Background: Hemophagocytic lymphohistiocytosis (HLH) is a life threatening condition caused by inappropriate immune activity. Infection is often the trigger, both in genetically predisposed and in sporadic cases. Although more commonly seen in the paediatric population, patients of all ages can be affected. Case presentation: A 26-year-old male patient with Crohn’s disease, treated with ustekinumab, presented with high fever, epistaxis and anorexia. Laboratory results showed pancytopenia, and a high serum levels of ferritin and LDH. Colonoscopy revealed only mild signs of disease activity. CT-scan showed splenomegaly and multiple lymphadenopathies. Bone marrow aspirate was suggestive for hemophagocytosis. PCR & serology for parvovirus B19 came back positive. Treatment with ustekinumab was temporarily put on hold and supportive care was given. Viral replication decreased and he recovered completely. Conclusion: There is a known association between HLH and Crohn’s disease. This is probably because they are more susceptible to infections with CMV, EBV and parvovirus B19, all known as triggers for HLH. The role of ustekinumab is unclear: did it play a role in the pathophysiological evolution of this primo-infection with parvovirus B19? On the other hand, did it contribute to the rather mild course of the disease, acting as a immunomodulator that works on interleukin-12, a cytokine that plays a role in HLH? Further study is warranted to answer these questions.

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