Don't Rule Out Strongyloides stercoralis Infection Among Allergies

Strongyloides stercoralis, a soil-transmitted parasite, infects as many as 100 million individuals across the globe — endemic to tropical and subtropical regions, including Southeast Asia, sub-Saharan Africa, the Caribbean, South America, the Southeastern United States, and Southern Europe.

Strongyloides undergo a complex life cycle within humans, and may result in chronic infections that last for several decades even in individuals that have not recently traveled to endemic areas.

Strongyloides infections may involve the lungs (cough, wheezing, shortness of breath), gastrointestinal tract (nausea, vomiting, diarrhea, abdominal pain), and skin (hives, itching). Infected individuals often suffer from mild symptoms, although those with compromised immune systems may experience the life-threatening hyperinfection syndrome.

Hyper infection represents disseminated disease, and can manifest with severe gastrointestinal and pulmonary symptoms, gram-negative bacterial infection, meningitis, or septic shock. Typical risk factors for hyperinfection syndrome include organ transplantation, hematologic cancers (leukemia, lymphoma), the presence of human T-lymphotropic virus 1 (HTLV-1) infection, and immunosuppressive medications such as corticosteroids.

As a result of the extremely high mortality rate of hyperinfection syndrome, health care providers must consider Strongyloides infection before prescribing corticosteroids for patients who have traveled to endemic regions.

Strongyloides infection must be promptly diagnosed and treated in individuals suffering from severe allergies and asthma, particularly since corticosteroids are the mainstay of treatment in these conditions.

Furthermore, the variable symptoms (wheezing, itching) associated with Strongyloides may masquerade as allergic diseases.

While Strongyloides might be difficult to diagnose, important clues that may suggest infection can include chronically elevated peripheral blood eosinophil counts and/or eosinophil counts that do not improve in the setting of steroid therapy.

We recently published the case of a 71-year-old man from Honduras that was referred to us for the evaluation of severe and generalized itching. This patient’s eosinophil count began to increase five years before our evaluation.

Interestingly, his eosinophil count continued to increase after he was started on oral corticosteroids for a presumed neurologic condition three years before our initial visit. We promptly diagnosed Strongyloides infection, and prescribed anti-parasitic therapy. The patient’s presenting symptoms and increased eosinophil count resolved just one month following treatment.

Given the heterogeneous clinical presentations associated with Strongyloides, infections are sometimes difficult to diagnose since laboratory testing can have inconsistent results, so they could also be identified by evaluating stool samples for parasitic larvae.

However, quite commonly, few larvae are excreted into the stool, and single stool tests may identify Strongyloides in less than one-third of cases. Due to the potential limitations of laboratory testing in the identification of Strongyloides, providers must also obtain a comprehensive medical (including travel) history in order to help guide the decision to recommend anti-parasitic treatment.

Until appropriately treated, Strongyloides infections may persist for many decades. Following diagnosis, the preferred treatment is ivermectin, although the optimal dosing schedule remains uncertain. In our clinical practice, we typically prescribe ivermectin 200 mcg/kg/day for two days. It is often difficult to gauge when the infection has been completely eradicated following treatment. However, published studies indicate that 90% of patients have experienced presenting symptom (e.g. hives) improvement and eosinophil count normalization within 12 months after treatment completion.

While often not common, providers are encouraged to consider Strongyloides infection in patients receiving or being planned for corticosteroid therapy. Strongyloides may masquerade as allergic and other conditions, and can have devastating consequences if not promptly treated.

In order to avoid complications, a high index of suspicion must be maintained in any individual that presents with increased eosinophil counts and a history of travel to an endemic region.