Specialist in Internal Medicine, Infectious Diseases, Tropical Medicine
A question I get very often: “Doctor Changa, do I have a parasite?”Parasitic infections are common especially in South and Southeast Asia, and remain under diagnosed for a number of reasons. Not all parasites are harmful; a few are considered commensals. However, for the traveler or the expatriate, even these commensals (aka ‘good guys’) can cause troublesome symptoms, while people living in these areas long term may not have any symptoms at all.
Diagnosing parasitic infections in stool samples can be challenging due to several factors:
1. Intermittent Shedding: Parasites may not be consistently present in the stool. The shedding of parasites can be intermittent, and a single stool sample may not capture the presence of parasites, leading to false negatives.
2. Low Parasite Burden: In some infections, the number of parasites in the stool may be low, making it difficult to detect them even with sophisticated diagnostic techniques.
3. Variability in Egg and Cyst Morphology: Parasite eggs and cysts can exhibit diverse morphologies (shapes and sizes). Identifying these structures accurately requires expertise, and variations in morphology may lead to misinterpretation or oversight.
4. Mimicry of Other Substances: Some non-parasitic structures or artifacts in the stool can resemble parasite eggs or cysts, leading to false-positive results. Distinguishing between genuine parasitic structures and artifacts requires careful examination.
5. Diagnostic Techniques: Traditional diagnostic methods, such as direct microscopy, may have limitations in sensitivity and specificity. Newer molecular techniques, like PCR (polymerase chain reaction), have improved sensitivity but may not be readily available in all settings.
6. Periods of Prepatency: There may be a delay between the time of infection and when the parasites start producing detectable forms (prepatent period). During this time, even if the person is infected, the parasites may not be detectable in the stool.
7. Multiplicity of Parasites: Infections with multiple parasite species are not uncommon. Identifying and distinguishing between different parasites in a mixed infection can complicate the diagnostic process.
8. Host Immune Response: The host's immune response can impact the presence and detection of parasites in stool samples. In some cases, the immune response may limit the release of parasite eggs or cysts into the stool.
9. Patient Compliance: Proper collection and handling of stool samples are crucial for accurate diagnosis. Patient non-compliance with sample collection instructions can lead to inadequate samples and affect the sensitivity of diagnostic tests.
10. Delays in sample processing: Stool samples much be analyzed for parasites right away (ideally within 15 minutes of collection). Samples that sit for hours on a collection counter, or delays in transportation, will cause the cysts to degenerate by the time the sample is examined, resulting in false-negative results.
11. Asymptomatic Carriers: Some individuals may harbor parasites without showing symptoms. In such cases, clinicians may not consider parasitic infections in the absence of clinical signs and symptoms, leading to underdiagnosis of carrier-status.
Due to these challenges, a combination of clinical evaluation, patient history, and multiple stool samples collected over several days may be necessary to increase the likelihood of accurately diagnosing parasitic infections. Additionally, advanced laboratory techniques and expertise are essential for reliable and precise identification of parasites in stool samples. Physicians who undergo training in countries with low incidence of parasitic infections are typically unfamiliar with the diagnosis and management of parasitic infections. As with all things, experienced and competent medical practitioners are a critical component in accurate diagnoses and successful treatment; this hold true especially in the case of parasitic infections.