Parasite Primer

Symptoms and Sources of Infection

It is estimated that three out of five Americans will be affected by parasites at some point in their lives. From protozoa to roundworms, tapeworms to flukes, there are four main classifications of parasites that infect humans. Symptoms may be mild or severe,

Learn about the various types of parasites, symptoms, and the sources of infection and spread.

Ancylostoma hookworm, illustration

Kateryna Kon / Science Photo Library / Getty Images


Protozoa, microscopic single-celled organisms, are the most common type of parasite in the United States. Unlike other kinds of parasites, they reproduce very rapidly. They do this in the intestines, and can travel to other organs such as the liver, lungs, pancreas, and heart. Protozoa have an indefinite life span.

  • Giardia (Giardia lamblia) originates in infected humans and animals. It is transmitted through water (especially in the Rocky Mountains), food, and contact with feces, often due to poor hygiene and handwashing. Giardia infects the small intestine. It can take up to three weeks before symptoms of giardia appear. Giardia can result in symptomless intestinal infection. It can also produce watery diarrhea, foul-smelling stools, nausea, stomach cramps, bloating, gas, low energy, and weight loss. Multiple stool samples are often necessary for diagnosis.
  • Cryptosporidium parvum is often transmitted by contact with human feces that contain infectious cysts, for example through poor handwashing. Cryptosporidium parvum is transmitted in water. It is the most prevalent waterborne parasite in the United States. In people with healthy immune systems, it can be symptomless or symptoms can include watery diarrhea that lasts about 10 days, nausea, cramps, and fever. In people who are immunocompromised, there can be severe diarrhea with weakness and weight loss, which can be life-threatening.
  • Cyclospora species is mainly seen in travelers. Recent outbreaks have been associated with contaminated water sources, such as Central American raspberries and basil. It is transmitted through contact with feces. Symptoms are similar to giardiasis. Symptoms come and go and can include diarrhea, frequent watery stools, weight loss, fatigue, bloating, or symptomless gallbladder disease.
  • Entamoeba histolytica is spread through water or food. Insects, such as flies and cockroaches, can carry the cysts. It may take up to three months after infection before symptoms appear. It can spread through the digestive tract and travel to other organs. Most often, the infected person is a symptomless carrier. It can cause stomach pain, bloating, and diarrhea. If there is tissue destruction in the large intestine, there can be low-grade fever with bloody diarrhea.
  • Toxoplasma gondii is an infection that generally comes from cats. Another source of toxoplasma is undercooked meat. Symptoms include flu-like symptoms such as fever, headache, swollen lymph nodes, and fatigue.
  • Trichomonas vaginalis is transmitted through sexual contact or from contaminated toilet seats, towels, or bathwater. There are often no symptoms. It can cause vaginal discharge, yeast infections, and painful urination in women. In men, it can cause an enlarged prostate gland and urinary inflammation.

Roundworms and Hookworms

These are unsegmented worms. They produce eggs that require incubation in soil or in another host before becoming potentially infective to humans.

  • Roundworm (Ascaris lumbricoides) is symptomless in many people. It is estimated that over 1 billion people in the world may be infected with roundworm. The source of infection is contamination of soil and vegetables with feces. Adult roundworms live in the small intestine and can exit through the mouth or nose of the infected person. Occasionally, there is obstruction of the pancreatic or bile duct, appendix, or small intestine. Dry cough, fever, and sleep disturbance may occur. Diagnosis is by stool exam for eggs and a blood test.
  • Necator americanus (hookworms) is transmitted through unbroken skin by walking barefoot. Hookworms travel into blood and through the lung and intestines. Hookworm infection is usually symptomless. There may be itching at the area of skin penetration. There can be digestive symptoms. The worms attach to and suck the blood from the mucus of the small intestine, leading to iron deficiency anemia, low energy, and peptic ulcer–like symptoms in severe infections.
  • Enterobius vermicularis (pinworm) infection is common in the United States. It is transmitted through contaminated food and water. The worms live in the intestines near the rectum and travel at night outside to the skin around the anus. From there it can be transmitted through person-to-person contact. It can be symptomless. There is often itching at night around the anus. There can also be unusual symptoms such as hyperactivity, vision problems, vaginitis, and psychological disturbances. To help make a diagnosis, tape is often applied to the anal area at night. When the tape is removed, adult worms may be seen with the unaided eye. At least five to seven tests are required to rule out infection.
  • Trichuris trichiura (whipworm) is a large intestine parasite that rarely shows symptoms. It is transmitted by ingestion of the eggs in soil or on vegetables. Symptoms of heavy infection include diarrhea, stomach pain, rectal prolapse, and stunted growth.
  • Trichinella (Trichinella spiralis) infection is often due to eating undercooked pork. Worms travel from the intestines into the muscles of the chest, diaphragm, jaws, and upper arms. Symptoms include diarrhea, nausea, severe muscle pain, facial swelling, difficulty breathing or chewing, and enlarged lymph nodes.


Tapeworms are flat, segmented, and shaped like ribbons. Tapeworms are the largest intestinal parasites and can grow to up to several feet or more in length. Tapeworm larvae can be found in undercooked meat or fish. From larvae, worms develop in the body and attach to the small intestine. They survive there by absorbing nutrients from ingested foods. Tapeworm infection is often symptomless, and treatment involves medication that targets the worm.

  • Taenia solium (pork tapeworm) infection most commonly occurs after eating undercooked pork, smoked ham, or sausages containing larvae. Adult worms attach to the intestines. Symptoms are similar to infection with beef tapeworm (see below). Larvae can travel to subcutaneous tissue, muscle, the central nervous system, and/or the eye, where they eventually form cysts to which the person infected responds with an inflammatory response—this can happen after four or five years. It can develop into blindness, seizures, neurological deficits, and hydrocephalus (swelling of the head).
  • Taenia saginata (beef tapeworm) infection occurs most commonly after eating undercooked beef containing the larvae. It can live in the intestines for up to 25 years and grow to a length of eight feet. It is usually symptomless, although occasionally presents as abdominal discomfort, loss of appetite, weight loss, and diarrhea. Segments can crawl out of the anus.
  • Diphyllobothrium latum (fish tapeworm) infection is most commonly due to eating freshwater fish containing larvae. Fish tapeworms can grow up to 15 meters (about 49 feet) in length. Symptoms are nonspecific abdominal symptoms, such as loss of appetite, heartburn, diarrhea, and nausea. Vitamin B12 deficiency can also occur, leading to macrocytic anemia and neurological symptoms such as muscle twitches.
  • Clonorchis sinensis is another worm transmitted by eating raw fish. The worms live in the gallbladder area, so complications can include bile duct stones, gallbladder stones, and other gallbladder disease.

Flukes or Flatworms

Flukes or flatworms are leaf-shaped worms that attach to the host using abdominal suckers. They usually begin their life cycle as snails, and then as larvae they infect fish, vegetation, or humans. Flatworms can travel to the lungs, intestines, heart, brain, and liver. Eggs can cause inflammation by releasing toxins that damage tissues.

  • Intestinal fluke (Fasciolopsis buski) worms live in the small intestine. They can cause intestinal ulcers and allergic reactions. Common symptoms are vomiting, diarrhea, nausea, and stomach pain. Intestinal fluke contamination comes from eating infected water vegetables, such as water chestnuts, bamboo shoots, and watercress.
  • Oriental lung fluke (Paragonimus westermani) is found predominantly in Asian countries. These worms can penetrate the intestines and travel to the brain or lungs. Symptoms of infection include irrepressible coughing fits and bloody sputum. Sources of these worms include undercooked crabs and crayfish.
  • Sheep liver fluke (Fasciola hepatica) is most commonly transmitted from fresh watercress. The worm attaches to the gallbladder and bile ducts, causing inflammation and local trauma. Symptoms include jaundice, fever, coughing, vomiting, and abdominal pain.
  • Blood flukes are transmitted by swimming in contaminated water. They burrow into the skin and migrate to the heart, lungs, liver, or bladder. They can live in the body for up to 30 years.

Also read about pargonimusschistosomiasis, and cryptosporidium

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  1. Garcia LS, Arrowood M, Kokoskin E, et al. Laboratory Diagnosis of Parasites from the Gastrointestinal TractClin Microbiol Rev. 2017;31(1):e00025-17. doi:10.1128/CMR.00025-17