Microsporiasis ( Glugea hertwigi)

Volume 2, Issue 1
March 2000
Updated November 2002

Glugea hertwigi is one species of several intracellular microsporans found to infect freshwater, marine, and anadromous fishes. The phyla, Microspora, includes the following genera of similar organisms infecting fish: Glugea, Ichthyosporidium, Loma, Microgemma, Microsporidium Mrazekia, Nosema, Pleistophora, Spraguea, Tetramicra, and Theragra.

Glugea infection in Rainbow Smelt

G. hertwigi, like G. stephani, is probably an obligate intra-cellular parasite with a direct life cycle needing only fish. Crustacean prey may act as reservoirs of infection. Ingested spores hatch in the gut and enter epithelial cells for development or transport to the developmental site. A minute tube, the polar filament, then everts through the spore wall and penetrates a host cell. The infective agent, or sporoplasm, passes via the filament to the host cytoplasm, where multiplication or merogony occurs. The second development stage, sporogony, produces two spores packed in vesicles. Glugea spores may be released from the skin, in feces, in urine, or on death of the host. Juvenile fish are particularly susceptible to infection.

The gross diagnosis of microsporiasis is made by the appearance of tumor-like cysts (called xenomas) in the fish’s tissue. G. hertwigi xenomas are usually found on internal organs including the gastrointestinal tract, liver, and reproductive organs. The xenomas contain many microscopic spores of the G. hertwigi parasite which are approximately 7.5μm long by 3.5 μm wide.

G. hertwigi has been associated with large Rainbow smelt (Osmerus mordax) fish kills on several occasions. It has been found in Maine waters, Lake Erie, Lake Ontario, and eastern parts of Canada.




Glugea exists in Maine.
There is no cure for Glugea in Rainbow smelt.
Glugea hertwigi will not cause disease in humans.
Fish with xenomas may have Glugea sp. or one of several other Microspora.

Control Measures

There is no current practical way of removing Glugea sp. from a wild fish population without complete extirpation of all fishes in the entire water body. The Maine Department of Inland Fisheries and Wildlife screens populations of rainbow smelt and other fishes for parasites, bacterial pathogens, and viral pathogens prior to live fish or egg transfers. Rainbow smelt transfers commonly are fertilized egg movements which reduces the likelihood of transferring sick or infected fish. Individuals fishing in Maine can help prevent the spread of Glugea and other fish pathogens by following these simple good fishing practices:

Don’t release live baitfish into water.

Don’t dispose of fish heads, skeletons or entrails in any body of water. Fish parts should be disposed of in the garbage, by deep burying or by total burning.

Don’t transport live fish between bodies of water. This practice could spread disease and is strictly illegal.


Delisle, C.E. 1972. Monthly variations of Glugea hertwigi (Sporozoa: Microsporida) in different tissues and organs of freshwater smelt and the consequences of this infection on the annual massive mortality of this fish. Can. J. Zool. Dec;50 (12): 1589-1600.

Hoffman, G.L. 1967. Parasites of North American Freshwater Fishes, University of California Press, Los Angeles.

Lassee, B. 1995. Introduction to Fish Health Management. USFWS, LaCrosse, Wi.

Nepszy, S., J. Budd, and A. O. Dechtiar. 1978. Mortality of young-of-the-year rainbow smelt (Osmerus mordax) in Lake Erie associated with the occurrence of Glugea hertwigi. J. Wild. Diseases 14:233-239.

Stoskopf, M. 1994. Fish Medicine. Saunders Publishing, Philadelphia.

Woodward, W. 1973. Glugea hertwigi. Maine Fish and Wildlife Magazine. Spring Issue.

glugea hertwigi cycts

Images were made possible in part by a grant from the Maine Outdoor Heritage Fund.