Ova and Parasite Exam

Also Known As:
O&P

Formal Name:
Parasitic Examination
stool

This article was last modified on March 24, 2021

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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.

Related Images and Infographic

Giardia lamblia as seen under a microscope. Image credit: CDC
Giardia as seen under a microscope. Image credit: CDC
Lifecycle of Giardia. Image credit: CDC
Entamoeba histolytica cyst. Image credit: CDC
Cryptosporidium (arrows). Image credit: CDC

View Sources

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