An ova and parasite (O&P) examination looks for parasites and their eggs in a sample of your stool. Parasites usually infect the lower digestive tract and can live there for years. They often come from unpurified water or food that has been contaminated by the parasite eggs (ova), particularly in countries where there is poor sanitation. 

Your sample will be sent to a pathology laboratory where it is smeared onto a slide, stained and investigated under a microscope to identify and count any parasites, ova and cysts that may be present. Other tests can be required to make a diagnosis such as antigen tests which look for specific proteins on the surface of the suspected parasite and genetic molecular tests to look for the DNA of the parasite. 

Treatment may include drugs that target a specific parasite. Some infections disappear naturally over time. 

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 (including their eggs (ova) or cysts), may be detected and identified under the 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. Some parasites spend part of their life in a different animal, such as a sheep, cow, or snail, before infecting humans. 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. Some parasites are hardy and can remain infectious for some time in the environment without living in a host.

The majority of people who are infected by gastrointestinal parasites become infected by drinking water or eating food that has been contaminated with the parasite eggs. 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 attention to hygiene (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.  Parasites are different in different parts of the world so it is important to let your doctor know your travel history- even many years in the past. Some parasites such as Strongyloides can cause disease years after they were first acquired.

The most common symptoms of a parasitic infection are prolonged diarrhoea, bloody diarrhoea, 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 diarrhoea lasts more than a few days, it may lead to weight-loss, dehydration and electrolyte imbalance. These can be dangerous conditions, particulary in children, the elderly and those with weak immune systems.

The most common parasites in Australia are the single-cell parasites (protozoa) such as Giardia, Dientamoeba  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 a common intestinal parasite affecting humans in Australia. Worldwide, it infects 2% of adults and 6-8% of children in developed nations and almost 33% of people living in developing countries. Dientamoeba fragilis is also found worldwide and often affects children. Although most infections are asymptomatic, it can cause watery diarrhoea much like Giardia.  Cryptosporidium (often called crypto) is the most common cause of recreational water-related disease outbreaks in Australia; there are over 2000 cases of cryptosporidiosis each year in Australia. 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.

Strongyloides stercoralis, a roundworm, is one of the most common parasites to cause infection in rural and remote Australia. The worm can reinfect its human host and thus persist for many years. Strongyloidiasis can cause skin eruptions, gut symptoms, and asthma type symptoms. In persons with lowered immunity (such as someone receiving chemotherapy) the infection can be life threatening. A simple O&P exam cannot usually identify Strongyloides as the worms are not excreted in the faeces, if suspected serology and a Strongyloides stool culture should be requested.

Those who travel outside the Australia, 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. There are 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 a fresh salad or ice cubes in a drink. Some of the tiny worm-like parasites including Strongyloides and Schistosoma, cause infection after penetrating the skin.  E. histolytica and Schistosoma are some common and important parasites that can be acquired on overseas trips.

Entamoeba histolytica is a protozoan parasite commonly acquired after travel to developing countries. In most cases infection is asymptomatic but in some people can become quite unwell with bloody diarrhoea (dysentery) or liver abscess.  Microscopy alone is not sufficient to diagnose this as it cannot be differentiated from similar but harmless parasites such Entamoeba dispar.

The trematode flukes called Schistosoma are common parasites that cause infection after freshwater exposure in Africa, the Middle East or Asia. The distinctive eggs can be identified in a stool O& P examination. One species- S. haematobium, causes bladder infection and urine, should be submitted for parasite examination. 

How is it used?

The ova and parasite (O&P) exam is used to help diagnose the cause of prolonged diarrhoea. It is ordered to determine whether there are parasites present in the lower digestive tract and, if so, to identify them. Since there are many other causes of diarrhoea, the O&P is often ordered along with other tests, such as a stool culture, which identifies the presence of disease-causing bacteria in the stool.

O&P tests may also be ordered to monitor the effectiveness of treatment for a parasitic infection.

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

When is it requested?

The O&P is ordered when a person is suspected of having ingested contaminated food or water and has intestinal symptoms such as:

  • Prolonged diarrhoea
  • Abdominal pain, cramping
  • Nausea, vomiting
  • Blood and mucus in his stool
  • Weight loss or failure to grow properly in an infant or child

It may be ordered when a person has these symptoms and has recently consumed stream or lake water, been exposed to someone who has a parasitic infection (like a family member), or has travelled recently.

O&P tests may also be ordered when someone has been diagnosed with a parasitic infection to monitor the effectiveness of treatment.

What does the result mean?

If there are no ova or parasites seen, then the symptoms may be due to another cause. There may also be too few parasites to detect in the sample tested. A doctor may want to order additional O&P tests to confirm this finding and/or may order other tests to search for the cause of the persistent symptoms.

If a parasite is identified, then the person does have a parasitic infection. Ova and parasites present in the stool samples are identified and counted. Numbers may range from many to very few or rare.

The type and duration of treatment will depend on what kinds of parasite(s) are found and on the person's general state of health. The number of parasites seen may give the doctor general information about how heavy or extensive the infection is.

Is there anything else I should know?

Drug therapies are usually used to treat Giardia and E. histolytica infections. They may resolve themselves after several weeks, but they may also cycle, with symptoms subsiding and then worsening again. In those with healthy immune systems, Cryptosporidium generally goes away after a few weeks and antiparasitic treatment is not required. In those with weakened immune systems (such as those with HIV/AIDS, organ transplant, cancer, etc.), parasitic infections may persist and cause severe symptoms.

Common questions

  • How can I prevent a parasitic infection?

The best way is to avoid food and water that is potentially contaminated. This is especially true if you travel to developing 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. As Strongyloides is acquired via skin penetration, always wear shoes when walking outside.  Avoid fresh water exposure (eg swimming, water sports) when travelling to countries where parasites are common. 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 handwashing after going to the bathroom or handling soiled nappies can help prevent passing the parasite on to others. The infected person should avoid preparing food for others until their symptoms are gone.


  • Will an O&P detect all parasites?

No, only 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.


  • 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 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 transmitted until everyone is effectively treated.


  • 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 characteristics of the parasite and make the infection harder to detect. Only a very fresh, unpreserved sample can be examined for the motile (moving) parasite forms which can help make a quick identification. Preserved samples may not be suitable for adjunct tests such as antigen tests and DNA tests and cannot be used for culture.


  • 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 fibres. The only way to be sure though is to look at the sample under the microscope. Parasites have characteristic external and internal structures that fibres 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 specific 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. Antigen tests, which detect the presence of specific parasite proteins have been developed to detect some parasites such as Giardia, Cryptosporidium and E. histolytica. Some laboratories use molecular methods to detect the DNA of parasites in stool as alternative way to identify their presence.


  • Why shouldn't I take an over the counter anti-diarrhoea medicine?

You should only take this on the advice of your doctor. Diarrhoea 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-diarrhoea medication, you can prolong the amount of time that you are ill and may make your infection worse.

Last Updated: Thursday, 1st June 2023

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