Ova and parasites

What is being tested?
The ova and parasites (O&P) test is a microscopic examination used to look for parasites that have infected the gastrointestinal tract. Parasites pass from the gastrointestinal tract into the stool. When a thin smear of fresh or preserved stool is placed on a slide and stained, the parasites and their eggs or cysts (the form in which the parasite is surrounded by a tough coating or capsule) can be detected and identified under the microscope. Different eggs and parasites have distinct shapes, sizes and internal structures that are characteristic of their species.

There is 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”. Most parasites have an adult form and a cyst/egg form. Some also pass through a larval stage between egg and adult. The eggs are tough and can survive in the environment for some time without a host.

Most people infected with intestinal parasites become infected by drinking water or eating food contaminated with the eggs. This contamination is not visible; the food and water will look, smell and taste completely normal. Because the stool of an infected person will often contain eggs, this is also a source of infection. Without careful hygiene (hand washing and care in food preparation), the infection can be passed on to others. This is especially a problem for children in nurseries and older people in care homes. In these settings, a parasitic infection can spread easily, and the immune systems of the people infected may be less effective at clearing the infection.

The three most common parasites in the USA are single-celled parasites: Giardia lamblia, Entamoeba histolytica (E. histolytica) and Cryptosporidium parvum. The Centers for Disease Control and Prevention (CDC) estimate that there are about 2 million giardia infections a year in the USA, and that Cryptosporidium, or “crypto”, is the most common cause of water-related illness during holidays. These parasites are found in mountain streams and lakes all over the world and can contaminate swimming pools, hot tubs and sometimes public water supplies. Cryptosporidia are resistant to chlorine and can survive for several days in swimming pools. Most parasitic infections in the USA are due to these three parasites, but other parasites, such as roundworms and tapeworms, can also cause infections.

People who travel outside the USA, especially to developing countries, may be exposed to a much wider variety of parasites. In hot climates and in places where water treatment and sewage systems are less effective, parasites are more common. As well as giardia, crypto and E. histolytica, there is a wide range of flatworms, roundworms, tapeworms, Strongyloides and flukes that can affect the gastrointestinal tract and other parts of the body. Travellers usually become infected by eating or drinking something contaminated with parasite eggs – even something as simple as ice cubes in a drink or a fresh salad – but some parasites can also enter through the skin, for example through the soles of the feet when walking barefoot.

The most common symptoms of a parasitic infection are persistent diarrhoea, bloody diarrhoea, mucus in the stool, abdominal pain and nausea. Patients may also have headache or fever, or few or insignificant symptoms.

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 collected, the stool sample should either be taken to the laboratory within an hour or transferred into special transport vials containing preservative solutions. Once the stool has been preserved, it should be taken to the laboratory as soon as possible for testing. If the doctor has ordered multiple samples, all the stool samples can be collected and transferred into the preservative vials before returning them to the laboratory. When multiple samples are requested, they should be collected at different times and on different days, because parasites are shed intermittently and may not be present in the stool at all times. Each vial should be labelled with the patient’s name and the date and time of collection.

How is it used?
When is it requested?
What does the result mean?
Is there anything else I should know?

How is it used?
The O&P test is used to diagnose the causes of persistent diarrhoea. It is requested to find out whether parasites are present in the gastrointestinal tract and, if so, to identify them. Because there are many other causes of diarrhoea, the O&P test is often requested together with other tests, such as a stool culture, which identifies pathogenic bacteria in the stool. Traditionally, O&P tests were requested in multiples, such as three samples from three different bowel movements, often on different days. This was considered the best way to detect possibly small numbers of eggs in the stool, giving a better chance of not missing an infection. The doctor may request a giardia, cryptosporidium or E. histolytica antigen test if one of these parasites is suspected of causing the diarrhoea. These tests detect protein structures in the parasites and can identify an infection even if no parasites or eggs are actually seen in the stool. Because antigen tests detect only a few specific parasites, they are not a substitute for a full O&P, which will detect a wider variety of parasites. O&P tests may also be requested to monitor the effectiveness of treatment for a parasitic infection.

When is it requested?
When you have persistent diarrhoea, abdominal pain and/or blood and mucus in the stool. When you have symptoms and have recently drunk water from a stream or lake, perhaps while camping, have been exposed to someone with a parasitic infection (such as a family member), or have travelled abroad.

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

What does the result mean?
If no eggs or parasites are seen, the diarrhoea may be due to another cause. It may also be that there are too few parasites to detect. The doctor may order further O&P tests and/or other tests to look for the cause of the symptoms.

If a parasite is identified, there is a parasitic infection. The type and length of treatment will depend on which parasite or parasites are found and on the patient’s general health. The number of parasites seen can give the doctor general information about the severity and extent of the infection.

Is there anything else I should know?
If diarrhoea lasts more than a few days, it can lead to dehydration and electrolyte imbalance, which are dangerous conditions in children and older people.

Medication is usually used to treat giardia and E. histolytica infections. They may clear up on their own within several weeks, but they can also become cyclical, with symptoms that ease and then get worse again. There is no effective treatment for cryptosporidium. In people with a healthy immune system, crypto usually clears up after a few weeks. In people with a weakened immune system (such as patients with HIV/AIDS, transplant recipients, cancer patients, etc.), crypto can be dangerous, becoming chronic and causing wasting and malnutrition.

Parasitic infections are monitored at community level. Apart from travel-related cases, public health officials want to find out where an infection came from so that any potential public health problem can be dealt with. For example, if crypto or giardia is due to contaminated swimming pool water or mains water, measures will need to be taken to stop the infection spreading. Parasites can be removed from mains water by chlorination and/or proper filtration.

1. How can I prevent a parasitic infection?
2. Will an O&P detect all parasites?
3. Why is a fresh stool sample needed?
4. Are the things I can see in my stool sample parasitic worms?
5. Are there other ways to test for parasites besides microscopic examination?
6. Once I have had a parasitic infection, can I be reinfected?
7. Why shouldn’t I take an over-the-counter antidiarrhoeal medicine?

1. How can I prevent a parasitic infection?
The best way is to avoid food and water that you suspect may be contaminated. This is particularly true if you travel to developing countries, where the ice in a drink or a salad at dinner can expose you to parasites. But even the clearest mountain stream should be treated with suspicion: it could be contaminated with giardia. Most parasites are invisible; you cannot even smell or taste them in water. If a family member has a parasitic infection, careful hand washing after using the toilet can help prevent the parasite spreading to others. The infected person should avoid preparing food for others until the symptoms have gone.

2. Will an O&P detect all parasites?
No, only those that live in the intestine and whose eggs are passed in the stool. There are other specific tests for other parasites, such as pinworms or the blood parasites that cause malaria.

3. Why is a fresh stool sample needed?
The structure of parasites can deteriorate in unpreserved stool, which would destroy the parasite’s identifying features and make the infection harder to detect.

4. Are the things I can see in my stool sample parasitic worms?
Most common parasites are too small to see with the naked eye. What you are probably seeing is undigested food fibre. The only way to be sure, however, is to examine the sample under the microscope. Parasites have characteristic internal and external structures that fibres do not have.

5. Are there other ways to test for parasites besides microscopic examination?
Yes, antigen tests have been developed for several common parasites, including giardia, cryptosporidium and E. histolytica. Antigen tests detect protein structures on the parasite and can detect fragments of the parasite in a stool sample. This has the advantage of allowing that particular parasite to be detected even if it is not seen on microscopic O&P examination. Blood antibody tests may be ordered to establish whether a person has been exposed to a parasite in the past. This may indicate a past or chronic infection but is not used to detect a current infection. Sometimes a biopsy of the small intestine is taken and the small amount of tissue is examined for parasitic infestation.

6. Once I have had a parasitic infection, can I be reinfected?
Yes, you can be reinfected if you are exposed again. This can happen if a family member has an asymptomatic parasitic infection, such as giardiasis (caused by giardia), and keeps shedding the organisms and reinfecting others, who will then need to be treated.

7. Why shouldn’t I take an over-the-counter antidiarrhoeal medicine?
You should only take one on your doctor’s advice. Diarrhoea is one of the ways the body helps to get rid of an infection. If you slow down or prevent this process by taking an antidiarrhoeal, you may prolong the illness and make the infection worse.