Chlamydia psittaci pneumonia in a patient using ustekinumab therapy for Crohn’s disease

ABSTRACT Background Ustekinumab, a monoclonal antibody targeting the p40 subunit of interleukin-12 and interleukin-23, is widely used for immune-mediated diseases such as Crohn’s disease. By modulating Th1 and Th17 pathways, ustekinumab may increase susceptibility to intracellular pathogens. We report the first case of Chlamydia psittaci pneumonia in a patient treated with ustekinumab. Case Summary A 27-year-old woman with Crohn’s disease presented with severe community-acquired pneumonia, progressive hypoxemia, and pulmonary embolism 3 weeks after initiating ustekinumab therapy. Despite broad-spectrum antibiotics, clinical improvement was limited. Her exposure history revealed contact with multiple pet birds, several of which died recently. Polymerase chain reaction testing confirmed C. psittaci genotype A infection. Targeted treatment with doxycycline led to clinical recovery. Ustekinumab therapy was temporarily postponed. Conclusion This case highlights a potential association between ustekinumab and susceptibility to C. psittaci , a zoonotic intracellular pathogen. It underscores the importance of exposure history and considering atypical pathogens in immunocompromised patients. Clinicians should maintain vigilance for opportunistic infections in individuals receiving targeted biologic therapies.