Marked eosinophilia in Giardia lamblia infection

Introduction: Eosinophilia, defined as an absolute eosinophil count >500 cells/µl, is most commonly secondary to allergic diseases, infections, or drug reactions. While parasitic helminth infections are a well-recognized cause, protozoal infections are less frequently associated. Reports linking Giardia lamblia infection with eosinophilia remain rare, in particular in the context of severe eosinophilia. Case description: A 77-year-old woman presented with acute watery diarrhoea for 3 days without systemic symptoms. Laboratory evaluation revealed marked eosinophilia (4.5 g/l at initial presentation, which rose to 7.9 g/l). The initial diagnostic workup, including stool polymerase chain reaction for bacterial and viral pathogens, helminth serologies, celiac screening, and abdominal imaging, was all negative. However, endoscopic evaluation with biopsies demonstrated mild eosinophilic gastrointestinal infiltration, and stool antigen testing was positive for Giardia lamblia. No other cause of eosinophilia could be identified. Treatment with metronidazole resulted in rapid clinical improvement and progressive reduction of eosinophil counts, with complete normalization during follow-up. Additional investigations only revealed mild hepatopathy, most likely related to statin therapy. Conclusion: This case highlights Giardia lamblia as a potential cause of marked secondary eosinophilia and thus may have to be considered as a differential diagnosis in patients presenting with unexplained severe eosinophilia and gastrointestinal symptoms.