Hexamitiasis is a parasitic disease that can affect both freshwater and saltwater fish. In some cases, lesions appear on the head and flanks of the fish, hence its alternative name: Hole in the Head disease. Hexamita infections may also be responsible for Head and Lateral Line Erosion (HLLE) disease in fish.
Fish infected with Hexamita commonly produce white, stringy feces, and their coloration becomes more subdued than normal. The fish eventually lose their appetite and gradually become emaciated. Often (but not always), lesions appear on the fish’s head. Sometimes, these lesions are also apparent on the fish’s flanks.
Among freshwater fish, Hexamitiasis and Hole in the Head disease most commonly affect perciform fish, particularly cichlid fish. Carps, including koi and goldfish, are also vulnerable. A wide variety of saltwater fish can suffer from head and lateral line erosion, but angelfish and surgeonfish seem to be particularly susceptible.
Hexamitiasis is caused by various species of the protozoan parasite genus Hexamita. These are likely present at low levels in the intestines of many aquarium fish where they do no harm. It is only when the fish is weakened in some way that the parasites can multiply rapidly and spread into the rest of the fish’s body. Once the parasites become established in the other internal organs, the fish soon dies.
Lab work on infected salmon and trout suggests that increased mucus production by the intestine as a response to the burgeoning parasite population is the cause of the copious and pale-colored feces associated with the disease. How and why the parasites are able to switch from their normal intestinal form into the systemic form that can spread throughout the body is not known.
The relationship between Hexamita and hole in the head disease is reasonably well-established. The lesions are caused by Hexamita parasites that have migrated into the sensory pores on the head where they cause the surrounding tissue to decay. Because of the damage to the epidermis, secondary infections easily set in, making things worse.
By contrast, it is not certain that all cases of head and lateral line erosion disease in fish are caused by Hexamita parasites. Malnutrition in particular has been proposed as a possible cause of the disease in saltwater fish at least. Because many fish likely carry low-level Hexamita infections, anyway, the disease isn’t normally considered to be contagious.
The prime medication used to treat hexamitiasis is metronidazole (often referred to by the trade name Flagyl). The most effective approach is using medicated fish food, especially when early infections are being dealt with. But seriously affected fish may not be eating, in which case you need to add the medication to the water, typically at a dosage of 250 mg per 10 gallons, once per day for at least three days. Alongside the medication, improvements to diet and environment are absolutely critical.