An allegedly 15-year-old male was seen at the emergency unit on 1 January 2018 for cold sweats, hyperthermia, cough, and fatigue occurring for 1 week. This Senegalese migrant had traveled across Libya and Italy before arriving in France 11 weeks before, where he resided in a migrant settlement with several persons who had the same symptoms. Physical examination disclosed centimetric cervical and axillary adenopathy. Chest radiography and thoracic scan ruled out influenza and tuberculosis. Complete blood count was normal. An “atypical lymphocyte” on the Sysmex XN-10 analyzer prompted a blood smear. This disclosed 2.78 × 10⁹/L neutrophils, 1.51 × 10⁹/L lymphocytes with 3% of activated lymphocytes, 0.19 × 10⁹/L monocytes, and no eosinophilia (0.09 × 10⁹/L) or basophilia. Yet attentive examination at low magnification (×10) detected microfilaremia, identified as Mansonella perstans by microscopy and serology (panels A-D, May-Grunwald Giemsa stain; original magnification ×10 [panels A-C]; original magnification ×50 [panel D]). HIV seropositivity was later diagnosed, explaining the superficial lymphadenopathy. Treatment with ivermectin and doxycycline was initiated.

The absence of hypereosinophilia, usually present in such parasitemia, is surprising. Microfilaremia is an endemic vectorized helminthiasis (Culicoides) in sub-Saharan Africa, usually asymptomatic in nonimmunocompromised patients. A blood smear examination is recommended for sick migrants, even if the differential count is normal, to avoid delaying treatment in immunocompromised patients.