The stool examination is a laboratory study in which stool (feces) is examined in order to detect intestinal parasites. It is one of the simplest and oldest laboratory techniques, being initially developed by Anton Van Leeuwenhoek in the 18th century.
Anton Van Leeuwenhoek, considered the father of microbiology, used the "direct" coproparasitoscopic method to observe his own feces and described what years later was identified as the trophozoites of Giardia lamblia, a protozoan that invades the small intestine of man.
Ascaris lumbricoides adult form (Source: Wikimedia Commons)
Parasitic diseases affect millions of people in the world, especially in poor or underdeveloped countries, where there are poor sanitary conditions related to the disposal of excreta and the consumption of contaminated water.
The diagnosis of these diseases is important for an adequate treatment, being the coproparasitoscopic examination is an indispensable tool for it. It is a simple, fast and inexpensive laboratory test.
The stool examination includes several techniques that, in addition to allowing direct visualization and quantification of eggs, trophozoites, cysts, or larvae, allow the structures of the microorganism to be identified and thus identify the parasite.
Techniques used for coproparasitoscopic examination include methylene blue staining techniques, concentration methods, Faust, Richie technique, sedimentation techniques, and direct, single, or serial examinations.
Unfertilized egg of Ascaris lumbricoides. (Source: Centers for Disease Control and Prevention, via Wikimedia Commons).
Sampling
To carry out this study, the patient is required to take a fresh stool sample that is not contaminated with urine, water, blood (menstrual) or soil. The sample must be the size of a walnut or, if it is liquid, it must be at least the volume corresponding to two tablespoons.
The patient must not have taken parasiticidal medications for at least three days prior to taking the sample or for the period indicated by their doctor. You also shouldn't use laxative medications.
Samples should be placed in a dry, wide-mouthed, lidded container or a specially designed disposable container (available from your preferred pharmacy). Samples should be placed in a cool environment, should not be refrigerated for more than 24 hours, and should not be stored near heat sources or frozen.
When the indicated examination is serial, at least three samples are required, which must be taken every 24 hours or more, as indicated by the physician. For these cases, laboratories generally provide a set of flasks containing solutions with preservatives.
When the patient observes some "worm" in the stool, if possible, he should place it in a closed bottle with water and take it to the laboratory together with the bottle of the stool sample.
The vials with the samples or with the "worms" must be labeled and identified with the name of the patient, his age, sex and the date and time of the sample collection.
It is very important to adequately instruct the patient on all these aspects of taking and handling the samples, since it depends on this that the elements that may exist in said sample remain viable for observation, identification and diagnosis.
Techniques
There are direct coproparasitoscopic examinations and suspension and concentration techniques of the samples that are used many times to avoid false negatives and observe much cleaner samples. Some staining techniques are also used to identify certain parasites.
Direct exams
The direct stool examination, using the pending drop technique, consists of diluting the stool sample with physiological solution (0.9% NaCl) and placing a drop of this solution in a concavity that has a special used slide. for that end.
Once the drop is placed on the slide, it is covered with a coverslip and observed under the microscope. This technique allows us to observe eggs and cysts, but it also allows us to observe any mobile element such as flagellates, larvae, trophozoites, ciliates, etc.
Suspension and concentration techniques
Suspension techniques use a solution that is denser than the elements to be observed, so that these float on the surface of the liquid and can be collected, since they remain concentrated in the surface layer of the solution.
This technique has the advantage that it allows to have a fairly clean sample of debris, since these, being more dense, remain at the bottom of the bottle. The relative disadvantage is that the solution shrinks and deforms the microorganisms in a short time.
These methods are not used for helminth and cestode eggs because they are very heavy and do not float in these solutions. They are widely used to observe protozoa in their tropozoic form or their eggs and for the observation of larvae, such as those of Strongyloides stercoralis.
Another technique widely used because it does not deform the microorganisms in the sample and is simple and inexpensive is the formalin sedimentation technique.
Examples of concentration techniques include Faust's and Richie's techniques.
The different techniques that allow the microscopic visualization of the eggs, larvae or other elements of the different intestinal parasites, combined with the staining techniques, allow the identification and diagnosis of these diseases.
Examples
Next, a clinical case is described and some images are shown that illustrate the usefulness of the stool examination for diagnosis and evaluation of the benefits of treatment.
Trichuris trichuria egg (Source: CDC / Dr. Mae Melvin, Courtesy: Public Health Image Library via Wikimedia Commons)
An 18-year-old male patient came to the doctor's office for colicky abdominal pain, more intense in the periumbilical area, nausea and episodes of watery diarrhea.
When questioning the patient, the doctor noted two salient points: 1) the patient reported having bathed in a lake in a rural area and 2) he was struck by the fact that his feces floated in the toilet. After examining the patient, the doctor suspects the presence of Giardia lamblia.
Giardia lamblia life cycle (Source: LadyofHats via Wikimedia Commons)
This protozoan is housed in the small intestine of man and interferes with the absorption of fat, which generates very greasy stools that tend to float. Pollution frequently occurs from polluted water in lakes or streams in rural areas or from poorly maintained pools or hot tubs.
Giardia lamblia trophozoites (Source: Photo by Eva Nohýnková, Department of Tropical Medicine, 1st Faculty of Medicine, Charles University in Prague and Hospital Bulovka, Czech Republic. Image in paper by Marie Lipoldova, Laboratory of Molecular and Cellular Immunology, Institute of Molecular Genetics, Academy of Sciences of the Czech Republic, Prague, Czech Republic via Wikimedia Commons)
The doctor orders a stool examination and the results confirm the presence of Giardia lamblia. After the end of the treatment, another stool examination is indicated that confirms the absence of cysts or trophozoites of Giardia lamblia.
References
- Buonfrate, D., Mena, MA, Angheben, A., Requena-Mendez, A., Muñoz, J., Gobbi, F.,… & COHEMI Project Study Group. (2015). Prevalence of strongyloidiasis in Latin America: a systematic review of the literature. Epidemiology & Infection, 143 (3), 452-460.
- by Haro Arteaga, I., & Ruiz, AEC (2014). Diagnosis of. Medical parasitology (4a, 347.
- Mendoza, D., Nunez, FA, Escobedo, AA, Pelayo, L., Fernandez, M., Torres, D., & Cordovi, RA (2003). Usefulness of 2 coproparasitological methods and their utilization in an antigiardiasis therapeutic trial. Cuban Journal of Tropical Medicine, 55 (3), 174-178.
- Price, -DL (2017). Procedure manual for the diagnosis of intestinal parasites. CRC Press.
- Sahin, I., Kiliç, H., Ozca, M., & Orhan, R. (1984). A copro-parasitological study on the wrestlers of the national team. Mikrobiyoloji bülteni, 18 (2), 114-118.