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The Hole-In-The-Head/Lateral line erosion FAQ
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Q: What causes HLLE?

A: Nobody knows. :-P There is not enough research to authoritatively say what causes HLLE, however, there are several theories:

HEXAMITA SPECIES: These flagellates can cause problems in tropical fish, but their association with HLLE is not conclusive. It is suggested that they may spread through the fish's system, causing damage directly (1), or interfere with absorption of nutrients, with the malnutrition of the fish being the actual cause of HLLE. (2) In favor of this theory is the presence of Hexamita in many fish with HLLE, and that treating with anti-hexamita medications is often effective against HLLE. However, cases of HLLE with no Hexamita present indicate that while Hexamita may be one cause of HLLE, it is apparently not the cause in all cases.

THE CALCIUM/PHOSPHORUS DEFICIENCY THEORY: HLLE may be the result of deficiencies in these minerals. This theory is supported by low levels of calcium/phosphorus in affected fish and reports of adding calcium/phosphorus supplements being beneficial to the recovery of fish with HLLE. It is suggested that the deficiency is caused by an infestation of Hexamita interfering with absorption of nutrients. (2) That most flake and pellet foods provide a great deal of both minerals, yet HLLE occurs in fish on such diets suggests that other factors are involved (listed in the other theories) that interfere with the absorption and/or utilization of these minerals.

VITAMIN C DEFICIENCY: It has been noted that adding vitamin C can aid in the recovery of fish suffering from HLLE, leading to speculation that lack of vitamin C is the original cause of the disease. While vitamin C is very important to the immune system and the healing of wounds, fish fed vitamin C deficient diets show no signs of HLLE but do show deformities not found in HLLE affected fish. (3) Because it is such an important vitamin in the recovery from illness and wounds, adding a vitamin C supplement to the food of fish being treated for HLLE is recommended. Fish fed large doses of vitamin C have still developed HLLE (5), so the mere presence of C in the diet doesn't appear to offer protection from HLLE, although it still has other benefits.

VITAMIN D DEFICIENCY: Insufficient levels of vitamin D have also been proposed as the cause of HLLE. This theory has a certain 'neatness' to it, as vitamin D has been identified as the precursor to a hormone important in maintaining levels of calcium and phosphorus in the blood and bones (3) which explains low levels of both minerals in affected fish. In support of this theory are reports that adding D to the diet of fish with HLLE is beneficial (2), as well as observations that exposure to ultraviolet light (which is used to manufacture D in the fish's skin) may be beneficial. (4) Since little vitamin D is found in most ingredients of fish foods, vitamin D should be added as a supplement in most commercial fish foods. (3) Like vitamin C, however, HLLE has NOT been produced in the laboratory as a result of feeding vitamin D deficient diets.

THE STRESS THEORY: HLLE has also often been associated with poor tank maintenance, and been suggested as a reaction to constant stress. (1) This might occur as a result of higher nutritional needs while under stress not being met, or by simply weakening the fish to the point where they are vulnerable to infections like Hexamita (or other unidentified pathogens.) My personal experience supports the association with poor water conditions, although anecdotal evidence should always be treated with some skepticism.

THE CARBON THEORIES: There are several theories that the use of activated carbon in tanks may cause HLLE.

One version of this theory is that the carbon could be causing nutritional deficiencies by adsorbing vitamins or trace minerals either directly from the water, or by adsorbing molecules that the needed nutrients could then bind to. This theory, however, has several problems: There aren't any vitamins in tap water to begin with, and minerals like calcium, phosphorus, and iron should be available in large quantities in most prepared fish foods. Additionally, it is reported that fish rely on food for nutrients, trace and otherwise, making the presence of nutrients in the water irrelevant (4) although fish have been shown to be able to extract various minerals from the water (calcium, iron, zinc, etc.)

A second carbon theory is that some types of carbon may release toxins which cause HLLE. Perhaps a more specific variant of this theory is that some poorly manufactured carbons might release chemicals that cause stress to the fish. (See THE STRESS THEORY.) Another theory is that loose particles of carbon dust in the water have some caustic affect, but there is no support for this that I have been able to find.

All carbon-related theories regarding the cause of HLLE suffer from one other significant shortcoming: HLLE can be found in tanks which have never had carbon of any sort used in them, yet is NOT found in many tanks using carbon. While it remains possible that carbon may be involved in some cases, there is no reason to believe it is a factor in most cases, if any. There is no sound evidence for a link, or even a logical specific theory--at best, the carbon-HLLE theories amount to a handful of individual cases where HLLE was seen in tanks using carbon, but appeared to improve after the carbon was removed. Since there is little to no information on what other steps where taken in the treatment of the HLLE, the association with carbon is rather suspect.

ASSORTED THEORIES: Electrical currents in the tank, a virus, and a few other factors have also been suggested as causes of HLLE. No data available on their likelihood as a cause.


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