Intestinal Parasites

Intestinal parasites in humans live a part of their life cycle within a person, in certain tracts and systems having to do with digestion.

What Are Intestinal Parasites?

Parasites are organisms such as worms or insects that must live on or inside another 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. Human intestinal parasites specifically live a part of their life cycle within a person, in certain tracts and systems having to do with digestion. These are: first, the gastrointestinal tract, also known as the alimentary canal, which consists of the mouth, esophagus, stomach, small intestine, colon, and anus; and second, the digestive system, which consists of the gastrointestinal tract and the associated, and attached, organs of digestion, including the liver and pancreas. Various intestinal parasites may infest the alimentary canal, the liver and biliary system, the pancreas, and in rare cases other organs, such as when ascarids invade the respiratory tract. Intestinal parasites may cause symptoms associated with the gastrointestinal tract such as diarrhea or states of malnutrition despite a healthy diet.

In humans, there are four major types of intestinal parasites: tapeworms (cestodes), roundworms (nematodes), flukes (trematodes), and microscopic organisms called protozoa. These parasites complete a part of their life cycle within the gastrointestinal tract, and some also travel to other parts of the body and invade other organs. While some intestinal parasites are so small they can only be seen under a microscope, others may be many feet long. There are several ways that parasites can infiltrate the human body, such as through oral ingestion or directly through the skin. A parasite life cycle involves different stages of development, such as eggs or larvae, and the adult form. Some parasites are able to enter the body through multiple routes and at multiple life cycle stages. For example, most tapeworms and roundworms lay their eggs within the human gastrointestinal tract. The eggs then pass out of the body through feces and if ingested can hatch and infest other humans with the parasite. Other parasites may enter the human body through egg and larval forms.

What Makes Intestinal Parasites a Health Issue?

Intestinal parasites are especially prevalent in the tropics. 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. A recent WHO report estimated that one-third of all deaths worldwide are due to infectious and parasitic diseases. Children are most frequently infected with these parasites. Intestinal parasites spread in areas with poor sanitation and are most common in developing countries on the African, Asian, and South American continents.


Though it is often reported that parasitic infections are not a large problem in the United States, most often impacting Americans who travel abroad, the CDC reports that parasitic infections do occur in the United States, affecting millions of people. The majority of cases are not caused by intestinal parasites. Giardiasis is the most common intestinal parasite infection in the United States, affecting about 20,000 people each year. Five infections have been targeted as Neglected Parasitic Infections (NPIs) in the United States. They include Chagas disease, cysticercosis, toxocariasis, toxoplasmosis, and trichomoniasis. These infections may have few symptoms and as a result go unnoticed, yet they cause serious illnesses, including seizures, blindness, heart failure, and even death. For example, 60 million people in the United States are infected with toxoplasmosis, and trichomoniasis infects 3.7 million people.

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 directly through the skin or if contaminated soil is ingested accidentally on food items (such as unwashed fruit or vegetables). Other parasites live in animals, such as pigs and cows. People can become infested by eating undercooked meat or drinking unpasteurized milk (milk that has not been processed with heat to kill parasites and bacteria). The eggs of some intestinal parasites pass through the human gastrointestinal tract into the feces. The parasites then spread to other people through unintentional contact with the fecal matter. 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 (such as when changing a diaper or doing laundry) or a contaminated area. Drinking water or swimming in water that has been contaminated are other common routes of infestation. These infestations often occur in outbreaks, where several people have been exposed to the same source and display symptoms at the same time, which is especially likely if many people come into contact with the same supply of contaminated food or water.

What Are Some Common Intestinal Parasites?

Tapeworms (Cestodes)

There are three common species of tapeworms: Taenia saginata (beef tapeworm), Taenia solium (pork tapeworm), and Diphyllobothrium latum (freshwater fish tapeworm). After humans ingest contaminated tapeworm-encysted meat or fish, the larvae travel to the intestines, where they latch onto the intestinal lining and gradually grow into adults. The largest tapeworms can measure more than 20 feet long (6 m) in some cases. The worms shed their eggs into the feces, from which they find their way into soil and water, are ingested by animals or fish, and reenter the cycle.

Symptoms of a tapeworm infestation are often mild or nonexistent but can include abdominal pain, diarrhea, and malnutrition. The fish tapeworm is often noted for causing vitamin B12 deficiencies. The pork tapeworm is known for leaving the human gastrointestinal tract in larval form and migrating to other parts of the body to create cysts of worm larvae in human body tissues. While cysts may form in any body tissue, the most commonly affected part of the body is the central nervous system * . The condition of having a tapeworm cyst in the brain is known as neurocysticercosis and may cause seizures * , blindness, or other nervous system abnormalities. Cysts may also form in the heart, eye, or other locations. If the larvae in the cysts die, the body may calcify the region as part of the healing process. Calcified cysts may also cause health problems.

Ascariasis (Nematode)

Ascariasis is caused by Ascaris lumbricoides, an intestinal roundworm. It is one of the most common intestinal parasites in areas with poor sanitation, affecting people in all parts of the world. In the United States, ascariasis is rare but occurs in rural parts of the Southeast. 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 two years. When people eat raw food containing this contaminated soil, they may ingest the eggs, which hatch in the stomach as larvae. Part of the life cycle of the larvae is to migrate outside the gastrointestinal tract. The larvae invade the walls of the gastrointestinal tract, migrate through the blood to the lungs and then to the throat, where they are swallowed. Eventually, they pass into the intestines, where adult worms form and begin the cycle again. The adult can grow to be more than 12 inches (30 cm) long and can live one to two years in the small intestine of the gastrointestinal tract.

Strongyloidiasis/threadworm (Nematode)

Strongyloidiasis is caused by a type of roundworm commonly referred to as the threadworm, Strongyloides stercoralis. Although the route of infestation can be fecaloral, it is most commonly from contact with contaminated soil where the larvae of the parasite can burrow directly through the skin. The larvae travel to the lungs, are coughed up into the mouth, swallowed, and enter the intestines. In the intestines, the worm matures to adulthood and begins laying eggs. What is special about this parasite is that the eggs can hatch inside the intestines and the worm can continue to cycle through many generations without leaving the body (called the autoinfective cycle), which causes an infestation that can last for decades. In individuals with weakened immune systems * , particularly those taking drugs such as corticosteroids that depress the immune system, strongyloidiasis can become overwhelming. A great number of larvae may invade the lungs and other organs. This problem is called the hyperinfection syndrome and, although rare, can be fatal.

Strongyloidiasis is endemic * to the tropics and is especially prevalent in West Africa, the Caribbean, and Southeast Asia. Symptoms may include a rash and itch at the site of penetration, diarrhea that may be intermittent with constipation, abdominal discomfort, and vomiting. During the period that larvae are migrating through the lungs there may be cough, difficulty breathing, and low-grade fever. Chronic strongyloidiasis may cause a physical sign known as larva currens (racing larva), a wavy-shaped wheal on the skin that extends from the region around the anus as far as the upper thighs and abdomen. These wheals are indicative of larvae invading the skin in the anal region.

Hookworm (Nematode)

Hookworms are a type of roundworm and a common intestinal parasite. The Centers for Disease Control and Prevention estimates that 1 billion people worldwide have hookworm infestations, although improved sanitation reduced the number of cases in the United States. Two species can infest humans: Ancylostoma duodenale and Necator americanus. The hookworm eggs hatch into larvae in the soil. Hookworms can directly penetrate human skin. Humans may become parasitized by walking barefoot in or touching contaminated soil, as well as ingesting food with contaminated soil on it. 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 the feces. Hookworms can live for one to two 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 may cause serious problems, including malnutrition and anemia (low red blood cell count) from intestinal bleeding. Newborns, young children, pregnant women, and malnourished individuals are most susceptible to these complications.

Dogs and cats may carry their own types of hookworm— Ancylostoma ceylanicum and Ancylostoma braziliense. These hookworms occasionally pass on to humans who come in contact with soil contaminated with cat or dog feces. This type of infestation, called cutaneous * larva migrans or creeping eruption, involves the hookworm larvae burrowing into the skin and causing severe itching, but the worms do not invade any deeper into the tissues. The condition resolves without treatment after several weeks or months.

Enterobiasis/pinworm (Nematode)

Enterobiasis, 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 day care centers. From there, infested children may spread the worms to their family members.

Pinworms live in the rectum, at the end of the large intestine. They come out at night to lay eggs on the perineum, the area around the anus and genitals. The eggs can spread to sheets and clothing, where they remain with the potential for about two weeks to invade a new host. Infestation occurs when people touch a contaminated area and then their mouths. Itching of the perineum is the most common symptom of pinworm infestation, which can lead to sleeplessness and irritability. Frequently, however, infected individuals show no signs of infestation.

Trichinosis (Nematode)

Trichinosis arises from several varieties of roundworms of the genus Trichinella. Although once very common, trichinosis was in the early 2000s 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. Trichinella larvae live encysted in the tissues of pigs and wild animals. When people eat their meat raw or undercooked, the larval cysts travel to the stomach, where acid dissolves the walls of the cysts and releases the immature, larval worms. They migrate to the small intestine, mature to adults, and lay eggs. Once the eggs hatch, the worms travel through the bloodstream to muscles, where they burrow in, forming new cysts. This ends the cycle in humans.

Giardiasis (Protozoan)

Giardiasis is the most common waterborne parasitic infection in the United States. Caused by Giardia intestinalis, 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 accidental oral contact with the feces of an infected person in food or from the hands. Infested individuals may quickly 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 or the elderly in diapers, which is especially a problem in day care centers and 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 those who do develop symptoms, stomach pain, bloating, flatulence (gas), and watery diarrhea usually start one to two weeks after infection. About half the people who are infected also lose weight because the parasite interferes with the absorption of fat in the diet, which results in fatty, foul smelling stools. The illness often lasts two to six weeks, or longer in people who are sick with another disease. People most at risk for infection include children, travelers, homosexual men, and those with a compromised immune system. Giardiasis is diagnosed by examining a stool sample. If this method fails and giardiasis is still suspected, then tests for special enzymes in the stool are performed.

Amebiasis (Protozoan)

Amebiasis is caused by a single-cell protozoan parasite called Entamoeba histolytica and occurs mainly in areas with poor sanitary conditions. The Entamoeba parasite has two life cycle forms: the motile (moving) form is responsible for the damage caused inside the gastrointestinal tract, whereas the cyst form is responsible for transmission of amebiasis. Amebiasis spreads when eggs are ingested through contaminated food or water or by touching surfaces contaminated with feces and then touching the mouth. It also can spread through certain types of sexual contact. Symptoms such as mild diarrhea and stomach pain may occur one to four weeks after an infestation, but only 1 in 10 infested individuals develops symptoms.

Amebic dysentery, a more severe form of the illness, is caused when the parasites physically invade and damage the intestinal walls, which causes bloody diarrhea, extreme stomach pain, and fever. Rarely, the infestation spreads to other body organs, particularly the liver. In the liver, amebas may form large abscesses, areas of infection walled off from the rest of the body. Abscesses may also form in the brain. Cases in the United States usually are seen in people who have recently arrived from or traveled to remote areas.

Cryptosporidiosis (Protozoan)

Cryptosporidiosis is caused by the protozoan Cryptosporidium parvum. Infestation usually occurs from ingestion of soil or water contaminated with fecal matter. Symptoms of cryptosporidiosis include diarrhea with frequent, watery, sometimes explosive bowel movements; loss of appetite; stomach cramps; bloating; nausea; fever; vomiting; and weight loss. The diagnosis can be made by examining a fecal sample under a microscope to view the organism. If the illness is not treated, its symptoms may return. Individuals with weak immune systems, such as those with AIDS * , have the most severe health problems from cryptosporidiosis and usually require treatment. However, the organism can infect people with normal immune systems and is one of the most common causes of protozoal diarrhea in the world. In individuals with healthy immune systems, the symptoms usually resolve on their own without treatment.

How Are Intestinal Parasites Diagnosed?

Physicians may use fecal samples, 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 adult forms. Blood samples may also be taken to check for antibodies made by the body against specific parasites. Physicians may also use a medical instrument called an endoscope * that allows them to view the internal structures of the gastrointestinal tract and to examine the intestines for infection. To detect pinworms, physicians 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, the time right 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 Parasites Treated?

Some cases of intestinal parasites require little or no treatment, and the parasites eventually disappear on their own. Other cases require drug intervention with antibiotics or anthelmintics (drugs used to fight parasitic infections). Medication used to treat the illnesses varies with the type of infestation. In most cases, patients can remain at home and maintain a normal schedule. Children with infestations must stay out of day care until they have been treated adequately and can no longer spread the parasites. Patients experiencing diarrhea are usually advised to drink plenty of fluids to avoid dehydration * . Infants and young children are particularly vulnerable to dehydration and nutrition problems when they become infested. Antidiarrheal medicine is usually not recommended because it may keep the parasites in the body longer. More severe cases of infestations may require treatment in the hospital. In individuals with weak immune systems (such as people with AIDS and those undergoing chemotherapy * ), and for infants and the elderly, parasitic infestations can be fatal.

How Is Intestinal Parasite Infestation Prevented?

Intestinal parasite infestation is best prevented through high standards of personal hygiene, which includes frequent and thorough handwashing, especially after changing diapers, after going to the bathroom, and before handling food. In areas known for parasites that live in the soil and burrow directly through the skin, wearing shoes may prevent parasites from entering the body. Individuals who travel to foreign countries known to have parasite problems are advised to 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. Avoiding swimming in bodies of fresh water such as ponds, rivers, and lakes in these areas can minimize the risk of contact with contaminated water.

See also Cyclosporiasis and Cryptosporidiosis • Giardiasis • Hookworm • Parasitic Diseases: Overview • Roundworm Infection • Tapeworm • Trichinosis


Books and Articles

Centers for Disease Control and Prevention. Intestinal Parasite Guidelines for Domestic Medical Examination for Newly Arrived Refugees. National Center for Emerging and Zoonotic Infectious Diseases, Division of Global Migration and Quarantine. November 6, 2013. (accessed July 21, 2015).

Firger, Jessica. “CDC Warns of Common Parasites Plaguing Millions in U.S.” CBS News. May 8, 2014. (accessed July 21, 2015).

Sjøgen, Kristian. “DNA Study: Vikings Were Plagued by Intestinal Parasites.” Science Nordic. January 5, 2015. (accessed July 21, 2015).


Centers for Disease Control and Prevention. “Parasites: Water.” (accessed July 21, 2015).

New England Biolabs. “Parasitic Infections in Humans.” (accessed July 21, 2015).

Organizations (accessed July 21, 2015).

Centers for Disease Control and Prevention. 1600 Clifton Rd., Atlanta, GA 30333. Toll-free: 800-311-3435. Website: (accessed July 21, 2015).

* seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.

* central nervous system (SEN-trul NER-vus SIS-tem) is the part of the nervous system that includes the brain and spinal cord.

* immune system (im-YOON SIS-tem) is the system of the body composed of specialized cells and the substances they produce that help protect the body against disease-causing germs.

* endemic (en-DEH-mik) describes a disease or condition that is present in a population or geographic area at all times.

* cutaneous (kyoo-TAY-nee-us) means related to or affecting the skin.

* AIDS, or acquired immunodeficiency (ih-myoo-no-dih-FIH-shensee) syndrome, is an infection that severely weakens the immune system; it is caused by the human immunodeficiency virus (HIV).

* 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.

* chemotherapy (KEE-mo-THER-apee) is the treatment of cancer with powerful drugs that kill cancer cells.

Disclaimer:   This information is not a tool for self-diagnosis or a substitute for professional care.

(MLA 8th Edition)