Ultrastructural characterization of the agent of systemic yeast infection of owl monkeys

Abstract

Systemic infection by an unclassified yeast-like organism has been encountered sporadically in wild-caught owl monkeys (Aotus sp.) from South America. The infection is presumably acquired in the wild; the incubation period ranges from months to years. The disease is indolent and clinical signs are non-specific. The diagnosis is based on histopathologic observation of yeast-like cells in multiple internal organs. Most cells appear to be phagocytized by macrophages, however, many are apparently free in the extracellular space. Other inflammatory infiltrates, including neutrophils, lymphocytes, plasma cells, and multinucleated giant cells, are conspicuously absent. Cells are thick-walled, globose to oval, range from 5 to 8 micron in diameter, and reproduce by narrow-based budding of single daughter cells. Attempts to cultivate the organism on artificial media have failed. Yeast cell ultrastructure was studied using transmission electron microscopy. The cell wall is multilayered, and the internal structure is markedly heterogenous. In some cells, the cytoplasm is lightly electron-opaque, finely granular and lacks recognizable organelles or nuclei. In others, the cytoplasm is electron dense and contains mitochondria, ribosome-like granules, and a multilobulated nucleus. This organism differs from other recognized pathogenic yeast in its combined light microscopic appearance, organ involvement and host response. Ultrastructurally, it most closely resembles Loboa loboi.