An unusual presentation of actinomycosis in a dairy cow

first lactation dairy cow (in late gestation) presented with a hard round swelling of the right olecranon and slight lameness. A lateromedial radiograph of the right olecranon had a mottled appearance with indistinct borders. Bone biopsy was performed post calving. Histology showed fibrosis and reactive changes, however this sample was considered too superficial and not representative of the whole lesion. Based on the differential diagnoses treatment was not possible and the cow was culled at the end of her lactation. At slaughter the forelimb was retained for further analysis. Grossly, the olecranon was severely enlarged, irregular with multiple yellow granulomas in the cortex and medulla of the bone. Histology showed multifocal pyogranulomas containing bacteria, Splendore-Hoeppli material and surrounded by remodelled bone and fibrosis. Despite displaying biochemical properties characteristic of Actinomycesspecies this could not be confirmed by partial 16s rRNA sequencing and the organism could not be definitively speciated.