Ova and Parasites Examination (008623)

Establish the diagnosis of parasitic infestation.

One negative result does not rule out the possibility of parasitic infestation. Stool examination for Giardia may be negative in early stages of infection, in patients who shed organisms cyclically, and in chronic infections.¹ The sensitivity of microscopic methods for the detection of Giardia range from 46% to 95%.² Tests for Giardia antigen may have a higher yield.³

Administration of barium, bismuth, Metamucil®, castor oil, mineral oil, tetracycline therapy, administration of antiamebic drugs within one week prior to test. Purgation contraindicated for pregnancy, ulcerative colitis, cardiovascular disease, child younger than five years of age, appendicitis or possible appendicitis.

Amebas and certain other parasites cannot be seen in stools containing barium. Amebic cysts, Giardia cysts, and helminth eggs are often recovered from formed stools. Mushy or liquid stools (either normally passed or obtained by purgation) often yield trophozoites. Purgation does not enhance the yield of Giardia. Formalin will preserve protozoan cysts and larvae and the eggs of helminths. It is used for concentration procedures. PVA will preserve the trophozoite stage of protozoa. A trichrome-stained smear may be prepared from PVA-fixed material. PVA cannot be concentrated; therefore, they should always be accompanied by a portion of the specimen in formalin.

Parasites commonly identified in the stool of AIDS patients include Cryptosporidium, Isospora, Entamoeba histolytica, and Giardia lamblia.

The pathogenic nature of Blastocystis hominis, which is commonly observed in stool of healthy and symptomatic patients, is controversial. A review of the literature by Miller and Minshew indicated that there was no convincing proof of a causal relationship between B hominis and symptoms, that there was no correlation between resolution of symptoms with therapy or with the disappearance of the organism from stool, and that treatment directed at the indication of B hominis is not indicated.⁴

Doyle et al have observed a role for Blastocystis in acute and chronic gastroenteritis but are unable to conclude whether the role is one of association or causation.⁵

In a large children's hospital study of nosocomial diarrhea rotavirus, C difficile and enteric adenovirus were recovered. Stool for ova and parasites and bacterial stool cultures yielded no pathogens.⁶ Optimal diagnostic yield is obtained by the examination of fresh, warm stool by an experienced technologist. Formalin will preserve protozoan cysts and larvae and the eggs of helminths. It is used for concentration procedures. PVA will preserve the trophozoite stage of protozoa. A trichrome-stained smear may be prepared from PVA-fixed material. Specimens submitted in PVA cannot be concentrated; therefore, they should always be accompanied by a portion of the specimen in formalin. Formed stools may be preserved in formalin or refrigerated in a secure container until they can be added to the formalin and PVA container for transport to the laboratory.

1. Brooke MM, Melvin DM. Morphology of Diagnostic Stages of Intestinal Parasites of Humans. 2nd ed. Atlanta, Ga: US Department of Health and Human Services. Centers for Disease Control and Prevention; 1989. CDC Publication N° 89-8116.

2. Janoff EN, Craft JC, Pickering LK. Diagnosis of Giardia lamblia infections by detection of parasite-specific antigens. J Clin Microbiol. 1989 Mar; 27(3):431-435. PubMed 2715318

3. Chappell CL, Matson CC. Giardia antigen detection in patients with chronic gastrointestinal disturbances. J Fam Pract. 1992 Jul; 35(1):49-53. PubMed 1613475

4. Miller RA, Minshew BH. Blastocystis hominis: An organism in search of a disease. Rev Infect Dis. 1988 Sep-Oct; 10(5):930-938 (review). PubMed 3055191

5. Doyle PW, Helgason MM, Mathias RG, Proctor EM. Epidemiology and pathogenicity of Blastocystis hominis.J Clin Microbiol. 1990 Jan; 28(1):116-121. PubMed 2298869

6. Brady MT, Pacini DL, Budde CT, Connell MJ. Diagnostic studies of nosocomial diarrhea in children: Assessing their use and value. Am J Infect Control. 1989 Apr; 17(2):77-82. PubMed 2729660

Brasitus TA. Parasites and malabsorption. J Clin Gastroenterol. 1983 May; 12(2):495-510. PubMed 6409470

Patterson M, Schoppe LE. The presentation of amoebiasis. Med Clin North Am. 1982 May; 66(3):689-705 (review). PubMed 6281593

René E, Marche C, Regnier B, et al. Intestinal infections in patients with acquired immunodeficiency syndrome: A prospective study in 132 patients. Dig Dis Sci. 1989 May; 34(5):773-780. PubMed 2714152

Smith JW, Wolfe MS. Giardiasis. Annu Rev Med. 1980; 31:373-783. PubMed 6994619