The external form of Hexamita is referred to as: Hole in the head disease. This disease affects susceptible fish such as discus, angelfish and oscars, which seem to be the most popular carriers of this problem (note that this disease may affect all fish). This disease usually starts out as a small "pimple" on the head, and as the condition proceeds in severity, ends up to be a very large sore causing lesions in the epithelium and... eventually ending up in death of the fish. Sometimes you are able to see small white nodules, sticking up out of the sore. The lateral line is also another area where this parasitic protozoan can be seen.
This disease is best treated early on with Metronidazole, or something stronger like Quinine Sulfate.
Environmental causes can be divided up into two major sub categories, those being water conditions, and stress. Poor water conditions have been linked to many cases of HITH and can be a major factor in its onset. It is important to say that this is pretty much a blanket category encompassing all aquarium disease, as problems are rarely seen where water quality is not an issue. It is possible to have diseased fish and have perfect water but this it is uncommon at best. That being said, proper water filtration and movement is a must. As a rule of thumb your ammonia should read 0, nitrites 0, and nitrates should be less then 40 ppm, and less then 10 ppm is ideal. Nitrates have been linked in some studies to the development of HITH where a fish is kept for long periods of time in water with levels exceeding 40 ppm. Stress can come in many different forms, but competition between tank mates is one to watch in the aquarium. This usually manifests itself in two ways, either unfair competition for food, or pestering. If you have fish of different aggression levels in the same tank (not necessarily fish of different species, as each fish has a different personality), one or both is likely. Either the more aggressive fish will pick at or straight out beat up the less aggressive fish or it will simply eat more then its share of food leaving the other with less then adequate portions. This is highly stressful and over time will wear at the fish’s immune system and can contribute directly to the next factor.
Diet is the most commonly accepted and supported cause of HITH. It is believed that the disease is more akin to scurvy, or rickets; both vitamin deficiency diseases in humans (scurvy is caused by not having enough vitamin C in your diet over extended periods; early sailors would get it because fresh fruit and vegetables could not be stored for long voyages). Just as the human body degenerates if proper diet is neglected for too long so do aquarium fish. If an unbalanced, or an out right improper diet is fed over a long period of time the fish can suffer greatly. Don't mistake a fat fish with a healthy fish, just as you wouldn't mistake a fat person for a healthy person. Simply eating food is not good enough; it has to be the right food. A link has been found between the development of HITH and a lack of calcium, phosphorous, and vitamins C & D. Many of the common fish foods sold in the hobby today are enriched with vitamins to make them more balanced, though supplemental vitamins can be added to them as well. Know the diet of your fish and be consistent, and varied. As a side note for carnivorous and semi-carnivorous fish such as Oscars, red devils, and Jaguars, feeder fish should never be chosen as a primary food source because they have virtually no nutritional value. In addition feeders contain the enzyme thaimase which breaks down thiamine. Thiamine is an important vitamin and if you use feeders as a large portion of your fish’s diet it WILL develop a thiamine deficiency. Fish fed exclusively or largely diets of feeder fish are extremely likely to develop HITH. They are also in high risk of contracting other diseases, such as ich, numerous other parasites, or fungal infections.
