A variety of parasites can cause infections of the digestive system (gastrointestinal (GI) infections). An ova and parasite (O&P) exam is a microscopic evaluation of a stool sample that is used to look for parasites that may infect the lower digestive tract, causing symptoms such as diarrhea. The parasites and their eggs (ova) 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 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.

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. (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. People who ingest contaminated food or water can become infected and, without careful sanitation (hand washing 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. 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. According to the World Health Organization (WHO), foodborne and waterborne diarrheal diseases, including those caused by parasites, kill about 2 million people a year.

Three of the most common parasites in the United States are the single-cell parasites Giardia species, Entamoeba histolytica, and Cryptosporidium species. 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 species are some of the most common intestinal parasites in the United States.
• 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 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.

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

To aid diagnosis, an ova and parasite exam may be done in conjunction 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.

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 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 intermittently and may not be in the stool at all times. Multiple samples can increase the likelihood that parasites will be detected.

No test preparation is needed.