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Myxosporidiosis: Henneguya sp. infestation

Volume 4, Issue 5
May 2002

Fish live in environments inhabited by thousands of types of parasitic organisms, varying from small, unicellular protozoa, to much larger and more complex worms and to other fishes. Parasites are organisms that live in or on another organism (the host) and depend on the host for food, have a higher reproductive potential than the host, and may harm the host when present in large numbers. Generally, parasites do not kill their host outright as it would destroy its source of ‘dinner.’ However, heavy parasitic loads can be fatal, or cause the host to be vulnerable to other fatal threats.

Henniguya spores

Figure 1. Henneguya spores 1000x phase contrast microscopy. [—–] approximately 5 μm.

Henneguya is a fish parasite that seldom causes severe harm to the host. A wide variety of freshwater fishes have been reported with infections of Henneguya sp. Some susceptible species in Maine include, brook trout (S. fontinalis), landlocked Atlantic salmon (S. salar), rainbow trout (O. mykiss), chain pickerel (E. niger), northern pike (E. lucius) , and brown bullheads (I. nebulosus) . Henneguya is among the most cosmopolitan of all the Myxosporidian parasites. Species in this genus have been found worldwide, infecting hundreds of different fish species.

Various Henneguya species produce different lesions in different hosts. Henneguyiasis is suspected when an opaque mass is found in various tissues of the fish’s body.

Chain pickerel gill arch with Henneguya xenoma

Figure 2. Chain pickerel gill arch with Henneguya xenoma.

External examination may reveal cysts in the skin and gills (Fig. 3); whereas, internal lesions may be found on the liver, heart, kidney, spleen or any other organ. Microscopic examination of the Henneguya sp. “xenomas” reveal tadpole shaped unicellular organisms with two eye-like polar capsules inside (Fig. 1&2). Infections are usually not life threatening to the fish unless they impair the function of a vital organ.

Henneguya spread through the water. When a xenoma ruptures, millions of Henneguya spores are released. The spores drift in the water and attach to a new host with a grappling hook-like organ called a polar filament. Once attached to a new host, the organism forms a new xenoma and begins to multiply. Fish veterinarians, culturists, biologists and others concerned with fish health may treat infected fish with chemotherapeutic agents or surgical removal. Unfortunately, many parasites, including Henneguya, are not easily controlled by any therapeutic procedure, thus prevention remains the best medicine.

Special points of Interest:


Henneguyiasis is caused by a Myxosporidian parasite.

Henneguya infects many species of fishes, but does not infect people. People may spread the parasite unwittingly on fishing equipment, or by improper disposal of fish entrails.

For more information read: Hoffman 1999 Parasites of North American Freshwater Fishes.

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