Ichthyobodo necatrix, formerly and still commonly called Costia, is a flagellated protozoal ectoparasite and a normal inhabitant of fish skin, although in very small numbers. Ichthyobodo does its damage by feeding on host epithelial tissues. Poor water quality and other stresses (especially crowding) may allow this mutualistic parasite to reproduce rapidly and overwhelm the fish host.
Although primarily a problem of freshwater species, this disease has been reported from several marine fishes. Infection usually follows a recent stress or the addition of unquarantined fish to an aquarium or pond.
The skin of affected fish may appear "cloudy" or covered with a thin mucus layer. Infected fish commonly have clamped fins, gulp at the surface for air, and "shimmy" or "shake" back and forth in the water column. Since the individual parasites are so small, you generally won't observe white spots or raised lesions. Severe gill infestations may lead to respiratory compromise and this condition is frequently fatal.
With the aid of a microscope and some basic surgical instruments, your veterinarian can confirm a diagnosis of ichthyobodosis by taking biopsies that reveal numerous, actively mobile, tear drop or comma-shaped flagellated protozoans that are less than 10 microns in size. The parasites may be attached to epithelial (surface) tissue or quickly rolling and turning in the water close to the skin and gills. Depending on the life stage, between two and four flagella are present, which are difficult to distinguish because of their small size and quick movements. Other problems which resemble ichthyobodosis include water quality disorders (like ammonia toxicity), bacterial skin disease, and other protozoal ectoparasitic disease.
Fish that are infected but not yet showing clinical signs stand a reasonable chance of recovery. Ichthyobodosis is responsive to treatment with formaldehyde and malachite green but it is harder to eradicate than most protozoal ectoparasites (external parasites) and is generally resistant to salt therapy. You should only attempt treatment of this disease once an accurate diagnosis has been made and then only under the supervision of your veterinarian.
Once your veterinarian or fish health professional has diagnosed ichthyobodosis, treatment should begin immediately. You should also perform frequent water changes, reduce crowding, and optimize water temperatures if possible.
All new fish should be quarantined for at least four weeks prior to being placed into any aquarium or pond. You should also carefully examine all fish in the aquarium at the pet store for signs of this disease.
By maintaining good water quality and not crowding your fishes (in other words, keeping environmental stressors to a minimum) you can help prevent transfer of this disease from one fish to another.