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

## TL;DR

A patient with Crohn’s disease developed Chlamydia psittaci pneumonia after starting ustekinumab therapy, highlighting a possible link between the drug and increased infection risk.

## Contribution

This is the first reported case linking ustekinumab therapy to Chlamydia psittaci pneumonia in an immunocompromised patient.

## Key findings

- A 27-year-old patient on ustekinumab developed C. psittaci pneumonia after bird exposure.
- Doxycycline treatment led to recovery, but broad-spectrum antibiotics were ineffective initially.
- The case suggests a possible increased susceptibility to intracellular pathogens with ustekinumab.

## Abstract

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.

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.

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.