Osteomyelitis, arthritis and myositis associated with <i>Yersinia pseudotuberculosis</i> in village weavers ( <i>Ploceus cucullatus</i> ) and an oriental magpie robin ( <i>Copsychus saularis</i> )

Avian pseudotuberculosis infection usually presents as well-demarcated visceral necrotic foci, typically affecting the gastrointestinal tract, liver and spleen. This case series describes an atypical presentation of <i>Yersinia pseudotuberculosis</i> (Yptb) characterized by severe chronic myositis, arthritis and osteomyelitis in five village weavers (<i>Ploceus cucullatus</i>), and acute osteomyelitis and myositis associated with septicaemia in an oriental magpie robin (<i>Copsychus saularis</i>) from a zoological collection. Clinical signs of the weavers included lethargy, poor flying ability and focally extensive periarticular and muscular swelling, whereas the magpie robin was found dead without premonitory signs. Radiography revealed focal lytic and proliferative bone lesions with loss of articular congruity and increased radiopacity of skeletal muscles, which was compatible with severe necrotizing, granulomatous osteomyelitis and polyphasic myositis with large intralesional bacterial colonies on histology. Most (<i>n</i> = 4/5) birds with available histology exhibited only mild to moderate heterophilic to histiocytic inflammatory lesions in their intestines, spleen and liver. Bacterial cultures typically yielded Yptb from joint and muscle samples (3/3), and less consistently from visceral organs (6/11) and bone marrow (0/5). Bacterial typing using Fourier-transform infrared (FT-IR) spectroscopy suggested that weaver Yptb strains were closely related. Whole genome sequencing of two Yptb strains identified one as ST14 serotype O:2a and the other ST42 serotype O:1a, with the presence of virulence genes including plasmid-borne <i>yadA</i> and chromosomally encoded virulence genes <i>ail</i> and <i>invA</i>. Weavers may be prone to develop atypical pseudotuberculosis with the musculoskeletal system as a predilection site for bacterial growth and associated granulomatous lesions.