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Protozoal parasite: Myxidium salvelini

Volume 4, Issue 2
February 2002

Myxidium salvelini

Figure 1. Urinary bladder epithelium infested with multinucleate, polysporic plasmodia of M. salvelini.

Parasitic protozoa probably cause more disease in fish culture than any other type of animal parasite. Under environmental circumstances, minor protozoan infestations produce little obvious damage, but in large numbers they can cause extensive internal and external injury and death. Protozoan parasites have been found in nearly every fish tissue including even the fish’s urinary bladder!

isolated myxidium salvelini

Figure 2. Myxidium salvelini isolated from S. fontinalis urine and ovarian fluid.

Myxidium salvelini is one such parasite. M. salvelini spores were found in the urine and ovarian fluid of brook trout brood fish in Maine.

Further investigation of the source of this parasite revealed immature parasite life stages developing in brook trout kidney tissue and urinary bladder epithelium (Fig. 2 & 3).

The life cycle of M. salvelini likely resembles that of other Myxoxporidia parasites: Myxobolus sp., Henneguya sp., Ceratomyxa sp., and Kudoa sp. Spores (Fig. 2) are released from the fish during urination. The spores remain viable in the water and sediments for many years. When the spores are ingested by a tubificid worm, they further develop into a Triactinomyxon.

S.fontinalis kidney from fish infested with Myxidium salvelini

Figure 3. S. fontinalis kidney from fish infested with Myxidium salvelini.

The Triactinomyxon completes its own developmental cycle inside the oligochaete, producing spores with long caudal appendages which after contact with the trout host initiate a new infection.

After ingestion, myxosporean spores extrude the polar filaments in the fish’s digestive tract, the shell valves open and the sporoplasm escapes. The sporoplasm probably migrate across the intestinal wall and reaches the bloodstream or lymphatic duct and through them, the final sight of infection (kidney, bladder).

There is a period of endogenous cell division where a single cell multiplies into tens if not hundreds of new parasites (Figure 1). Cysts containing large numbers of Myxosporea are grossly visible.

Special points of interest:


Fish with Myxidium cannot transmit the disease to people.

Myxidium is a Myxozoan.

Myxidium species infest many fish species some cause disease others are seem benign.

For more information read: Lom and Dykova 1992 Protozoan Parasites of Fishes.

Images were made possible by a grant from the Maine Outdoor Heritage fund.