Strongyloidiasis: why screening is important before steroids and biological therapy

Introduction

Strongyloidiasis, caused by Strongyloides stercoralis, is a neglected parasitic infection that can persist silently for decades thanks to its unique autoinfection cycle. Most carriers feel well, yet the risk becomes critical when the immune system is suppressed.

In patients receiving steroids, chemotherapy, transplantation drugs, or biologics, latent infection can flare into hyperinfection syndrome, with mortality approaching 60–80%. This threat is entirely preventable.

A pre-immunosuppression checklist, including serology and timely ivermectin 200 mcg/kg (Stromectol® 3 mg) treatment, can save lives. This article explains how infection occurs, which tests to order, how to calculate the correct dosage, and what pitfalls to avoid.

How do you get infected and what is the danger of hyperinfection

Strongyloides stercoralis infects through skin contact with contaminated soil, often when walking barefoot in warm, humid climates. After penetrating the skin, larvae travel via the lungs to the intestine, where they mature. Unlike most helminths, Strongyloides can cause autoinfection. Some larvae re-enter the bloodstream from the intestine or perianal skin, allowing infection to persist for decades without new exposure. Most carriers feel healthy or report only mild itching or abdominal discomfort.

The danger comes with immunosuppression. Corticosteroids, chemotherapy, or transplant drugs can unleash massive replication, producing hyperinfection syndrome with pneumonia, intestinal bleeding, and sepsis. Mortality reaches 60–80% if untreated.

The paradox is that patients at risk often had no warning signs. A migrant or traveler decades removed from exposure may suddenly develop overwhelming sepsis after starting steroids.

Patient vignette: “I never felt sick, but before my kidney transplant my doctor tested me for Strongyloides. A short course of tablets spared me from a fatal complication.”

Diagnostics: serology vs. PCR vs. microscopy (decision tree)

Diagnosing Strongyloides stercoralis is more challenging than treating it. Because parasite numbers in stool are usually low, routine ova-and-parasite exams frequently miss infection.

Stool microscopy can demonstrate larvae but only in heavy infections, making it unsuitable for screening. It is best used for confirmation or for monitoring cure after therapy.

Serology (ELISA and related assays) is the recommended first-line test in non-endemic settings. With sensitivity up to 95%, it reliably detects chronic, low-level infection. Its drawback is occasional cross-reactivity with other helminths, which can produce false positives, but in the context of immunosuppression, a positive result generally warrants treatment.

PCR testing of stool detects parasite DNA with high specificity and is increasingly available. It is particularly helpful when serology is negative but suspicion remains high, such as in migrants with unexplained eosinophilia or prior exposure.

The practical decision tree is straightforward. At-risk patients, i.e. those with epidemiologic exposure or unexplained eosinophilia, should undergo serology before starting steroids or biologics. A positive result requires treatment regardless of symptoms. If negative but suspicion persists, order PCR or repeat serology. Microscopy should not be relied on for screening.

Ivermectin treatment: regimens, repeats, cure monitoring

The treatment of choice for strongyloidiasis is ivermectin 200 mcg/kg, usually supplied as 3 mg tablets (Stromectol®). Albendazole is less effective and reserved only when ivermectin is unavailable. For uncomplicated infections, ivermectin is given once daily for one to two days. In immunosuppressed patients or when hyperinfection risk is high, a second course is advised after two weeks. The aim is to break the parasite’s autoinfection cycle.

Dosing must be calculated by weight. A 60-kg adult requires 200 mcg/kg × 60 = 12 mg, equal to four 3 mg tablets. Because tablets cannot be split, the dose is rounded to the nearest whole tablet. Many centers now use conversion tables or online calculators to avoid errors.

Follow-up testing with stool or PCR after two to four weeks helps confirm clearance, though intermittent shedding may limit sensitivity. In high-risk cases, repeating ivermectin even without definitive evidence of persistence is safer than risking relapse.

In practice, treatment is simple: a short weight-based course of ivermectin, correctly rounded into 3 mg tablets, reliably cures infection and prevents fatal hyperinfection.

Risk groups, pitfalls, interactions

Strongyloidiasis does not respect time or distance. Patients can carry the parasite silently for decades after leaving an endemic area. The highest-risk groups are migrants and long-term residents from tropical and subtropical regions, as well as travelers who had prolonged barefoot exposure, sometimes many years earlier. Another important red flag is unexplained eosinophilia, especially if it preceded the need for steroids or biologics.

The greatest danger arises when immunosuppression is introduced. Corticosteroids, calcineurin inhibitors, cytotoxic chemotherapy, and modern biologics such as anti-TNF agents can all precipitate hyperinfection and disseminated disease. In these patients, even low-level chronic carriage can rapidly become fatal.

Special pitfalls complicate management. Co-infection with Loa loa, seen in parts of Central and West Africa, is a recognized contraindication because ivermectin can trigger severe encephalopathy in such cases. In pregnancy, evidence on safety is limited; while untreated strongyloidiasis poses a risk, most guidelines recommend postponing therapy if possible, or proceeding only with expert consultation. In children under 15 kg, ivermectin use remains off-label; some clinicians employ it cautiously, while others consider albendazole as a compromise.

Drug interactions are rare, as ivermectin has a wide safety margin, but clinicians should remain aware of its CNS penetration and avoid combination with potent depressants in fragile patients.

Patient FAQ

What are the symptoms of strongyloidiasis?

Many people feel completely well and never suspect an infection. Others notice only mild itching, abdominal discomfort, or episodes of diarrhea. The absence of symptoms does not mean safety: the infection can stay hidden for decades.

Why do I need treatment if I feel fine?

The danger comes when your immune system is lowered by steroids, chemotherapy, or transplant medication. In that situation, Strongyloides can suddenly multiply uncontrollably, leading to hyperinfection, a complication that is often fatal. Treating now prevents that risk.

Does itching or eosinophilia always mean I have it?

Not necessarily. Itching and raised eosinophils can have many causes. They may raise suspicion, but only proper testing can confirm or exclude infection.

What if I miss a dose of ivermectin?

If you remember soon after, take the tablet as directed. If the next scheduled dose is already close, skip the missed one and continue. Missing a single tablet is unlikely to compromise the entire treatment, but do not stop the course early without medical advice.

Can infection come back after treatment?

If ivermectin is given at the right dose and repeated as needed, the cure rate is very high. However, follow-up testing is usually recommended, especially for people with weak immunity. Reinfection is possible if you return to areas with contaminated soil.

Is treatment safe?

Yes, ivermectin at 200 mcg/kg is generally well tolerated. Side effects are uncommon and usually mild, such as dizziness or abdominal upset.

Practical tools / Callouts

Pre-Immunosuppression Checklist: Strongyloides stercoralis

Who should be screened?

• Patients with planned long-term corticosteroids, biologics, chemotherapy, or transplantation
• Migrants, refugees, or long-term residents from tropical/subtropical regions (Latin America, Africa, Southeast Asia, Pacific)
• Travelers with barefoot soil contact in endemic areas, even decades ago
• Patients with unexplained eosinophilia

What test to order?

• First-line: Strongyloides serology (ELISA or equivalent)
• If negative but high suspicion: repeat serology or PCR stool test
• Avoid relying on single stool microscopy

When to treat?

• Any positive result, regardless of symptoms
• If high suspicion and immunosuppression imminent → discuss empiric ivermectin

Treatment regimen

• Ivermectin 200 mcg/kg orally, rounded to the nearest 3 mg tablet
• Uncomplicated infection: 1–2 days
• Immunosuppressed/high-risk: repeat after 14 days
• Monitor cure with stool/PCR at 2–4 weeks

Contraindications / cautions

• Loa loa co-infection risk (Central/West Africa)
• Pregnancy: weigh risk/benefit, consult specialist
• Children <15 kg: off-label use; consult pediatric infectious diseases

Do not start immunosuppression until screening and treatment are addressed.

Referral Template for Laboratory / Infectious Disease Service

Conclusion

Strongyloidiasis remains a “forgotten” infection that can persist silently for decades and only reveal itself when it is already too late. For patients preparing for immunosuppressive therapy, this is not a benign parasite but a potential trigger of fatal hyperinfection syndrome.

Prevention is straightforward: screen with serology in patients from endemic regions, migrants, travelers with past barefoot exposure, or anyone about to receive steroids or biologics. If results are positive, proceed with ivermectin 200 mcg/kg (Stromectol® 3 mg), repeating as needed and following up with laboratory confirmation.

This simple measure is inexpensive, requires only a few days, and can be lifesaving. Before prescribing high-dose steroids, scheduling a transplant, or initiating biologic therapy, every clinician should ask: “Have I ruled out hidden strongyloidiasis?”

References

1. Centers for Disease Control and Prevention. (2024). Clinical Care of Strongyloides. Retrieved from https://www.cdc.gov/strongyloides/hcp/clinical-care/index.html
2. Nutman, T. B. (2017). Human infection with Strongyloides stercoralis and other related Strongyloides species. Parasitology, 144(3), 263–273. https://doi.org/10.1017/S0031182016000834
3. Requena-Méndez, A., Buonfrate, D., Bisoffi, Z., Gutiérrez, J. M., Muñoz, J., & Krolewiecki, A. J. (2017). Evidence-based guidelines for screening and management of strongyloidiasis in non-endemic countries. American Journal of Tropical Medicine and Hygiene, 97(3), 645–652. https://doi.org/10.4269/ajtmh.16-0923