Do helminth infections cause peripheral blood eosinophilia?

Peripheral blood eosinophilia—defined as an absolute eosinophil count (AEC) ≥0.5 × 10⁹/L—is a hallmark clinical finding in helminthic infections. While eosinophilia has multiple etiologies, helminths are recognized as the primary parasitic drivers, whereas most protozoan infections do not typically elicit a significant eosinophilic response.

Clinical evidence

Helminthic infections are globally identified as the most common cause of secondary or reactive eosinophilia.

• Pathogen specifics: Common culprits include Strongyloides stercoralis, Schistosoma species, and hookworms. In these infections, eosinophilia is often the first and sometimes the only clinical indicator of disease.
• Response magnitude: Most helminthic infestations result in mild to moderate eosinophilia (AEC <5,000 cells/μL). However, the degree of eosinophilia often fluctuates based on the parasite's life cycle; for instance, it typically peaks during tissue-migratory phases (e.g., Loeffler’s syndrome) compared to established intestinal phases.
• Diagnostic utility: Due to the high prevalence of these infections in certain populations, current guidelines recommend comprehensive parasitic screening for any unexplained eosinophilia, particularly in patients with a history of travel to or residence in endemic regions.

Mechanistic explanations

The relationship between helminths and eosinophilia is mediated by a specialized Type 2 (Th2) immune response.

• Cytokine signaling: Helminthic antigens activate CD4+ Th2 cells and innate lymphoid cells (ILC2s), which release interleukin-5 (IL-5). IL-5 is the definitive cytokine for eosinophil homeostasis, stimulating the differentiation and proliferation of eosinophil progenitors in the bone marrow.
• Recruitment and function: Following marrow release, chemokines such as eotaxins direct the migration of eosinophils to the site of infection. Once localized, eosinophils execute antibody-dependent cellular cytotoxicity (ADCC), releasing granules (e.g., major basic protein) to damage the parasite's tegument.

Bottom line

Helminth infections are a classic and scientifically established cause of peripheral blood eosinophilia, driven by an IL-5-mediated Th2 immune response. This finding remains a critical diagnostic marker for identifying tissue-invasive parasitic worms.