How is an intestinal parasitic infection diagnosed and treated?

Diagnosis of Intestinal Worms

For most intestinal helminths (worms), collect three consecutive stool samples and perform concentrated stool microscopy with ova and parasite (O&P) examination including permanent stained smears—this is the gold standard diagnostic approach. 1, 2

Primary Diagnostic Strategy

Concentrated stool microscopy with O&P examination is the cornerstone test for identifying intestinal helminths including nematodes (roundworms like Ascaris, hookworm, Strongyloides), cestodes (tapeworms), and trematodes (flukes). 1, 2

Critical Collection Requirements

• Collect at least 3 consecutive stool specimens over consecutive days, as parasite shedding is intermittent and sensitivity increases dramatically with repeated testing. 2, 3 A single negative stool test does NOT rule out parasitic infection. 2

• The optimal specimen is a diarrheal stool sample (one that takes the shape of the container). 1 Fresh stool is preferred for protozoal agents. 1

• Permanent stained smears (such as trichrome stain) must be included in addition to wet mounts, as this significantly improves detection rates and allows for species identification. 1, 2

Species-Specific Diagnostic Modifications

Pinworm (Enterobius vermicularis)

Do NOT use routine stool O&P for pinworm diagnosis—this is a common error that leads to missed diagnoses. 2, 4 Instead:

• Perform the cellophane tape test (sellotape test) by placing the sticky side of tape on the perianal skin in the morning before bathing, then examining it under microscopy for ova. 1, 4 This is the preferred diagnostic method because gravid female worms migrate to the perianal region at night to lay eggs. 4

• Eggs and adult worms are not typically passed in stool, making stool examination ineffective. 4

Tapeworms (Taenia species)

• Look specifically for eggs or proglottids (segments) in concentrated stool microscopy. 1, 2 Eggs are eliminated intermittently, so repeat specimens increase diagnostic yield. 1

• Species identification is critical—distinguish between Taenia saginata (beef tapeworm) and T. solium (pork tapeworm), as T. solium carries risk of neurocysticercosis. 1, 2

• If T. solium is identified or suspected, order cysticercosis serology to assess for systemic involvement. 1, 2 Do NOT treat with praziquantel without first excluding neurocysticercosis, as killing intestinal worms may worsen CNS disease. 2

Strongyloides stercoralis

For Strongyloides, serology is recommended in addition to stool testing, as concentrated stool microscopy has very low sensitivity. 2 Specialized techniques may be required:

• Detection in stool may require the Baermann technique or agar plate culture. 1

• Specialized Strongyloides stool culture or fecal PCR are more sensitive than routine microscopy. 2

Advanced Molecular Testing

Nucleic acid amplification tests (NAAT) or multipanel gastrointestinal PCR assays may be used when available and can detect multiple parasites simultaneously. 1, 2 However:

• Clinical correlation is essential when interpreting NAAT results, as these assays detect DNA and not necessarily viable organisms. 1, 2

• NAAT is particularly useful for organisms difficult to detect by microscopy, such as Strongyloides and certain protozoa. 2

Additional Testing Based on Clinical Context

For Patients with Eosinophilia or Systemic Symptoms

• Perform complete blood count with differential to assess for eosinophilia, which is common in tissue-migrating helminths. 2

• Consider serology for schistosomiasis, strongyloidiasis, or other tissue-invasive parasites if there is travel history to endemic areas. 2

For Immunocompromised Patients

A broader parasitic workup is essential, including testing for Cryptosporidium, Cyclospora, Cystoisospora, and microsporidia in addition to helminths. 1, 2

Treatment Considerations

Once diagnosis is confirmed:

• Ascaris: Albendazole 400 mg single dose 1
• Hookworm: Albendazole 400 mg single dose 1
• Pinworm: Albendazole 400 mg or mebendazole 100 mg single dose 1
• Tapeworm (T. saginata): Praziquantel 10 mg/kg single dose 1
• Strongyloides: Ivermectin 200 mcg/kg is FDA-approved for intestinal strongyloidiasis 5

Public Health Reporting

Tapeworm cases, particularly T. solium, should be reported to local public health authorities, and household contacts should be screened. 2 All specimens testing positive for bacterial pathogens should be cultured to ensure outbreak detection. 1