What is the recommended diagnostic work‑up and first‑line treatment for common intestinal parasites, including considerations for pregnancy and immunocompromised patients?

Intestinal Parasites: Diagnostic Work-Up and First-Line Treatment

Diagnostic Approach

For suspected intestinal parasites, obtain concentrated stool microscopy as the primary diagnostic test, with specific testing tailored to clinical presentation and exposure history. 1

Initial Diagnostic Testing

• Collect a single diarrheal stool specimen for optimal laboratory diagnosis of intestinal parasites, as this provides the highest diagnostic yield 2
• Request concentrated stool microscopy for most helminth infections including Ascaris, hookworm, Strongyloides, and tapeworms 1
• Order faecal PCR when available for improved sensitivity, particularly for Strongyloides and other helminths 1
• Perform perianal swab or adhesive tape test for suspected pinworm (Enterobius vermicularis) rather than stool examination, as eggs are deposited perianally 1

Protozoal Parasite Testing

• Use stool microscopy with direct fluorescent antibody testing for Giardia lamblia diagnosis, which provides optimal sensitivity and specificity 3
• Employ microscopy with immunofluorescence for Cryptosporidium, as this method is both sensitive and specific 3
• Utilize microscopy or PCR assays for Cyclospora diagnosis in patients with persistent diarrhea 3

Serology Indications

• Order serology for Strongyloides in patients with eosinophilia and negative stool microscopy, as sensitivity reaches 94-98% 1
• Request serology for schistosomiasis when geographic exposure history suggests risk, with sensitivity of 90-99% 1
• Obtain serum antibody testing for Trichinella infection when clinical presentation suggests this diagnosis 3

Special Diagnostic Considerations

• Exclude Loa loa infection through appropriate testing in patients from endemic regions BEFORE administering ivermectin, as treatment can cause severe adverse reactions 1
• Perform repeat stool examinations for tapeworm diagnosis, as eggs are eliminated intermittently and multiple specimens increase diagnostic yield 1

First-Line Treatment by Parasite

Geohelminths (Soil-Transmitted Helminths)

Albendazole 400 mg single dose is first-line treatment for Ascaris, hookworm, and uncomplicated Strongyloides in immunocompetent patients 1

• Mebendazole 500 mg single dose serves as an alternative for Ascaris and hookworm 1
• Ivermectin 200 μg/kg single dose is equally effective for these geohelminths and can be combined with albendazole for empiric treatment of eosinophilia with negative stool microscopy 1

Strongyloides (Critical for Immunocompromised)

Ivermectin 200 μg/kg single dose for immunocompetent patients; for immunocompromised patients, administer ivermectin 200 μg/kg on days 1,2,15, and 16 to prevent hyperinfection syndrome 1

• Seek specialist advice for hyperinfection syndrome, which requires prolonged treatment and may present with negative serology 1
• Screen all immunocompromised patients (including those on steroids, chemotherapy, or with HTLV-1) for Strongyloides before initiating immunosuppression, as hyperinfection carries high mortality 1

Pinworm (Enterobius vermicularis)

Albendazole 400 mg single dose, repeated in 2 weeks is first-line treatment 1

• Pyrantel pamoate is an equally effective alternative 3
• Treat all household members simultaneously to prevent reinfection 4

Tapeworms

Praziquantel 10 mg/kg single dose for Taenia saginata and T. solium intestinal infection 1

• Identify the tapeworm species when possible, as T. solium may coexist with neurocysticercosis requiring different management 1
• Consider cysticercosis serology if T. solium is identified or species cannot be determined 1
• Niclosamide 2 g is an alternative when species identification is not possible 1
• Praziquantel 25 mg/kg single dose for Hymenolepis nana (dwarf tapeworm), which requires higher dosing 1

Schistosomiasis

Praziquantel 40 mg/kg single dose for S. mansoni and S. haematobium 1

• Praziquantel 60 mg/kg in two divided doses for S. japonicum 1
• Repeat treatment at 8 weeks to treat residual worms that have matured to adults, as eggs and immature schistosomulae are relatively resistant 1

Protozoal Infections

Metronidazole is first-line treatment for Giardia lamblia and Entamoeba histolytica 3, 4

• Nitazoxanide or tinidazole are effective alternatives for Giardia 3
• Nitazoxanide is the treatment of choice for Cryptosporidium when symptoms persist beyond 2 weeks, though infection is often self-resolving in immunocompetent patients 3
• Sulfamethoxazole/trimethoprim (co-trimoxazole 960 mg twice daily for 7 days) for Cyclospora in patients with persistent diarrhea 1, 3

Special Populations

Pregnancy

Defer antiparasitic treatment during pregnancy unless clinically or epidemiologically necessary, as most intestinal parasites do not cause serious medical or obstetric complications with adequate nutrition and prenatal monitoring 5

• Provide supportive care, adequate nutrition, and careful monitoring rather than chemotherapy for most intestinal parasites during pregnancy 5
• Consult Table 5 in the UK guidelines for specific pregnancy and lactation safety information when treatment is deemed necessary 1

Immunocompromised Patients

Perform broad differential diagnosis with stool culture, viral studies, and examination for parasites in all immunocompromised patients with diarrhea 2

• Test for Cryptosporidium, Cyclospora, Cystoisospora, Microsporidia, Mycobacterium avium complex, and Cytomegalovirus in AIDS patients with persistent diarrhea 2
• Use extended ivermectin regimens (days 1,2,15,16) for Strongyloides in immunocompromised patients to prevent hyperinfection 1
• Seek expert advice for all immunocompromised patients with parasitic infections, as treatment duration and regimens often require modification 1

Empiric Treatment Considerations

Administer albendazole 400 mg plus ivermectin 200 μg/kg for empiric treatment of possible prepatent or undetected geohelminth infection in patients with eosinophilia and negative stool microscopy, aged >24 months 1

• Exclude Loa loa before ivermectin administration in anyone who has traveled to endemic regions (Central/West Africa) 1
• Discuss with an expert for children aged 12-24 months before empiric treatment 1

Critical Pitfalls to Avoid

• Never administer ivermectin without excluding Loa loa in patients from endemic areas, as this can cause fatal encephalopathy 1
• Do not use standard stool culture panels for Yersinia, as it must be specifically requested 2
• Avoid treating STEC infections with antibiotics, as this increases hemolytic uremic syndrome risk 6
• Do not rely on single stool specimens for tapeworm diagnosis, as egg shedding is intermittent 1
• Screen for neurocysticercosis before treating intestinal T. solium, as praziquantel alone is inadequate for CNS disease 1