Hypothesis of Head and Lateral Line Erosion in Fish

Part - 1

HLLE, Stress and Immune Function

By: Terry D. Bartelme

Controversy

Few diseases or conditions in captive fish inspire as much speculation, controversy, opinion and conversation as Head and Lateral Line Erosion (HLLE). Although the primary etiology of HLLE has not been established, it seems like everyone has a different idea of what causes this syndrome. There is little in the way of conclusive scientific evidence about what actually causes HLLE. However, conflicting anecdotal evidence as to the cause of this condition is plentiful. The disease is not particularly significant to commercial fish enterprise so it does not foster a great deal of scientific research. This places at least a portion of the burden of finding viable treatments and/or a cure to hobbyists and those in the tropical fish industry. Some say HLLE is caused by stray electrical voltage in the aquarium, others suggest that the condition is brought on by dietary deficiency. Pathogens including fungal, bacterial, viral and parasitic have been implicated as causing HLLE. Toxins or heavy metals in the water are other reported causes as well as poor water quality in general. Poor water quality would include a high dissolved organics level, suspended solids, high nitrate level and other impurities. Carbon that is used to remove impurities from the water has also been suggested as suspect.

What is Head and Lateral Line Erosion?

You may be asking yourself; just what is Head and Lateral Line Erosion? This syndrome could be described as a disease or condition that causes skin, scale and tissue loss in the head region and/or along the lateral line sensory system or flanks of fish. The lesions first appear as pinpoint foci that grow in size and numbers. These lesions are non-hemorrhagic ulcers that are devoid of pigmentation and a normal epidermis.

What is the Lateral Line Sensory System?

What is the Head and Lateral Line Sensory System? The lateral line system consists of a row of pores that runs along the flank of fish. Shorter rows of pores can be found in the head region and other areas. The pores contain sensory cells and hair-like projections. Together the subcutaneous pits, sensory cells and hair-like projections make up the lateral line system.

What is the Function of the Lateral Line Sensory System?

The lateral line system gives fish the ability to sense movement and water flow. This helps them to synchronize with their shoal (school of fish), feel the proximity of objects in the water, perceive movement, and orient to the direction of the water flow. It also helps fish with hearing and feeding.

What Happens to Fish with HLLE?

Effected fish generally continue to eat and behave normally, unless the condition becomes greatly advanced. The syndrome itself is usually not fatal, but it is unattractive and makes it difficult for retailers to sell these fish. The loss of tissue and the normal protective barrier along the Head and Lateral Line System causes several complications. It makes maintaining hydro mineral balance between the internal fluids of fish and the surrounding ambient water more difficult. Performing osmoregulation becomes more costly energy-wise. Fish are also more susceptible to opportunistic pathogens, especially those that are bacterial, or viral in nature.

Some Reported Causes for HLLE

A 1991 report written in The Journal of Aquatic Animal Health by P.W. Warner and D.H. Lewis, concludes that a virus is the cause of HLLE (3:198-205). Steve Collins, curator of the Indianapolis Zoo-Aquarium, wrote and article in summer 1995 issue of SeaScope about controlling HLLE with dietary changes. No obvious pathogens were found in the fish studied. Supplementing the diet with vitamin C did not get positive results. However, vitamin A in the form of broccoli and carrots appeared to improve or cure the condition. Head and lateral line erosion has also been associated with an acid-fast bacterial infection (Edward Noga, unpublished data.). Bacteria that cause fish tuberculosis fall into this category.

Like I said previously, everyone seems to have a different idea for what may cause HLLE. I see one common theme that ties all of these various suspected causes together: stress. The importance of stress as the underlying theme cannot be overemphasized.

Stress and Immune Response

Chronic stress can come in many forms. These would include but are not limited to: low pH, toxins in the water, inappropriate water temperature, chemicals, heavy metals, crowding, low dissolved oxygen level, a high dissolved organics level, the presence of pathogens at an intolerable density, a photo period that is either too long or too short and suspended solids. Acute stressors would include photo-shock, aggression from tank-mates, electrical shock, handling, transport and netting. An argument can be made for including an inadequate diet as a stressor or stress factor.

Hormones are released into the blood in response to stressors. Chronic production of cortisol (a stress hormone) does inhibit immune function. These "stress hormones" are adaptive under natural circumstances. However, in captivity, fish are subjected to stressors much more frequently than what would be considered natural circumstances. Under these conditions, the stress response and subsequent release of stress hormones becomes maladaptive in nature. This reduces immune function efficiency and causes other complications. Thermal stress alone can chemically change and reduce mucus production by fifty-percent. This is significant since the mucus layer is the first line of defense against potential pathogens.

Chronic stress causes fish to become more susceptible to infectious diseases caused by bacterial, fungal, parasitic and viral pathogens. I believe stress can also open the way for diseases that are non-infectious in nature such as autoimmune disease.

What is Autoimmune Disease?

A healthy immune system normally defends the body against invading pathogens and disease. Autoimmune disease (we will refer to this as "AD") is when the immune system attacks self, targeting organs, cells and tissues of its own body.

What Causes AD and HLLE?

Viral infections have been known to trigger the onset of autoimmune disease in humans. It is possible that this is also true in fish. Could other pathogens such as bacteria, fungus and parasites also trigger the onset of AD and consequently HLLE in fish? Some other influences that affect the course of autoimmune disease include aging, hormones and stress. Stress does cause the release of "stress hormones" in fish and the fact that stress influences hormone production may serve to exacerbate the situation. When stress is chronic it negatively impacts immune function. Chronic stress in fish can lead to a generalized melanosis and the loss of tissue between fin rays. Could it also cause tissue loss in the lateral line sensory system?

What Fish are Susceptible?

Neither HLLE nor AD appears to be contagious. Genetics plays an established role in human autoimmune disease. Certain autoimmune diseases occur frequently among family members. Taking this a bit further, could we surmise that certain families or species of fish would also be more susceptible to autoimmune disease? HLLE is frequently seen in tangs (Acanthuridae) and angels (Pomacanthidae). Groupers, lionfish, damsels and clownfish and other species suffer from the condition less often. There appears to be a strong correlation between the incidence of this syndrome and the species of fish involved. Larger, older (long-term aquarium inhabitants) fish appear to be more frequently stricken with HLLE. This may be due to the fact that they have been fed an inadequate diet long enough to exhibit the clinical signs of HLLE. Perhaps the immune systems of these older fish simply do not function as well as those of fish that have been in captivity a shorter period of time.

Stress and Autoimmune Disease

It is a well-documented fact that stress suppresses immune function. Since stress does inhibit some aspects of immune function it is not a stretch to assume that it may cause the immune system to malfunction in other ways. This may be what happens in Head and Lateral Line Erosion. While stress may inhibit some aspects of immune function it may also over-stimulate still other aspects of immune system function. Could stress also cause the overproduction of reactive oxygen intermediate molecules that are made by macrophages and neutrophils in the immune system? This would lead to the destruction of normal tissue surrounding these immune system components. Could stress in fish cause T cells in the immune system to overproduce chemokines and cytokine molecules that attract destructive macrophages (white blood cells)? Is it possible that stress can cause B cells to mistakenly make antibodies against the body's own tissues? All of these suppositions are apparently true in human autoimmune disease.

The second installment in this three-part series will include my hypothesis of what causes Head and Lateral Line Erosion. I will also cover autoimmune disease, treatment and how to eliminate or reduce stress in fish. Part three will include sections on how immune function and nutrition is related, how we can enhance immune function, the treatment of diseases secondary to HLLE and the prognosis for recovery.