Mycobacterium microti infection associated with spindle cell pseudotumour and hypercalcaemia: a possible link with an infected alpaca

A 44-year-old woman who had recently been on immunosuppressive therapy presented with malaise, cough, fever, weight loss, lymphadenopathy, severe hypercalcaemia and a paratracheal mass on imaging. The initial impression was of disseminated malignancy, and lymphoma was suspected. A mediastinal biopsy showed a mycobacterial spindle cell pseudotumour containing acid and alcohol fast bacilli (AAFB). Sputum microscopy demonstrated AAFBs, confirmed as Mycobacterium tuberculosis complex by PCR. Prolonged culture grew Mycobacterium microti, an organism often associated with disease in small rodents and llamas. M microti isolates from postmortem samples of an alpaca at a nearby farm were genetically indistinguishable. Although the patient had not visited the farm, concurrent illness in her adopted stray cat suggested a possible zoonotic connection. The patient responded to antituberculous therapy, and rehydration and pamidronate for hypercalcaemia. We believe the hypercalcaemia was caused by a similar mechanism to raised calcium levels sometimes seen in tuberculosis.