Eosinophilia in the Tropics: Should Empirical Anthelmintic Treatment Be Considered Standard Practice? A Prospective Cohort Study

Background: Eosinophilia is a common clinical finding with diverse etiologies. Helminthiasis is a major aetiology in tropical regions, but data from haematology consultation services at tertiary centres in endemic settings are limited. Objectives: To determine the prevalence of helminthiasis among patients with eosinophilia at a tropical tertiary care centre and identify clinical and laboratory characteristics differentiating helminth from non-helminth etiologies. Methods: A prospective cohort study with predefined retrospective outcome classification was conducted at Siriraj Hospital, Thailand (January 2022 to November 2023) in adult patients with blood eosinophilia above 500 cells/μL. All patients underwent helminth screening through stool examination and serological testing for strongyloidiasis, gnathostomiasis, angiostrongyliasis, filariasis and cysticercosis, and received empirical ivermectin or albendazole with monthly follow-up for 6 months. Results: Among 131 patients, 76 (58.0%) were diagnosed with helminthiasis, comprising 49 definite (laboratory-confirmed) and 27 probable (treatment-response-defined) infections. Strongyloidiasis (53.1%) was most common among definite infections, followed by gnathostomiasis (40.8%) and cysticercosis (26.5%), with multiple infections in 24.5% of definite cases. The median time to absolute eosinophil count (AEC) normalization was 1.0 month (IQR: 1.0–2.0), with 93.4% normalizing within 3 months. Probable infections were significantly associated with soil contact and forest travel (all p < 0.05), while definite infections were associated with organism-specific exposures (all p < 0.05). Patients aged 75 years or above showed significantly lower AEC at presentation (adjusted geometric mean ratio 0.52, 95% CI: 0.31–0.89, p = 0.017). Leukopenia and hypoalbuminemia were more prevalent in non-infectious etiologies. Conclusions: Helminthiasis was identified in more than half of patients with eosinophilia at a tropical tertiary haematology service. Empirical anthelmintic therapy effectively normalized eosinophilia in the majority within 3 months, supporting its use as a standard initial approach in endemic settings. Detailed exposure anamnesis and concurrent evaluation for non-helminth etiologies, particularly in patients with leukopenia or hypoalbuminemia, should be routinely performed.