At a Glance
Why Get Tested?
To determine whether you have a parasite infecting your digestive tract.
When to Get Tested?
When you have diarrhoea that lasts more than a few days and/or have blood or mucus in your loose stools, especially if you have drunk unpurified water or have travelled recently.
A fresh or preserved stool sample, sometimes multiple samples collected on different days.
Test Preparation Needed?
The Test Sample
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 and/or their eggs (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. 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. 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 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 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, dangerous conditions 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; outbreaks cause approximately 1,500 cases of cryptosporidiosis each year. 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 ice cubes in a drink or a fresh salad. 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, however one species- S. haematobium, causes bladder infection and the urine, not stool, should be submitted for parasite examination.
How is the sample collected for testing?
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 an hour 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.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.