To determine whether you have a parasite infecting your gastrointestinal tract.
- When you have diarrhoea that lasts more than a few days and/or have blood or mucous in your loose stools
- Have drunk stream or lake water while camping, or have travelled outside of the United Kingdom
- Visited a farm or been in contact with sheep
A fresh or preserved stool sample, sometimes multiple samples collected on different days.
An ova, cysts and parasite (OCP) exam is a microscopic evaluation of a stool sample. It is used to look for parasites that have infected the gastrointestinal tract. These parasites and their ova (eggs, cyst form) are shed - passed out of the body through the faeces. When thin smears of stool are put onto glass slides and viewed under a microscope, the parasites and ova can be detected and identified under a microscope. Different ova and parasites have different 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 particular life cycle (maturation process) and a place that it is adapted to live in. Some spend time in an intermediate host (such as a sheep, cow, or snail) before infecting humans, some infect humans “by accident”, and some are not picky about who or what they infect. Most parasites have an adult form and a cyst/egg/ova form. Some also mature through a larval phase. Those parasites that infect the gastrointestinal tract are passed out of the body through the faeces. The parasite ova are hardy, they can exist in the environment for some time without a host.
Most people who are infected by parasites become infected by drinking water or eating food that has been contaminated with the ova. This contamination cannot be seen - the food and water will look, smell, and taste completely normal. Since an infected person’s stool is also infectious, without careful hygiene (handwashing and care with food preparation) they may pass the infection on to others. This is especially a concern with infants at day care centres and the elderly in nursing homes. Not only is a parasitic infection easily passed in these populations, but also the immune systems of those infected may be less effective at getting rid of the infection.
The most common parasites are three single cell parasites: Giardia lamblia, Entamoeba histolytica (E. histolytica), and Cryptosporidium parvum. They are found in mountain streams and lakes throughout the world and may infect swimming pools, hot tubs, and occasionally community water supplies. Cryptosporidium (called crypto), resists chlorine and can live for several days in swimming pools. They can be removed from the water supply, however, with adequate filtration. Most parasitic infections in the UK are due to these parasites, but other more worm-like parasites such as a roundworms or tapeworms do occasionally cause infections.
Those that travel outside the UK, 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 often much more prevalent. Besides giardia, crypto, and E. histolytica, there are also a wide range of flat worms, roundworms, hookworms, and flukes. 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 parasites can also penetrate the skin.
The most common symptoms of a parasitic infection are prolonged diarrhoea, bloody diarrhoea, mucous in stool, abdominal pain, and nausea. Patients may also have headaches and fever, and will sometimes have few, or no noticeable symptoms.
How is the sample collected for testing?
A fresh stool sample is collected in a clean container (or on a clean surface). The stool sample should not be contaminated with urine or water.
When multiple samples are requested, they are collected at different times, often on different days. The collection container must be labelled with the patient’s name and the date and time of the stool collection. These details are very important; if the sample is not labelled correctly the lab may refuse to examine it.
Is any test preparation needed to ensure the quality of the sample?
No test preparation is needed.
How is it used?
The OCP test is used to help diagnose prolonged diarrhoea. It is requested to determine whether there are parasites present in the gastrointestinal tract and if so, to identify them. Since there are many other causes of diarrhoea, the OCP is often requested along with other tests, such as a stool culture (which identifies the presence of pathogenic bacteria in the stool). Classically, OCP tests were requested in multiples, such as 3 samples from 3 different bowel movements, often on separate days. This was considered to be the best way to detect what might be small amounts of ova in the stool (better chance of not missing an infection).
OCP tests may also be requested to monitor the effectiveness of treatment for a parasitic infection.
When is it requested?
When you have prolonged diarrhoea, abdominal pain, and/or blood and mucous in your stool. When you have symptoms, and have recently travelled outside the U.K., drunk stream or lake water, or been exposed to someone who has a parasitic infection (like a family member).
OCP tests may also be requested when you have had a parasitic infection, to monitor the effectiveness of treatment.
What does the test result mean?
If there are no ova or parasites seen, your diarrhoea may be due to another cause. There may also be too few parasites to detect. Your doctor may want to request additional OCP tests and/or may request other tests to search for the cause of your symptoms.
If a parasite is identified, then you do have a parasitic infection. The type and duration of 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 doctor general information about how heavy or extensive your infection is.
Is there anything else I should know?
If diarrhoea lasts more than a few days it may lead to dehydration and electrolyte imbalance, dangerous conditions in children and the elderly. They cannot afford to lose large amounts of fluid.
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, then worsening again. There is no effective treatment for cryptosporidium. In those with competent immune systems, cryptosprodium generally goes away after a few weeks. In those with compromised immune systems (such as those with HIV/AIDS, organ transplant, cancer, etc.), however, cryptosporidium may be dangerous, becoming chronic and causing wasting and malnutrition.
Parasitic infections are monitored on a community level. Other than travel related cases, health officials want to try to determine where your infection came from so that they can address any potential public health concerns. For instance, if cryptosporidium or giardia is due to contaminated swimming pool water or community water supply, steps will need to be taken to prevent the spread of the infection.
How can I prevent a parasitic infection?
The best way is to avoid food and water that is suspected of being 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. 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 toilet can help prevent passing the parasite on to others.
Will an OCP test detect all parasites?
Why does it need to be a fresh stool sample?
Are those parasitic worms I'm seeing in my stool sample?
Usually no. 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 a characteristic structure, including digestive organs that fibres do not have.
Are there other ways to test for parasites?
Yes, antigen tests have been developed for several common parasites including giardia, cryptosporidium, and E. histolytica. The antigen tests detect protein structures on the parasite and they can detect the presence of even fragments of the parasite in a stool sample. This has the advantage of allowing detection of that particular parasite even if it is not seen in the OCP examination. Antigen tests are not commonly used in the U.K. Blood antibody tests may be requested to determine whether or not someone has been exposed to a parasite in the past (this 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, a small amount of tissue that is examined for parasitic infestation. Molecular tests are also becoming more common in larger laboratories. This type of test is able to detect extremely low numbers of parasites which may be present.
Once I've had a parasitic infection, can I be re-infected?
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 sometimes make your infection worse.