Ova and Parasite Exam

Also Known As

O&P

Formal Name

Parasitic Examination, stool

This article was last reviewed on

September 25, 2020.

This article waslast modified on September 25, 2020.

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 stool sample transported to the lab within 2 hours or a preserved stool sample; usually multiple samples are collected on different days.

Test Preparation Needed?

None

What is being tested?

An ova and parasite (O&P) exam is a test in which a laboratory professional uses a microscope to examine a stool sample and look for parasites. A variety of parasites can cause infections of the digestive system (gastrointestinal (GI) infections). When you have a parasite infecting your lower digestive tract, causing symptoms such as diarrhea, the parasites and their eggs (ova) are shed from your lower digestive tract into your stool.

For the exam, thin smears of your fresh or preserved stool are put onto glass slides and stained. The parasites and/or their ova or cysts (the form in which the parasite is surrounded by a resistant cover or capsule) may be seen and identified using a microscope. Different ova and parasites have distinct shapes, sizes, and internal structures that are characteristic of their species.

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, before infecting humans. Some infect humans "by accident." There are parasites that are 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.

Most people who are infected by gastrointestinal parasites become infected by drinking water or eating food that has been contaminated with the ova. (For more details, read Food and Waterborne Illness.) 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. If you eat contaminated food or drink contaminated water, you can become infected. Without careful sanitation (hand washing and care with food preparation), you can pass the infection to others.

The most common symptoms of a parasitic infection are prolonged diarrhea, bloody diarrhea, mucus in your stool, abdominal pain, and nausea. These symptoms typically arise days to weeks after infection and can continue for some time. You may also have headaches and fever, or few or no noticeable symptoms.

If your diarrhea lasts more than a few days, it may lead to weight loss, dehydration, and electrolyte imbalance. These complications are especially dangerous in children, the elderly, and those with weakened immune systems (e.g., people with HIV/AIDS, cancer, and transplant recipients). According to the World Health Organization (WHO), foodborne and waterborne diarrheal diseases, including those caused by parasites, kill more than 700,000 people a year.

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

Giardia species are some of the most common intestinal parasites in the U.S.
Cryptosporidium (often called crypto) is one of the leading causes of recreational water-related disease outbreaks in the U.S.
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 drink untreated water or contaminated food.

If you travel outside the U.S., especially to resource-limited nations, you 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. As a visitor, you would 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 your foot when you are walking barefoot.

If your illness is uncomplicated and goes away within a few days, your healthcare practitioner may not order testing. However, if your symptoms are severe, if there is bloody diarrhea or mucus present in your stool, or if it persists, then a stool culture may be ordered. This is especially true if you have been outside the U.S. and/or have eaten or drunk anything that has also made someone close to you ill.

To aid diagnosis, an ova and parasite exam may be done along with or following a GI pathogens panel that simultaneously tests for multiple disease-causing bacteria, viruses, and parasites. Other tests that may be done include a stool culture and antigen tests to identify specific microbes.

How is the sample collected for testing?

You collect a fresh stool sample in a clean container provided by the laboratory or your healthcare practitioner. The stool sample should not be contaminated with urine or water. Once it has been collected, your stool should either be taken to the laboratory within two hours 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 periodically and may not be in your stool at all times. Multiple samples can increase the likelihood that parasites will be detected.

Common Questions

How is the test used?

The ova and parasite (O&P) exam is used to detect the presence of parasites in your stool sample and help diagnose an infection of the digestive system (gastrointestinal, GI tract). Since there are many causes of GI infections, an O&P may be used in conjunction with other tests, such as a gastrointestinal (GI) pathogens panel or a stool culture, to help establish your diagnosis.

Many GI infections in the U.S. resolve with no specific treatment, with only supportive care, and may not require testing. If you are otherwise healthy, the infections are considered common illnesses that are not serious and sometimes thought of as "food poisoning" or "stomach flu." However, there are cases where it is useful to perform testing that identifies the cause of your GI infection – to guide its treatment, eliminate its source, and limit its spread.
When is it ordered?
The O&P is ordered when you are suspected of having ingested contaminated food or water and have signs and symptoms of a GI infection, such as:

Prolonged diarrhea

Abdominal pain, cramping

Nausea, vomiting

Blood and mucus in your stool

Not everyone who has these symptoms will necessarily have testing done or be treated. An O&P is more likely to be ordered when you:

Have severe symptoms, dehydration, electrolyte imbalance, and/or other complications

Are very young, elderly, or have a weakened immune system

Have prolonged signs and symptoms and/or infections that do not resolve without treatment

Have an illness during and following travel outside the U.S., especially to resource-limited nations

Have recently drunk stream or lake water

Have been exposed to someone who has a parasitic infection (like a family member)
What does the test result mean?
Results are typically reported as negative or positive.

A negative result means there were no ova or parasites seen and your diarrhea may be due to another cause. However, there may also be too few parasites to detect in the sample tested. A healthcare practitioner may order additional O&P tests to confirm this finding and/or may order other tests to search for the cause of your persistent symptoms.

A positive result means you have a parasite infecting your digestive tract. The ova and parasites present in your stool samples are identified and counted. Numbers may range from many to very few or rare.

Typically you will only have one type of parasite that is causing your symptoms, but there could be more than one. The most common and frequently detected parasites in the U.S. are:

Giardia species—some of the most common intestinal parasites in the United States.

Cryptosporidium (often called crypto)—one of the leading causes of recreational water-related disease outbreaks in the U.S.

Entamoeba histolytica—common but only causes illness in about 10% to 20% of those infected.

Some examples of other intestinal parasites that may be detected with an O&P include the following.

Single cell parasites:

Dientamoeba fragilis

Balantidium coli

Cyclospora cayetanensis

Worms (helminths):

Roundworms such as Ascaris, Strongyloides

Hookworms

Tapeworms such as Hymenolepis nana, Taenia solium and Diphyllobthrium latum

Flatworms and flukes such as Fasciolopsis buski

For more on parasites that cause GI infections, see the articles on Diarrhea, Food and Waterborne Illness, and Travelers' Diseases.
What other lab tests may be done?

Other tests for parasites may be used in conjunction with an O&P to help make a diagnosis. A healthcare practitioner may order a Giardia, Cryptosporidium, or Entamoeba histolytica antigen test if it is suspected that one of these parasites may be causing your infection. These tests detect protein structures on the parasites and can identify an infection, even if no actual parasites or ova are seen in the stool. Since antigen tests only detect a few specific parasites, they are not replacements for the complete O&P, which will detect a wider variety of parasites.

A GI pathogens panel may be performed on your stool sample to simultaneously test for the viruses, bacteria, and parasites that are common causes of GI infections. These panels are relatively new and laboratories and healthcare practitioners are still determining how to utilize them to provide the optimum patient results while being cost-effective. An O&P and a stool culture may also be done if a microbe is not identified by the panel and/or if there is suspicion of a parasite or bacteria other than those included in the GI panel.
If my O&P is positive, what is the treatment?

Medications are usually used to treat Giardia, E. histolytica and Cryptosporidium infections. The type and duration of your treatment will depend on what kinds of parasite(s) are found and on your general state of health. The number of parasites seen may give your healthcare practitioner general information about the extent of your infection.
How can I prevent a parasitic infection?

The best way is to avoid food and water that are potentially contaminated. This is especially true if you travel to resource-limited nations, where ice in a drink or a dinner salad may expose you to parasites. But the clearest mountain stream should also be suspect; it could be contaminated with Giardia. You cannot see most parasites; you won't be able to smell them or taste them in the water. If someone in your family has a parasitic infection, careful hand washing after going to the bathroom or handling soiled diapers can help prevent passing the parasite to others. The infected person should avoid preparing food for others until that person's symptoms are gone.
Will an O&P detect all parasites?

No, it will only detect those that live in the digestive tract and whose eggs are passed through the stool. There are other tests specific for other parasites, such as pinworms, or blood parasites that cause malaria.
Why does it need to be a fresh stool sample?

The structure of the parasites may deteriorate in unpreserved stool, which would destroy the identifying features of the parasite and make your infection more difficult to diagnose.
Are those parasitic worms I'm seeing in my stool sample?

Most of the common parasites are much too small to be seen with the naked eye. What you are probably noticing are undigested food fibers. The only way to be sure is for a trained health professional to look at the sample under the microscope. Parasites have characteristic external and internal structures that fibers do not have.
Are there other ways to test for intestinal parasites besides a stool exam?

Yes, blood antibody tests may be ordered to determine whether or not someone has been exposed to a parasite in the past. A positive result may indicate a past or a chronic infection but is not used to detect a current infection. Sometimes a biopsy of the small intestines is taken and the small amount of tissue is examined for parasitic infestation.
Once I've had a parasitic infection, can I be re-infected?

Yes, you can become re-infected if exposed again. This may happen if you eat or drink something that is contaminated or you have a family member who has an asymptomatic parasitic infection, such as giardiasis, and continues to shed the organisms and re-infects others. The infection can be spread to others until everyone is effectively treated.
Why shouldn't I take an over the counter anti-diarrhea medicine?

You should only take this on the advice of your healthcare provider. Diarrhea is one of the methods your body uses to help rid itself of the infection. If you slow down or prevent this from happening by taking anti-diarrhea medication, you can prolong the amount of time that you are ill and may make your infection worse.

See More Common Questions See Less Common Questions

Related Images and Infographic

View Sources

Sources Used in Current Review

(2020 February 26, Updated). Ova and Parasite Test. MedlinePlus Medical Encyclopedia. Available online at https://medlineplus.gov/lab-tests/ova-and-parasite-test/. Accessed on 01/19/2020.

(2018 July23, Updated). Diagnosis of Parasitic Diseases. Centers for Disease Control and Prevention. Available online at https://www.cdc.gov/parasites/references_resources/diagnosis.html. Accessed on 01/19/2020.

Connor, B. (2019 June 24, Reviewed). Posttravel Evaluation, Persistent Diarrhea in Returned Travelers. Centers for Disease Control and Prevention. Travelers Health Chapter 11. Available online at https://wwwnc.cdc.gov/travel/yellowbook/2020/posttravel-evaluation/persistent-diarrhea-in-returned-travelers. Accessed on 01/19/2020.

(2019 June 24, Reviewed). Family Travel, International Adoption. Centers for Disease Control and Prevention. Travelers Health Chapter 7. Available online at https://wwwnc.cdc.gov/travel/yellowbook/2020/family-travel/international-adoption. Accessed on 01/19/2020.

Couturier, M. et. al. (2020 February, Updated). Diarrhea. ARUP Consult. Available online at https://arupconsult.com/content/diarrhea. Accessed on 01/19/2020.

Guandalini, S. et. al. (2020 January 31, Updated). Diarrhea. Medscape Pediatrics: General Medicine. Available online at https://emedicine.medscape.com/article/928598-overview. Accessed on 01/19/2020.

(© 1995-2020). Ova and Parasite, Concentrate and Permanent Smear, Microscopy, Feces. Mayo Clinic Laboratories. Available online at https://www.mayocliniclabs.com/test-catalog/Overview/9216. Accessed on 01/19/2020.

Sources Used in Previous Reviews

Thomas, Clayton L., Editor (1997). Taber's Cyclopedic Medical Dictionary. F.A. Davis Company, Philadelphia, PA [18th Edition].

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

Hardy, D. (2002 January). Diarrhea Caused by Parasites. Clinical Laboratories Newsletter, Univ of Roch Med Center, Vol. 22, No. 1 [On-line newsletter]. Available online at http://www.urmc.rochester.edu/path/newsletter102.htm.

Diagnostic Procedures for Stool Specimens. DPDx, Laboratory Identification of Parasites of Public Health Concern [On-line information]. Available online through http://www.dpd.cdc.gov.

Kotton, C., Updated (2002 January 08, Updated). Stool Ova And Parasites Exam. MedlinePlus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003756.htm.

A.D.A.M. editorial, Updated (2001 September 14). Giardiasis. MedlinePlus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000288.htm.

Kott, C., Updated (2002 July 19, Updated). Cryptosporidium enteritis. MedlinePlus Health Information, Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/000617.htm.

(2001 March). Amebiasis. CDC, Division of Parasitic Diseases, Parasitic Disease Information, Fact Sheet [On-line information]. Available online at http://www.cdc.gov/ncidod/dpd/parasites/amebiasis/factsht_amebiasis.htm.

(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.

(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.

(2003 February, Updated ). Giardiasis. AAFP, familydoctor.org Fact Sheet [On-line information]. Available online at http://familydoctor.org/handouts/078.html through http://familydoctor.org.

Giardia Antigen by EIA. ARUP's Guide to Clinical Laboratory Testing (CLT) [On-line information]. Available online at http://www.aruplab.com/guides/clt/tests/clt_a247.htm#1149440.

Cryptosporidium Stain and Antigen. ARUP's Guide to Clinical Laboratory Testing (CLT) [On-line information]. Available online at http://www.aruplab.com/guides/clt/tests/clt_a174.htm#1060170.

Giardia lamblia. Ohio State University, College of Biological Sciences, Parasites and Parasitological Resources [On-line information]. Available online at http://www.biosci.ohio-state.edu/~parasite/giardia.html.

Cryptosporidium parvum. Ohio State University, College of Biological Sciences, Parasites and Parasitological Resources [On-line information]. Available online at http://www.biosci.ohio-state.edu/~parasite/cryptosporidium.html.

Wu, A. (2006). Tietz Clinical Guide to Laboratory Tests, Fourth Edition. Saunders Elsevier, St. Louis, Missouri. Pp 1590.

(2007 June 25, Reviewed). Risks from Food and Water (Drinking and Recreational). Travelers' Health: Yellow Book, CDC Health Information for International Travel 2008, Chapter 2, Pre- and Post-travel General Health Recommendations [On-line information]. Accessed on: 12/9/07. Available online at http://wwwn.cdc.gov/travel/yellowBookCh2-FoodWaterRisks.aspx.

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

Forbes BA, Sahm DF, Weissfeld AS. Bailey & Scott's Diagnostic Microbiology. 12th Edition: Mosby Elsevier, St. Louis, MO; 2007.

Vorvick, L. (Updated 2010 May 4). Stool ova and parasites exam. MedlinePlus Medical Encyclopedia [On-line information]. Available online at http://www.nlm.nih.gov/medlineplus/ency/article/003756.htm. Accessed February 2011.

(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. 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. 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. 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. 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. 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. Accessed May 2011.

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

Chamberland, R. (2014 March 5 Updated). Stool Ova and Parasite Test. Medscape Drugs & Diseases [On-line information]. Available online at http://emedicine.medscape.com/article/2117799-overview. Accessed on January 2016.

Lehrer, J. (2014 May 15, Updated). Stool ova and parasites exam. MedlinePlus Medical Encyclopedia [On-line information]. Available online at https://www.nlm.nih.gov/medlineplus/ency/article/003756.htm. Accessed on January 2016.

Barr, W. and Smith, A. (2014 February 1) Acute Diarrhea in Adults. Am Fam Physician. 2014 Feb 1;89(3):180-189. [On-line information]. Available online at http://www.aafp.org/afp/2014/0201/p180.html. Accessed on January 2016.

Pearson, R. (2013 November). Approach to Parasitic Infections. Merck Manual Professional Version [On-line information]. Available online at http://www.merckmanuals.com/professional/infectious-diseases/approach-to-parasitic-infections/approach-to-parasitic-infections. Accessed on January 2016.

(2014 March 10). Diagnosis of Parasitic Diseases. CDC [On-line information]. Available online at http://www.cdc.gov/parasites/references_resources/diagnosis.html. Accessed on January 2016.

Dowshen, S. (2014 September Reviewed). Stool Test: Ova and Parasites. KidsHealth from Nemours [On-line information]. Available online at http://kidshealth.org/parent/system/medical/test_oandp.html. Accessed on January 2016.

Ask a Laboratory Scientist

This form enables patients to ask specific questions about lab tests. Your questions will be answered by a laboratory scientist as part of a voluntary service provided by one of our partners, American Society for Clinical Laboratory Science. Please allow 2-3 business days for an email response from one of the volunteers on the Consumer Information Response Team.

Please indicate whether you are a

Country

I have read and agree to the Privacy Policy (read the Privacy Policy)

I have read and agree to the Terms of Use (read the Terms of Use)

I agree to the disclaimer (read the disclaimer).

Disclaimer

Thank you for using the Consumer Information Response Service ("the Service") to inquire about the meaning of your lab test results. The Service is provided free of charge by the American Society for Clinical Laboratory Science, which is one of many laboratory organizations that supports Lab Tests Online.
Please note that information provided through this free Service is not intended to be medical advice and should not be relied on as such. Although the laboratory provides the largest single source of objective, scientific data on patient status, it is only one part of a complex biological picture of health or disease. As professional clinical laboratory scientists, our goal is to assist you in understanding the purpose of laboratory tests and the general meaning of your laboratory results. It is important that you communicate with your physician so that together you can integrate the pertinent information, such as age, ethnicity, health history, signs and symptoms, laboratory and other procedures (radiology, endoscopy, etc.), to determine your health status. The information provided through this Service is not intended to substitute for such consultations with your physician nor specific medical advice to your health condition.
By submitting your question to this Service, you agree to waive, release, and hold harmless the American Society for Clinical Laboratory Science and its affiliates or their past or present officers, directors, employees, agents, and Service volunteers (collectively referred to as "ASCLS") and the American Association for Clinical Chemistry and its affiliates or their past or present officers, directors, employees, agents, and Service volunteers (collectively referred to as "AACC") from any legal claims, rights, or causes of action you may have in connection with the responses provided to the questions that you submit to the Service.
AACC, ASCLS and its Service volunteers disclaim any liability arising out of your use of this Service or for any adverse outcome from your use of the information provided by this Service for any reason, including but not limited to any misunderstanding or misinterpretation of the information provided through this Service.

See More See Less

On This Site

Elsewhere On The Web

Ask a Laboratory Scientist