What methods are used to diagnose parasites in the body?

How to Diagnose Parasites in the Body

The diagnostic approach depends on the suspected parasite type and clinical presentation, but for most intestinal parasites, order a single stool specimen for antigen detection or PCR/molecular testing rather than traditional microscopy, while blood parasites require thick and thin blood films examined under oil immersion microscopy. 1, 2

Intestinal Parasites

First-Line Testing Approach

• Order parasite-specific antigen tests (EIA) or PCR/NAAT on a single stool specimen for suspected intestinal parasites like Giardia, Cryptosporidium, and Entamoeba histolytica, as these methods have sensitivity and specificity exceeding 95% 2

• Molecular amplification assays (PCR/NAAT) are the most sensitive tests currently available and should be performed, especially when other diagnostic testing is unrevealing 1

• Avoid routine ova and parasite (O&P) microscopy as the primary diagnostic method—it requires three specimens collected on different days, has low sensitivity (74% vs 96% for immunoassay), is labor-intensive, and costs approximately $1,836 per significant organism identified 2

Specimen Collection Requirements

• The optimal specimen is a diarrheal stool sample (one that takes the shape of the container) 2

• Fresh stool is preferred for protozoal agent identification 2

• For molecular testing, samples can be inoculated onto filter paper and dried for preservation and easy transport 1

When to Test for Specific Parasites

• Test for Giardia in patients with chronic diarrhea (lasting >7 days), returning travelers with persistent symptoms, exposure to contaminated water, daycare exposure, men who have sex with men, or immunodeficiency states 2, 3

• Consider Cryptosporidium testing using microscopy with immunofluorescence, which is both sensitive and specific 3

• For Entamoeba histolytica, microscopic identification in stool is necessary, but must be distinguished from non-pathogenic Entamoeba species using specific antigen detection or PCR-based assays when available 4

Blood Parasites (Malaria and Babesiosis)

Microscopy Requirements

• Prepare both thick and thin blood films stained with Giemsa (Wright-Giemsa and rapid Field stains are also acceptable) 1

• Screen manually at low power using 10× objective first to identify microfilariae, then examine under oil immersion at 100× magnification 1

• Examine a minimum of 100 microscopic fields on both thick and thin films before reporting negative; examine at least 300 fields for patients without previous Plasmodium exposure since they may be symptomatic at lower parasite levels 1

• Collect ≥3 specimens drawn 12-24 hours apart (ideally during febrile episodes) if initial film is negative and malaria or babesiosis is strongly suspected 1

Rapid Diagnostic Tests for Malaria

• Malaria rapid diagnostic tests (RDTs) like BinaxNow Malaria (the only FDA-approved test) can be used for initial screening when qualified technologists are unavailable or immediate diagnosis is required 1

• RDTs are somewhat less sensitive than thick blood films and may be falsely negative with very low parasitemia and non-falciparum infections 1

• All positive RDTs must be confirmed by thick and thin blood film examination within 12-24 hours to confirm species and calculate parasitemia 1

Critical Pitfall

• Babesia and Plasmodium may be indistinguishable on blood films and both can be transmitted by transfusion—if parasites are identified without expertise for species identification, report as "Plasmodium or Babesia parasites" and send for confirmatory testing at a reference laboratory (CDC provides rapid telediagnostic services) 1

Tissue Parasites (Leishmaniasis)

Diagnostic Specimen Collection

• Collect tissue aspirates or biopsy specimens for smears, histopathology, parasite culture, and molecular testing 1

• For visceral leishmaniasis, bone marrow aspiration is the preferred first source (52-85% sensitivity) as it is safer than splenic aspiration (93-99% sensitivity but risk of life-threatening hemorrhage) 1

• Contact reference laboratories (CDC, Walter Reed Army Institute of Research) before collecting specimens to optimize collection and transport 1

Multiple Diagnostic Approaches

• Use multiple diagnostic methods simultaneously to maximize detection: microscopy for amastigotes (with Giemsa or Wright-Giemsa stains), parasite culture, and molecular testing 1

• DNA-based assays (PCR) should be performed as they are the most sensitive assays for leishmaniasis diagnosis, particularly useful when few parasites are present or experienced microscopists are unavailable 1

• Microscopy requires visualization of amastigotes including cell membrane, cytoplasm, nucleus, and the characteristic extranuclear rod-shaped kinetoplast at ×1000 magnification with oil immersion 1

• Histopathology sensitivity is estimated at 50-70% for Old World species and 15-30% for New World species due to the 3-5 µm tissue section thickness making kinetoplast confirmation difficult 1

Schistosomiasis

• Conventional diagnosis relies on detection of eggs in feces or urine, though sensitivity may be low, especially in low-prevalence areas 5

• Newer antigen detection tests (circulating cathodic antigen [CCA] and circulating anodic antigen [CAA] in urine or serum) are more sensitive and may reveal 6-10 times more infections than conventional methods 5

Reference Laboratory Support

• For complex cases or species identification, utilize reference laboratories: CDC (http://www.cdc.gov/dpdx/contact.html), Walter Reed Army Institute of Research, or National Reference Centre for Parasitology in Montreal 1

• Reference laboratories can perform isoenzyme analysis and DNA-based assays for definitive species identification once parasites are isolated in culture 1

Key Pitfalls to Avoid

• Do not order routine O&P examination for patients without travel history to or recent immigration from high-risk areas—the yield is extremely low and cost-ineffective 2

• Do not rely on automated hematology analyzers for blood parasite detection—they may fail to detect Plasmodium and Babesia species 1

• Do not use serology alone for visceral leishmaniasis diagnosis (rK39-based tests may provide supportive evidence but are not stand-alone diagnostic tests) 1

• Molecular testing may detect colonization rather than active infection in some cases, making clinical correlation essential 2

• For Strongyloides, use concentration techniques (Baermann apparatus) when examining stool, as larvae per gram may be very low; conduct at least three stool examinations over three months post-treatment to ensure eradication 6