Haematology, serum biochemistry, thoracic imaging, bronchoalveolar lavage (BAL), Ziehl-Neelsen staining of the BAL fluid and interferon gamma release assay (IGRA) were compatible with pneumonia caused by the less pathogenic member of the <i>Mycobacterium tuberculosis</i> complex, that is, <i>M microti</i> (the 'vole bacillus'), which is common in cats in the UK. This evidence concerns the gene IFNG and susceptibility to pneumonia measurement.