Algal Lymphadenitis in a Dog Caused by Scenedesmus Species

A 6-year-old, spayed female Labrador/Weimaraner cross-breed dog that had previously lived in Arizona presented in Montana for an annual examination with an incidentally enlarged popliteal lymph node, which was subsequently biopsied. Histologically, the lymph node was expanded by eosinophil-rich granulomas with both extracellular and intrahistiocytic green algae. These algae had intracytoplasmic, birefringent, and refractile granules; readily formed 2 to 3 mm green colonies on Columbia blood agar medium; and ultrastructurally had a multilayered cell wall and intracytoplasmic chloroplasts. Amplified product from the internal transcribed spacer and D1/D2 regions of the 28S ribosomal RNA gene had high sequence identity to Scenedesmus sp. Despite similar infection in the retropharyngeal lymph node 1 year later, the animal remained otherwise healthy with no clinical signs. To the authors' knowledge, this is the first case of Scenedesmus species infection in a dog and is a differential diagnosis for Coccidioides immitis.