Protozoal Parasite: Myxobolus subtecalis

Volume 4, Issue 8
August 2002

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.

Mummichug with four epidermal Myxobolus xenoma neaar tail

Figure 1. Mummichug with four epidermal Myxobolus xenoma near tail.

Myxobolus subtecalis spores were found in the grossly visible creamy white epidermal xenomas on the sides of a mummichug (Fundulus heteroclitus). Each white xenoma contained thousands of mature and immature spores.

Myxobolus subecalis spores

Figure 2. Thousands of Myxobolus subtecalis spores from one mummichug epidermal xenoma. 100x phase contrast microscopy. Protozoa unstained.

The life cycle of M. subtecalis likely resembles that of other Myxosporidia parasites: Myxobolus sp., Henneguya sp., Ceratomyxa sp., and Kudoa sp. Spores (Fig. 2) are released from the fish’s skin.

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. 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.

Myxobolus subtecalis salvelinus

Figure 3. Myxobolus subtecalis salvelinus 1000x phase contrast microscopy. Protozoa unstained.

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 site of infection (skin). There is a period of endogenous cell division where a single cell multiplies into tens if not hundreds of new parasites
(Figure 1).

Mummichugs are common baitfish in Maine. Anglers using live bait should be careful not to use baitfish infected with parasites like Myxobolus subtecalis.

Special points of interest:

Fish with Myxobolus cannot transmit the disease to people.

Myxobolus is a protozoan parasite.

Myxobolus infects many species of fish.

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