Fatal encephalitis caused by Shamonda virus Europe 1 (SHAV-EU1) in a patient with a severely weakened immune system, France, 2026

New Shamonda virus clades (SHAV-EU1 and SHAV-EU2) emerged in Europe in summer 2026, primarily affecting cattle. We report the first human infection with SHAV-EU1: a French patient with a severely weakened immune system developed fatal encephalitis in September 2026, 1 month after their allogeneic stem cell transplantation. Metatranscriptomics detected SHAV-EU1 in cerebrospinal fluid, confirmed by RT-PCR. Biopsies from central nervous system showed high viral loads. Investigations revealed SHAV-EU1 in serum, respiratory and stool samples at least for 2 months before encephalitis onset.