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. 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.
