Strongyloides Stercoralis

An infection acquired decades ago can become dangerous when you start corticosteroids or other immune-suppressing treatment. Strongyloides can persist without symptoms, especially after residence in an affected region or skin contact with contaminated soil. Its ability to reproduce inside your body makes it a particular concern before chemotherapy or transplantation.

Stool qPCR looks directly for the parasite's DNA. A positive result supports infection, but false positives can occur. A negative result can miss an infection, so this test often needs to be combined with a blood antibody test and sometimes additional stool testing.

What the Test Finds

Strongyloides stercoralis is a parasitic roundworm transmitted when infective larvae in contaminated soil penetrate your skin. It occurs mainly in tropical and subtropical regions, though transmission also occurs in some temperate areas. Adult females live in the lining of the small intestine. Their eggs usually hatch there, and larvae pass into stool.

This test uses quantitative polymerase chain reaction, usually called qPCR or real-time PCR. The method copies a selected stretch of parasite DNA and monitors the signal as copying proceeds. It can detect DNA without requiring live larvae in the specimen.

Stool qPCR has a clinical diagnostic role, but assays differ between laboratories and there is no universally standardized qPCR scale for infection severity. Published studies support its use alongside other diagnostic methods. They do not establish the performance of every laboratory's assay.

Some laboratories report a cycle threshold as well as a detected or not-detected interpretation. The cycle threshold records how many copying rounds were needed to produce a detectable signal. Within the same assay and processing conditions, a lower value generally indicates more DNA in the tested material. It is not a validated count of worms in your body or a reliable measure of your risk of severe disease. If your report is qualitative, there is no numerical level to trend.

Why Infection Can Persist for Decades

Some larvae become infective inside your bowel and penetrate the intestinal wall or the skin around the anus. This process is called autoinfection. It allows infection to continue without further contact with contaminated soil, which is why a childhood exposure or a long-ago move can still matter.

Travel duration alone doesn't determine whether you were exposed. Barefoot walking or other skin contact with contaminated soil matters, and a short trip can include that exposure. Chronic infection can inflame the intestine; more severe intestinal disease can damage its lining and interfere with nutrient absorption.

Hyperinfection and Disseminated Disease

Hyperinfection is an acceleration of autoinfection with greatly increased numbers of larvae, mainly affecting the intestine and lungs. Disseminated disease involves spread beyond those usual sites. Both can be life-threatening. Corticosteroids are a particularly important trigger; transplantation, certain cancers and other immune-suppressing treatments also increase concern.

A Brazilian study compared 46 kidney transplant recipients with severe strongyloidiasis with 92 transplant recipients without it. Among the severe cases, about 28% died within 30 days. Respiratory failure and bloodstream infection were associated with death. Receiving a deceased-donor kidney was associated with roughly six times the odds of severe infection, and greater cumulative corticosteroid exposure was also associated with it. These were observational findings about clinical disease, not relationships between stool qPCR values and outcomes.

Severe infection can also allow gut bacteria to enter the bloodstream, leading to sepsis or meningitis. A small Italian hospital study explored whether chronic infection might contribute to invasive bacterial infections even without hyperinfection. Fourteen of 57 participants had positive Strongyloides antibody tests, while none of the 43 who provided stool for PCR tested positive. Antibody positivity alone couldn't establish active infection in every case, and this study did not prove that the parasite caused the bacterial infections.

HTLV-1, HIV, and Other Risk Groups

HTLV-1 is a virus that infects immune cells and increases the risk of severe strongyloidiasis. A review included 14 studies, but the estimate for severe disease came from three studies: coinfection was associated with about 60 times higher odds of severe disease. That estimate was imprecise. A separate analysis found about five times higher odds of treatment failure across several drugs and regimens. These findings concern diagnosed infection and treatment outcomes; they do not provide a way to calculate risk from a stool DNA result.

Strongyloides also occurs in people with HIV. A review estimated an infection prevalence of about 5% across the included HIV populations, using varied diagnostic methods. That figure is not a stool qPCR detection rate. Although severe cases occur in people with HIV, observational studies have not established HIV alone as a risk factor for hyperinfection comparable to corticosteroid exposure or HTLV-1.

Antibody screening has found evidence of exposure among people receiving haemodialysis in Vienna and among Latin American migrants without symptoms in Spain. These studies support considering exposure history even when you live in Europe and feel well. Their antibody findings should not be treated as rates of stool qPCR-confirmed infection.

Symptoms of Chronic Infection

Long-standing infection is often silent. Possible symptoms include abdominal discomfort, diarrhea, itching and recurrent hives. Eosinophils are white blood cells that can increase with parasitic infections, allergies and other conditions. An unexplained increase can be a clue, but a normal count doesn't exclude strongyloidiasis, including severe disease.

A systematic review found symptoms in about half of infected people. In the three studies reporting eosinophil findings before and after treatment, about three quarters had an elevated count at diagnosis. Symptoms and eosinophilia became less common after treatment, but neither symptom improvement nor a falling eosinophil count reliably establishes cure.

How Accurate Stool qPCR Is

Sensitivity describes how often a test detects an infection when one is present. A meta-analysis estimated real-time PCR sensitivity at 56.5% when the reference tests included serology or stool methods, and 64.4% when comparisons were restricted to stool methods. Specificity describes how often a test is negative in someone classified as uninfected; the corresponding estimates were about 95% and 94%. The reference tests themselves were imperfect.

Performance varies with the assay, specimen processing, infection intensity and comparison method. Some field studies found substantially more infections with qPCR than with individual larval methods. Others found that combining methods detected infections missed by either alone. Baermann testing concentrates larvae from stool, while agar plate culture detects larvae through their movement and growth on a culture plate.

These studies used different laboratory protocols and definitions of infection. Their results explain why methods can complement each other; they are not interchangeable accuracy estimates for your test. Species-targeted PCR can also reduce confusion between Strongyloides and similar-looking hookworm larvae.

When Results Can Be Misleading

• Low or intermittent shedding: chronic infection may produce few larvae, and they aren't distributed evenly through every stool specimen. The small portion used for DNA extraction may contain no detectable parasite material.
• Early infection: larvae generally begin appearing in stool about 3 to 4 weeks after infection. Earlier testing can miss infection, and blood antibodies may also be absent at that stage.
• DNA extraction: in a Lao study, sensitivity against a combined reference was 31.3% with one extraction protocol and 77.5% with another. Longer digestion and different processing substantially changed detection.
• Reaction interference and preservation: substances in stool can inhibit PCR. An unsuitable preservative, including formalin for many molecular protocols, can also reduce detection.
• Assay specificity: some assays detect more than one Strongyloides species. A comparison of molecular assays found disagreement between methods and unintended bacterial DNA amplification in some assays. These findings do not establish that this particular laboratory assay has those problems.
• Contamination or a weak signal: laboratory contamination and nonspecific amplification can produce false positives. An unexpected result may need laboratory review or confirmation, especially when exposure is unlikely.
• Recent antiparasitic treatment: successful treatment can make stool DNA undetectable, but a single negative sample can also reflect missed shedding. A cycle-threshold change alone doesn't establish cure.