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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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