A five-year-old, female scaly-sided merganser was presented with a chronic recurrent conjunctivitisof the left eye. Physical examination showed a caseous nodular lesion at the palpebral conjunctiva associatedwith excessive tearing. Necropsy revealed caseous necrotic debris at the left conjunctiva andinfra-orbital sinus, but also multiple granuloma in several internal organs. Histological examinationshowed granulomatous splenitis, hepatitis, arteritis, fibrinonecrotic enteritis and conjunctivitis, andalso amyloidosis of liver and spleen. Bacterial and molecular tests of a conjunctival swab in the liveanimal showed the presence of a multiresistant Escherichia coli strain and Chlamydia psittaci, respectively.These bacteria had only a secondary pathogenic role. No premortem cytological examinationor Ziehl-Neelsen (ZN) staining of the histological section of the conjunctiva biopt or additional tests,such as blood test and radiographs, were performed. Finally, a Mycobacterium avium infection of theconjunctiva and internal organs was confirmed following necropsy by ZN staining and PCR analysis.Local and systemic antimicrobial treatments with fluoroquinolones, tetracyclines and aminoglycosideswere unsuccessful.