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Parasite Primer

Symptoms and Sources of Infection


Updated April 16, 2014

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

1. Protozoa
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 lifespan.
  • 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 intestines. 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 contains infectious cysts, for example poor handwashing. Cryptosporidium parvum is transmitted in water. It is the most prevalent waterbourne 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, 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 can carry the cysts, such as flies and cockroaches. 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 intestines, there can be low grade fever with bloddy 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.
2. 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 one 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 intestines 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 intestines. Dry cough, fever, and sleep disturbance may occur. Diagnosis is by stool exam for eggs and 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 mucous of the small intestines, 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. 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 5 to 7 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.

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