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Also known as: Ova and Parasite Exam
Formal name: Parasitic Examination, Stool
Related tests: Stool Culture, Giardia, Cryptosporidium, and Entamoeba histolytica antigen tests

At a Glance

Why Get Tested?

To determine whether you have a parasite infecting your digestive tract

When to Get Tested?

When you have diarrhea that lasts more than a few days and/or have blood or mucus in your loose stools, especially if you have drunk unpurified water while camping or have traveled outside of the United States

Sample Required?

A fresh or preserved stool sample, sometimes multiple samples collected on different days

Test Preparation Needed?


The Test Sample

What is being tested?

An ova and parasite (O&P) exam is a microscopic evaluation that is used to look for parasites that have infected the lower digestive tract. The parasites are shed from the lower digestive tract into the stool. When thin smears of fresh or preserved stool are put onto glass slides and stained, the parasites and/or their eggs (ova) or cysts, the form in which the parasite is surrounded by a resistant cover or capsule, can be detected and identified under the microscope by a trained laboratorian. Different ova and parasites have distinct shapes, sizes, and internal structures that are characteristic of their species.Giardia as viewed under a microscope

There are a wide variety of parasites that can infect humans. Each type of parasite has a specific life cycle and maturation process and may live in one or more hosts. Some parasites spend part of their life in an intermediate host, such as a sheep, cow, or snail, before infecting humans. Some infect humans "by accident." There are parasites that consist of a single cell while others are worms (helminths). Most parasites have more than one form through which they develop. Many have a mature form and a cyst and/or egg(ova) form. Some also have a larval phase, which is between the egg and the mature form. Ova are hardy and can exist for some time in the environment without living in a host and remain infectious.

The majority of people who are infected by gastrointestinal parasites become infected by drinking water or eating food that has been contaminated with the ova. Ova and parasites from infected people or animals are released in the stool and can then contaminate any water, food, or surfaces that come into contact with them. This contamination cannot be seen; the food and water will look, smell, and taste completely normal. People who ingest contaminated food or water can become infected and, without careful sanitation (handwashing and care with food preparation), the infection may be passed on to others.

Parasitic infections are especially a concern for certain groups such as infants, the elderly, and people with weakened immune systems such as those with HIV/AIDS. In these populations, a parasitic infection can result in serious symptoms and complications.

The most common symptoms of a parasitic infection are prolonged diarrhea, bloody diarrhea, mucus in stool, abdominal pain, and nausea. These symptoms typically arise days to weeks after exposure and persist. Some people may also have headaches and fever; others may have few or no noticeable symptoms. If diarrhea lasts more than a few days, it may lead to weight-loss, dehydration and electrolyte imbalance, dangerous conditions in children, the elderly and those with weak immune systems.

The three most common parasites in the United States are the single-cell parasites Giardia Entamoeba histolytica, and Cryptosporidium. Found throughout the world in even the most remote and pristine mountain streams and lakes, these parasites may infect swimming pools, hot tubs, and occasionally community water supplies.

  • Giardia (Giardia lamblia, Giardia intestinalis, Giardia duodenalis) is the most common intestinal parasite affecting humans in the United States. There are an estimated 2 million Giardia infections in the U.S. each year. Worldwide, it infects 2% of adults and 6-8% of children in developed nations and almost 33% of people living in developing countries.
  • Cryptosporidium (often called crypto) is the most common cause of recreational water-related disease outbreaks in the U.S.; outbreaks cause an estimated 300,000 cases of cryptosporidiosis each year.
  • Entamoeba histolytica is common but only causes illness in about 10% to 20% of those infected.

In many cases, Giardia and Cryptosporidium cysts can survive in water for weeks or months and are resistant to low levels of chlorine. The number of those infected with these parasites tends to increase during the summer months when more people are pursuing outdoor activities such as backpacking, hiking, and swimming and they unwisely or accidentally drink untreated water or contaminated food.

Those who travel outside the U.S., especially to developing nations, may be exposed to a much wider variety of parasites. In warm climates and places where water and sewage treatment are less effective, parasites are more prevalent. Besides Giardia, Cryptosporidium, and E. histolytica, there are also a wide range of flat worms, roundworms, hookworms, and flukes that can affect the digestive tract and other parts of the body. Visitors usually become infected by eating or drinking something that has been contaminated with the parasites' ova, even something as simple as ice cubes in a drink or a fresh salad, but some of the tiny worm-like parasites can also penetrate the skin, such as through the skin of the foot when someone is walking barefoot.

How is the sample collected for testing?

A fresh stool sample is collected in a clean container. The stool sample should not be contaminated with urine or water. Once it has been collected, the stool should either be taken to the laboratory within an hour after collection or transferred into special transport vials containing preservative solutions.

Often, multiple samples are collected and tested. These should be collected at different times on different days because parasites are shed intermittently and may not be in the stool at all times. Multiple samples can increase the likelihood that parasites will be detected.

NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.

Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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Article Sources

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.

Sources Used in Current Review

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(Modified 2010 June 2). Parasites and Foodborne Illness. USDA Food Safety and Inspection Service Fact Sheets [On-line information]. Available online at http://www.fsis.usda.gov/factsheets/Parasites_and_Foodborne_Illness/index.asp through http://www.fsis.usda.gov/factsheets. Accessed February 2011.

Pennardt, A. (Updated April 28). Giardiasis in Emergency Medicine. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/782818-overview through http://emedicine.medscape.com. Accessed February 2011.

(Modified 2009 July 20). Detection of Parasite Antigens. DPDx Diagnostic Procedures Stool Specimens, [On-line information]. Available online at http://www.dpd.cdc.gov/dpdx/HTML/DiagnosticProcedures.htm through http://www.dpd.cdc.gov. Accessed February 2011.

Fisher, M. et. al. (Updated 2010 November). Diarrhea, Parasitic Evaluation. ARUP Consult [On-line information]. Available online at http://www.arupconsult.com/Topics/ParasiticDiarrhea.html?client_ID=LTD through http://www.arupconsult.com. Accessed February 2011.

Mayo Clinic Staff (2010 May 29). Giardia infection (giardiasis). Mayo Clinic [On-line information]. Available online at http://www.mayoclinic.com/health/giardia-infection/DS00739 through http://www.mayoclinic.com. Accessed May 2011.

(© 2005-2010). Viruses, Bacteria and Parasites in the Digestive Tract. Children's Hospital Boston [On-line information]. Available online at http://www.childrenshospital.org/az/Site1959/mainpageS1959P0.html through http://www.childrenshospital.org. Accessed May 2011.

Mukherjee S . (Updated November 18, 2009). Giardiasis. eMedicine [On-line information]. Available online at http://emedicine.medscape.com/article/176718-overview through http://emedicine.medscape.com. Accessed May 2011

Kucik C, et al (March 1 2004). Common Intestinal Parasites. Am Fam Physician. 2004 Mar 1;69(5):1161-1169. Available online at http://www.aafp.org/afp/2004/0301/p1161.html through http://www.aafp.org. Accessed May 2011.

Pagana, K. D. & Pagana, T. J. (© 2011). Mosby's Diagnostic and Laboratory Test Reference 10th Edition: Mosby, Inc., Saint Louis, MO. Pp 911-912.

Forbes, B. et. al. (© 2007). Bailey & Scott's Diagnostic Microbiology, 12th Edition: Mosby Elsevier Press, St. Louis, MO. Pp 543-627.

Sources Used in Previous Reviews

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Pagana, Kathleen D. & Pagana, Timothy J. (2001). Mosby's Diagnostic and Laboratory Test Reference 5th Edition: Mosby, Inc., Saint Louis, MO.

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(2001 May). Cryptosporidiosis. CDC, Division of Parasitic Diseases, Parasitic Disease Information, Fact Sheet [On-line information]. Available online at http://www.cdc.gov/ncidod/dpd/parasites/cryptosporidiosis/factsht_cryptosporidiosis.htm through http://www.cdc.gov.

(2001 May). Giardiasis. CDC, Division of Parasitic Diseases, Parasitic Disease Information, Fact Sheet [On-line information]. Available online at http://www.cdc.gov/ncidod/dpd/parasites/giardiasis/factsht_giardia.htm through http://www.cdc.gov.

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(2007 February 27, Reviewed). About Parasites. CDC Parasitic Diseases [On-line information]. Available online at http://www.cdc.gov/ncidod/dpd/aboutparasites.htm through http://www.cdc.gov. Accessed on 12/24/07.

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