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Azathioprine hypersensitivity mimicking underlying inflammatory bowel disease

A 39-year-old waste hygienist with known Crohn’s colitis presented with 2 days of diarrhoea and vomiting associated with fever and rigors. His medications were azathioprine 100 mg daily, which had been commenced 3 weeks earlier, and a tapering prednisone dose of 9 mg daily. There was no significant dietary, travel or history of exposure to animals. Examination was normal other than dehydration, a fever of 39.8°C and general abdominal tenderness. In urinalysis, few white and red cells and mild proteinuria were observed. A full blood count (FBC) showed white cells 11.3 × 109/L (neutrophils 10.8), but was otherwise normal.The erythrocyte sedimentation rate (ESR) was 50 mm/h and C-reactive protein (CRP) was 178 mg/L. Electrolytes, liver function tests and amylase were normal, but creatinine was 168 μmol/L. Infection serology was negative. Blood, stool, sputum and urine microbiology were negative. Plain abdominal and chest films were normal. Abdominal computed tomography (CT) showed a thickened colon only. Oral medication was withheld. He was given i.v. fluids and his symptoms, apart from fever, settled after 2 days. The FBC and creatinine also had normalized. He then noticed lower back and bilateral knee ache and wished to be discharged on previous medication. Seven days later he re-presented with diarrhoea, vomiting and a fever of 38°C. Examination findings and blood tests were unchanged. Pelvic magnetic resonance imaging (MRI) found no perianal/pelvic lesion and a Tc-labelled white-cell scan found pancolonic uptake. His azathioprine was again discontinued and after 48 h of spiking temperatures to 39.4°C the pyrexia defervesced along with all his symptoms. He has since remained well.