA 30333. The CDC is the U.S. government authority for information about infectious and other diseases. It has fact sheets for the most common types of intestinal parasite infestations at its website.
Telephone 800-311-3435

U.S. Food and Drug Administration, Center for Food Safety and Applied Nutrition (CFSAN), 5100 Paint Branch Parkway, College Park, MD 20740. CFSAN has an online "Bad Bug Book" that gives facts and figures on many foodborne parasites and illnesses.
Telephone 888-723-3366

User Contributions:

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Sep 15, 2006 @ 1:01 am
i have been infected with Enterobiasis four times and its getting rather annoying i have treated it all four times successfully but i am afraid it might come back what can i do to prevent it ive tried everything else before but nothing worked they just keep coming back please reply.
patricia navarro
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Jan 5, 2007 @ 7:19 pm
Exellent article. I have been reading a bit on this topic for some time now, for i was a victim of some kind of parasite infestation. I took a holistic approach to my problem. I expelled hundreds of worms soon after i started the holistic regimen. Many were 5 inches long. Although, i feel that i have not yet reached the level of full recovery. I am currently being seen by UCSF GI Dept because I feel that something is always moving in my GI. I believe it is in my small intenstine. I have an edoscopy and a colonoscopy scheduled for Feb. Do you have any recommendations on how to guide my GI doctors with said procedures. They are not as familiar with parasite infections as they seem to be with other matters in their specialty area
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Jan 22, 2007 @ 6:18 pm

I am having the same exact problem Patricia. The movement has become unbearable though. It certainly has me on edge. I too am seeing someone from UCSF GI Dept and worry that they aren't hearing my concerns. I don't sleep at night due to "activity". There is also a sour smell present. Do you have the same problems? Which holistic approach did you use? I wonder if we have the same doctor?!

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