For about seven years now, Georgina Miller has been helping NIH investigators figure out what's wrong with their research animals. "It's very important to know if a sick animal has a research-related complication or not; it's important to find out why a research animal died. This is pretty obvious. Researchers certainly don't want to endanger the health of their colonies," says Miller, one of five veterinary pathologists in the Diagnostic Services Section of the Veterinary Resources Program.

Georgina Miller

For the past three years, Miller has also been pursuing a "mystery," an agent that has infected owl monkeys used by NIAID in malaria research. She spoke of her quest during an interview at the NIH Research Festival, where she presented a poster on "Ultrastructural Characterization of the Etiologic Agent of Systemic Yeast Infection of Owl Monkeys."

The owl monkeys—"the only ones that can be infected with human malaria"are caught wild in the tropical forests of Peru, and it's there, Miller said, that the yeast infection was most likely acquired before they were shipped here. Three years ago, Miller received a splenic biopsy from a sick but still living owl monkey whose clinical symptoms included weight loss, debilitation, and anemia.

"The spleen was filled with wall-to-wall yeast cells," she said, but, unlike most other fungal organisms, these could not be cultured (she sent samples to the NIH's Clinical Center Mycology Lab as well as to the Centers for Disease Control and Prevention; neither was able to grow the fungus on artificial media). The yeast cells infect almost every organ in the body without provoking an immunologic reaction. Another peculiarity is that the agent is dimorphic, with two forms discernible on electron microscopy—with and without a nucleus. "To my knowledge, no other fungus works like that," Miller said. Since finding that first case in 1995, she has found the infection in two other monkeys at necropsy and has identified through bone marrow aspirates two more cases in clinically healthy monkeys.

Infected animals will live for many years with their infection. The yeast, Miller said, is "engulfed by macrophages and just sits there, causing no further inflammation. The yeast cells continue to multiply and become so numerous thatn they displace blood-producing cells in the bone marrow," generating the clinical symptoms that bring the infection to the attention of researchers and the veterinarians caring for the animals.

The agent resembles a variety of Histoplasma capsulatumfound only in Africa (var. duboisii), but the differences are clear on electron microscopy, Miller said. Even more, however, the mystery yeast resembles a disease-causing agent called Loboa loboi, which is found only in humans living in tropical regions in Central and South America and dolphins in the waters of the southern Atlantic. The disease, lobomycosis, causes a disfiguring dermatitis in humans and is confined to the skin and draining lymph nodes.

Though larger than the owl-monkey yeast, Loboa loboi cannot be cultured, is dimorphic, and has a similar cell wall. "Loboa is not well researched; it's probably never been sequenced. If we could sequence Loboa loboi, and determine that it and the owl-monkey agent are closely related, infected owl monkeys might be used to find therapies for humans," Miller remarked, noting that there is currently no treatment now for the human disease.

Earlier this year, Miller and her colleagues published the first report in the literature on the Peruvian owl-monkey yeast and its antemortem diagnosis through bone marrow aspirates—"Systemic yeast infection in owl monkeys (Aotus vociferans): ante-mortem screening and diagnosis by examination of bone marrow aspirates," Lab. Anim. Sci., 48:391­394, 1998. Another manuscript, similar to the poster presentation at the NIH Research Festival, has been accepted for publication by The Journal of Medical Mycology. Miller is now investigating the reliability of a urine antigen assay as a less invasive diagnostic tool.

Although animals may appear clinically healthy, they can harbor infections that add undesirable variables and may compromise the validity of a study's findings. The owl monkey yeast infection cases, Miller said, "clearly illustrates the benefit of routine pathologic evaluation of even healthy-appearing animals used in biomedical research."

Miller can be reached by e-mail at <millerg@vrp.ncrr.nih.gov>. She and the VRP pathology service can be called at 496-4465.

—Fran Pollner