FIELD AND LABORATORY STUDIES OF GLUGEA HERTWIGI (MICROSPORIDA) IN THE RAINBOW SMELT OSMERUS MORDAX

¹ The Marine Biological Laboratory, Woods Hole, Massachusetts 02543
² The Department of Zoology and Physiology, Louisiana State University, Baton Rouge, Louisiana 70893

Glugea hertwigi-induced microsporidosis is a disease of the smelt Osmerus mordax. The yearly incidence of infection reaches over 50% in adult smelt and as high as 90% in juveniles. Primary infections localize as large intracellular colonies in submucosal cells of the digestive tract. Field observations indicate the ovaries of spawning females are the secondary site of infection. G. hertwigi was successfully transmitted to both laboratory-reared and collected young smelt at 20° C by small filter-feeding vectors and by direct ingestion of spores. Infections transmitted by spore-carrying vectors numbered hundreds per animal, and were visible along the intestine one week after feeding. Large parasitized host-cells (xenomas) extended from the intestinal serosa and were easily recovered. G. hertwigi infections acquired by direct spore feeding numbered one or two per animal; these fish have the capacity to develop many new infections by feeding on spore-carrying vectors. Microscopic study revealed that G. hertzwigi development was indeed within a single greatly hypertrophied host cell. After 1 week of growth, 20 to 5O-µm xenomas contained a few enlarged host nuclei and vegetative G. hertwigi; after 2 weeks, the xenomas measured 100 to 125 µm, exhibited multiple host nuclei and numerous G. hertwigi sporoblasts and spores.