A 5-mo-old Bassett Hound–Labrador Retriever cross was autopsied following a bout of lethargy, inappetence, and bleeding gums. Mucous membranes were white, and the small intestine was blue-black; the colon contained black feces. The spleen was swollen, and multiple lymph nodes were enlarged and hemorrhagic. Microscopically, the small intestine had focal crypt cell necrosis and circumferential transmural vasculitis, the latter the cause of infarction and the blue-black coloration. Lymphocytes were necrotic in spleen and lymph nodes, and erythrophagocytosis was present in some nodes. Vasculitis was present in brain, meninges, lung, liver, and kidneys. Electron microscopy revealed aggregates of 15–18 nm round viral particles in damaged crypt cells and in the endothelium of small blood vessels. Electron-dense intracytoplasmic inclusions consisting of paracrystalline-arrayed virus were demonstrated in macrophages in medullary lymph node sinuses. These virions were identified as circovirus, which was confirmed by real-time PCR and sequencing.

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A 27-mo-old, spayed female mixed-breed dog was presented with left forelimb pain, which progressed to full thickness necrosis of the soft tissues of multiple limbs. Clinical imaging and postmortem examination suggested multiple large arterial thromboemboli. Histologic examination of vascular lesions revealed markedly thickened tunica intima with polypoid intraluminal projections, which partially to entirely occluded the arterial lumen. The expanded tunica intima was comprised of intimal accumulation of Alcian blue–positive matrix with scattered spindle-to-satellite cells. These cells were positive for von Willebrand factor and vimentin but negative for α–smooth muscle actin, suggesting endothelial origin. Deposition of the intimal mucoid matrix was observed in the elastic and muscular arteries associated with regional ischemic changes. Mucoid emboli, likely from fragmentation of proliferative intimal tissue, were identified in smaller vessels supplied by affected arteries. Based on these findings, we diagnosed systemic mucoid degeneration of the arterial tunica intima. Such systemic arterial degeneration characterized by deposition of mucoid matrix in the tunica intima has not been reported previously in dogs, to our knowledge, and should be distinguished from thromboembolism and other degenerative vascular diseases.