Mycobacterium avium complex in captive swamp wallabies (Wallabia bicolor): a case series highlighting liver, lymphoid and bone predilection

Six captive adult swamp wallabies ( Wallabia bicolor ), aged between 2 and 11 years, including five females and a single male, presented with non-specific clinical signs including lethargy and loss of body condition, ultimately leading to their euthanasia. Ante-mortem haematology and biochemistry identified anaemia in two cases and increased blood urea nitrogen in one case. Ante-mortem radiography identified multiple, variably mineralized visceral nodules in the liver of two animals and lytic lesions of the right scapula in one case and the right femoral neck in another. Post-mortem examination identified multifocal caseonecrotic nodules, most frequently in the liver (6/6), spleen (4/6), intra-abdominal lymph nodes (4/6) and bone (2/6). Histological examination of these lesions mostly identified characteristic tubercle granulomas with variable numbers of intralesional acid-fast bacilli. Culture or polymerase chain reaction (PCR) combined with line probe assay speciation confirmed Mycobacterium avium complex (MAC) as the aetiological agent in all six cases. An additional faecal PCR did not detect M. avium subsp. paratuberculosis in 4/6 cases. This is the first case series describing the clinical and post-mortem findings associated with MAC in swamp wallabies. The ubiquitous identification of hepatic lesions alongside frequent splenic and abdominal lymph node involvement suggests a predilection for these sites. These findings suggest that identification of hepatic, splenic or lymphoid nodules or lytic bony lesions should warrant consideration of MAC in swamp wallabies, especially given the zoonotic potential.