Ova and Parasite exam (O&P)

Useful For

For the differential diagnosis of community acquired and travel associated acute and chronic diarrhea including colitis, rectocolitis, and dysentery.


Reflex Tests

Reporting Name Available separately Always performed
Iodine Wet mount YES NO
Trichrome Stain YES NO
Cryptosporidium IFA NO YES
Giardia IFA NO YES
Modified acid fast for Isospora and Cyclospora YES NO

Testing Algorithm

Stool specimens submitted in preservative are subjected to microscopy employing Trichrome staining (permanent stain) and an iodine wet mount from concentrated sediment for detection of Entamoeba histolitica and other intestinal parasites. All stool specimens are subjected to immunofluorescent stain targetting Cryptosporidium and Giardia.


If Isospora, Cyclospora, or Sarcocystis is suspected please contact the laboratory to request modified acid fast staining.


Indications for Testing

Community acquired and travel associated acute or chronic diarrhea including colitis, rectocolitis, dysentery, of acute and chronic presentation.

In-patients admitted to healthcare facilities for > 3 days are unlikely to present due to parasitic agents, O&P exam for in-patients is not indicated.


Method Name



Reporting Name

Ova and Parasite exam









Giardia lamblia

Giardia intestinalis

Giardia duodenalis

Beaver fever

Cryptosporidium parvum



Specimen Required

Stool specimens preserved in 10% formalin, sodium acetate-acetic acid-formalin (SAF) or ECOFIX® should be used.

At least 3 specimens, passed at intervals of 2 to 3 days to rule out most infections. If amoebic dysentery is suspected up to 6 specimens may be submitted. Specimens should not be withheld to submit in a batch as this will delay diagnosis.


Fecal specimens should be collected prior to administration of antibiotics and anti-diarrhea medications. Avoid use of barium, bismuth and mineral oil as these substances may interfere with the detection and identification of intestinal parasites.


Specimen Minimum Volume

5 ml


Transport Temperature

Specimen Room temperature Refrigerated Frozen
Preserved stool YES YES NO


Reject Due To

Specimens other than 10% formalin/SAF/Ecofix


Useful For

For the differential diagnosis of community acquired and travel associated acute and chronic diarrhea including colitis, rectocolitis, and dysentery.


Clinical Information

Diarrhea is defined by increased stool frequency, liquidity, or volume. For most individuals, the essential characteristic of diarrhea is the passage of loose stools. It is important to recognize that diarrhea is a symptom or sign, and can be caused by numerous conditions, infectious or non-infectious.


Acute diarrhea by definition lasts < 2 weeks and typically does not require stool testing. However, stool testing is indicated in acute diarrhea in the presence of certain clinical or epidemiological features, including age >65 years, immune compromise, volume depletion, hematochezia (blood-tinged stool), fever, severe abdominal pain, known or suspected inflammatory bowel disease, community infectious disease outbreaks, and employment as a food handler.


Chronic diarrhea (> 4 weeks duration) typically warrants a diagnostic evaluation, is less likely to resolve on its own, and presents a broad differential diagnosis. Red flag symptoms such as nocturnal diarrhea, weight loss, and gross blood in the stool should be soughts out, as should systemic symptons such as fevers, rashes, or joint pains, which indicate inflammatory bowel disease, or infections such as Whipple’s disease. A trabel history must be obtained to assist diagnosis.


Chronic diarrhea can be classified as inflammatory, fatty, or watery. Non-infectious causes must be considered in the differential diagnosis. Infectious agents causing inflammatory diarrhea may be bacterial (Clostridium difficile, Salmonella, Aeromonas, Pleisiomonas, Yersinia, and Mycobacteria) or parasitic (amoebiasis, Strongyloides stercoralis, Schistosoma, Trichuris trichuria). Fatty diarrhea may be caused Giardia, Cryptosporidium, Cyclospora, and very rarely Whipple’s disease. Watery diarrhea may be caused by the parasitic agents Giardia, Cryptosporidium, Cyclospora, and Isospora.


Amoebiasis. The clinical spectrum of intestinal E. histolytica infection ranges from an asymptomatic carrier state and acute colitis to fulminant colitis with perforation, depending on the host’s nutritional status and susceptibility, including age and virulence of infecting strain. Invasive intestinal amebiasis usually manifests as an acute rectocolitis. Most patients present with a nontoxic dysenteric syndrome, and constitutional symptoms are not as prominent as in Shigella dysentery. The onset of acute rectocolitis is gradual and 85% of patients have intense abdominal pain. Initially there are loose watery stools, but these rapidly become blood-stained and contain mucus. Tenesmus occurs in 50% of patients and is always associated with rectosigmoidal involvement. Watery diarrhea or loose stools without blood may be present for a few days, particularly if the distal colon is involved. Entamoeba histolytica and Giardia lamblia infections are two of the most common protozoal infections seen worldwide. The infection is acquired by ingestion of cysts in fecally contaminated food or water, or sexually (particularly in men who have sex with men). Excystation and infection occur in the large intestine from where trophozoites attach to the intestinal wall and liberate extracellular enzymes which enable invasion of the mucosa and facilitates spread to other organs, especially the liver and lung where abscesses develop. Diagnosis of invasive amebiasis, in particular amebic liver abscess, is supported by E. histolytica-specific serology.


Giardiasis. Giardia intestinalis/lamblia is an intestinal flagellate protozoan that infects the biliary tract and upper small intestine of both humans and animals. It is the most common cause of intestinal parasitosis in humans worldwide and is the causative organism of Giardiasis, sometimes known as “beaver fever.” After an incubation period of approximately 12 – 20 days, patients can experience nausea, chills, low-grade fever, epigastric pain, and a sudden onset of watery diarrhea. Diarrhea is often explosive and presents as foul smelling without the presence of blood, cellular exudates, or mucus. Individuals can develop subacute or chronic infections with symptoms such as recurrent diarrhea, abdominal discomfort and distention, belching, and heartburn. Humans are the principal reservoir but Giardia organisms can infect beavers, dogs, cats, and other animals. These animals can contaminate water with feces containing cysts that are infectious for humans. A Giardia vaccine is available for dogs and cats and may affect the prevalence in human infections. Giardiasis is passed via the fecal-oral route. People become infected directly by ingestion of cysts from the feces of an infected person or indirectly by ingestion of fecally contaminated water or food. Person-to-person transmission is common where personal hygiene may be poor. Children who are not toilet trained are often linked to day care and family outbreaks. The mean annual incidence reported in Canada from 2000 to 2004 was 14.6 per 100,000 population and for the same period the mean incidence rate in Newfoundland Labrador was 7.8 per 100,000.


Cryptosporidiosis. Cryptosporidium spp. are intracellular parasites that primarily infect epithelial cells of the stomach, intestine, and the biliary ducts. In severely immunocompromised persons, the respiratory tract is sometimes involved. Cryptosporidium spp. have a worldwide distribution, and their oocysts are ubiquitously present in the environment. Transmission of C. hominis is mostly from person-to-person (anthroponotic) where as C. parvum is typically zoonotic from livestock to humans. Exposure occurs through fecal-oral transmission by animal to human, waterborne (typically spring and late summer due to recreational water exposure), foodborne (mostly associated with fruits, vegetables, shellfish, and unpasteurized apple cider and milk) and person-to-person. Cryptosporidiosis occurs in patients with acquired immunodeficiency syndrome (AIDS) and as a self-limited moderate diarrhea in young children, especially daycare attendees and their relatives. Cryptosporidiosis can cause a profuse and watery diarrhea, abdominal cramps, anorexia, fever, nausea, general malaise and vomiting. The disease course can be variable, ranging from a self-limiting diarrhea to more severe and protracted syndrome more commonly seen in immunocompromised patient populations. The disease occurs worldwide and has become recognized as one of the most common causes of waterborne disease in humans in North America. Extensive waterborne outbreaks have been associated with contamination of drinking water; exposure to contaminated recreational water including swimming pools, water slides, hot tubs, and lakes; and consumption of contaminated beverages.


Reference Values





E. histolytica (pathogenic), E. dispar (nonpathogenic) and E. moshkovskii (nonpathogenic) may be indistinguishable by microscopy. In addition, asymptomatic E. histolytica often occur, complicating interpretation of microscopic findings. On occasion, ingested red blood cells may be detectable in E. histolytica (only) aiding definitive identification of pathogenic E. histolytica.


The role of Blastocystis hominis in human disease in controversial. When B. hominis  is present in large numbers in the absence of other pathogens, it may be the cause of gastrointestinal disease. The most common symptom is recurrent diarrhea without fever, vomiting, and abdominal pain. B. hominis is the only intestinal pathogen that is reported quantitatively to aid the clinician in determining its clinical significance.


The detection of the nonpathogenic amoeba (Endolimax nana, Entamoeba hartmanni, Entaboema coli, Dientamoeba fragilis, Iodamoeba bütschlii) may be considered evidence of exposure to contaminated food or water.



The majority of individuals infected with Giardia intestinalis/lamblia are asymptomatic. However, acute giardia may mimic other protozoan, viral, and bacterial pathogens. Diarrhea is typically eplosive and presents as foul smelling without the presence of blood, cellular exudate, or mucus. Individuals can develop subacute or chronic infections with symptoms such as recurrent diarrhea, abdominal discomfort and distention, belching, and heartburn. In patients with chronic cases of giardiasis, diarrhea can lead to dehydration, malabsorption, and impairment of pancreatic function.


The frequency of symptomatic disease in those infected with Dientamoeba fragilis ranges from 15 – 25% in adults, and symptomatic disease is more common in children, in whom up to 90% of those infected have clinical signs. It should be noted that the organism is isolated from patients with no apparent clinical symptoms. Chilomastix mesnili is not considered a pathogen, but its presence suggests exposure to contaminated food or water.



Infection with Balantidium coli is most often asymptomatic however, symptomatic infection can occur, resulting in bouts of dysentery similar to amebiasis.



Clinical Reference

Leber, A. L., and Novak-Weekley, S. 2007. Intestinal and Urogenital Amebae, Flagellates, and Ciliates, p. 2092-2112. In Murray, P. R., Baron, E. J., Jorgensen, J. H., Landry, M. L., and Pfaller, M. A. Manual of Clinical Microbiology, 9th ed., vol. 2. ASM Press, American Society for Microbiology, Washington, DC.


Petri, Jr., W. A., and Haque, R. 2010. Entamoeba Species, Including Amebiasis, p. 3411-3425. In Mandell, D., Bennett, J. E., and Dolin, R. Principles and practice of infectious diseases, 7th ed., vol. 2. Churchill Livingstone, Elsevier, Philadelphia, PA.


Martínez-Palomo, A. and Espinosa-Cantellano M. 1999. Amebiasis and other protozoan infections, 6.25.1 – 6.25.4. In Armstrong, D. and Cohen, J. Infectious Diseases, vol. 2. Mosby, Harcourt Publishers Ltd.


Sweetser, S. 2012. Evaluating the patient with diarrhea: a case-based approach. Mayo Clin Proc. 87(6):596-602.

Ova, Cyst and Parasite (O&P exam)

Status Days Analytic Time Maximum Laboratory Time Specimen Retention
Routine Monday – Friday 24h 48h 1 week


Method Description

Stool specimens are examined microscopically subsequent to Wheatly’s Trichrome stain, Cryptosporidium/Giardia immunofluorescent stain; and concentrated prior to staining with iodine for a wet mount.


Performing Laboratory Location

Newfoundland & Labrador Public Health Laboratory

St. John’s



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