Specimen Sources
To catch these tiny parasite invaders, we need to become detectives and know where to look. That’s where specimen sources come in. Are we talking about parasites lurking in the gut? Then it’s stool samples we need. Suspecting a blood-borne infection? Time to draw blood. Or maybe the parasite has invaded tissues, requiring a biopsy. In this section, we’ll see the common specimen sources – stool, blood, tissue, respiratory samples, and more
The Importance of Specimen Source
- Why it Matters: The right specimen is crucial. You could have the best microscope skills, but if you’re looking in the wrong place, you’ll miss the parasite
- Parasite Location: Parasites have preferred hangouts. Some live in the intestines, others in the blood, and some burrow into tissues. Knowing this helps guide your specimen selection
- Timing is Key: Parasite life cycles vary. Some are more easily detectable at certain times
Common Specimen Sources & What They Reveal
Stool, The King of Parasitology Specimens
- What We’re Looking For: Protozoa (trophozoites and cysts), helminth eggs and larvae
- Common Suspects: Giardia lamblia, Entamoeba histolytica, Ascaris lumbricoides, hookworm, Strongyloides stercoralis.
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Collection Considerations
- Number of Specimens: Usually, a series of 3 stool specimens collected over 10 days is recommended. Why? Parasite shedding can be intermittent
- Preservatives: Crucial for preserving parasite morphology. Common ones include formalin (for concentration) and PVA (for permanent stains)
- Consistency Matters: Liquid stools are more likely to contain trophozoites (the active, feeding stage), while formed stools are more likely to contain cysts (the dormant, infective stage)
Blood, A Window into Systemic Infections
- What We’re Looking For: Blood-borne protozoa and microfilariae (larval worms)
- Common Suspects: Plasmodium spp. (malaria), Trypanosoma spp. (sleeping sickness, Chagas disease), Wuchereria bancrofti (filariasis)
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Collection Considerations
- Timing: For malaria, collect blood during fever spikes when parasites are more numerous. For filariasis, nocturnal blood draws are often best as microfilariae may exhibit nighttime periodicity
- Anticoagulants: EDTA is generally preferred
- Smears: Both thick and thin blood smears are prepared. Thick smears concentrate parasites for increased detection, while thin smears preserve morphology for identification
Tissue, For the Invasive Parasites
- What We’re Looking For: Parasites that have invaded organs or tissues
- Common Suspects: Toxoplasma gondii (toxoplasmosis), Trichinella spiralis (trichinosis), Leishmania spp. (leishmaniasis)
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Collection Considerations
- Source: Biopsies from affected organs (e.g., liver, muscle, brain)
- Processing: Tissue is typically fixed, sectioned, and stained (e.g., with Giemsa or trichrome) for microscopic examination
Respiratory Specimens, When Parasites Go Airborne (Almost!)
- What We’re Looking For: Parasites that can infect the lungs
- Common Suspects: Pneumocystis jirovecii (pneumocystis pneumonia), Paragonimus westermani (lung fluke), Strongyloides stercoralis (in disseminated infections)
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Collection Considerations
- Specimen Types: Sputum, bronchoalveolar lavage (BAL), tracheal aspirates
- Processing: Staining is critical. Pneumocystis requires special stains like GMS or DFA
Key Takeaways
- Know Your Parasites: Understand where different parasites are likely to be found in the body
- Proper Collection is Paramount: Follow established protocols for specimen collection, preservation, and transport
- Communication is Key: Work closely with clinicians to understand the patient’s symptoms and travel history. This helps narrow down the list of potential parasites and guides specimen selection
Key Terms
- Specimen A sample of body fluid or tissue taken for diagnostic testing
- Stool Fecal matter; a primary specimen for detecting intestinal parasites
- Blood Smear A thin film of blood spread on a glass slide for microscopic examination
- Biopsy The removal and examination of a tissue sample from a living body for diagnostic purposes
- Respiratory Specimen Material collected from the respiratory tract, such as sputum or bronchoalveolar lavage
- Preservative A substance added to a specimen to prevent its decomposition and maintain the integrity of the parasites
- Trophozoite The active, feeding, and motile stage of a protozoan parasite
- Cyst The dormant, non-motile, and infective stage of a protozoan parasite
- Microfilariae The larval stage of filarial worms that circulate in the bloodstream
- Bronchoalveolar Lavage (BAL) A procedure in which fluid is injected into the lungs and then collected for examination