Head and lateral line erosion, also known as HLLE, hole-in-the-head disease, or HITH, is a fish disease that affects both freshwater and marine fish in captivity. Sometimes marine head and lateral line erosion or MHLLE is used to refer to the marine version, though the causes and treatments for each are similar. Among freshwater fish, it affects cichlid

Cichlid

Cichlids are fishes from the family Cichlidae in the order Perciformes. Cichlids are members of a group known as the Labroidei along with the wrasses , damselfish , and surfperches . This family is both large and diverse. At least 1,300 species have been scientifically described, making it one of...

s and angelfish

Pterophyllum

Pterophyllum is a small genus of freshwater fish from the family Cichlidae known to most aquarists as "Angelfish". All Pterophyllum species originate from the Amazon River, Orinoco River and Essequibo River basins in tropical South America...

, with Oscars developing the disease more frequently than other related fish. Angelfish, tangs, and grouper

Grouper

Groupers are fish of any of a number of genera in the subfamily Epinephelinae of the family Serranidae, in the order Perciformes.Not all serranids are called groupers; the family also includes the sea basses. The common name grouper is usually given to fish in one of two large genera: Epinephelus...

s, and to a lesser degree lionfish

Lionfish

Lionfish may refer to:* genus Pterois, collectively known as the lionfish* Red Lionfish , a significant invasive species off the East Coast of North America and in the Caribbean-Fish:...

, damselfish

Damselfish

Damselfish comprise the family Pomacentridae except those of the genera Amphiprion and Premnas. They can grow up to long. While most are marine, a few species inhabit the lower stretches of rivers in freshwater. Damselfish usually have bright colors. in tropical coral reefs, and many of those are...

, and clownfish

Clownfish

Clownfish or anemonefish are fishes from the subfamily Amphiprioninae in the family Pomacentridae. Twenty-eight species are recognized, one in the genus Premnas, while the remaining are in the genus Amphiprion. In the wild they all form symbiotic mutualisms with sea anemones...

 are susceptible in marine aquaria.

Symptoms

Beginning as small pits around the fish’s eyes, light colored lesions then develop along the fish’s lateral line system, onto the body and sometimes onto the unpaired fins. Rarely fatal, it does cause disfigurement, making the fish less suitable for public aquarium display. At least 20 families of fish have been identified as having developed HLLE in captivity. Not all species of fish show the same symptoms, and do not always develop lesions to the same degree (Hemdal 2006).

Causes

There is much debate as to the cause of HLLE. Very little scientific research

Scientific method

Scientific method refers to a body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. To be termed scientific, a method of inquiry must be based on gathering empirical and measurable evidence subject to specific principles of...

 has been done on the topic, and most information available is anecdotal.

Removing carbon from the filter is a commonly suggested remedy for the disease. It is suggested that the carbon may either a.) add fine carbon particles to the water that irritate the skin, b.) leach potentially harmful chemicals into the water, or c.) remove minerals that are important to the health of the fish. Though there have been many reports of fish being affected by carbon, only one study has substantiated this; http://www.coralmagazine-us.com/content/hlle-and-activated-carbon-looking-link

Some people believe that parasites of the genus Hexamita

Hexamita

Hexamita is a genus of parasitic diplomonads.It is related to Giardia.H. columbae and H. meleagridis lives in the intestines of birds.H. muris and H. pitheci lives in the intestines of mammals.H. salmonis and H...

, a flagellated protozoan

Protozoa

Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...

, is to blame for this syndrome in freshwater fishes. HLLE and Hexamita infections are often seen in the same specimens. However, HLLE can be found in many fish who do not have a Hexamita infection, suggesting that a Hexamita infection may cause stress or interfere with the absorption of vitamins and minerals causing malnutrition, which may be the actual cause of the disease. Hexamita may also be a secondary infection

Infection

An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 common in fish already weakened by HLLE.

One non-replicated study showed that improvement in nutrition

Nutrition

Nutrition is the provision, to cells and organisms, of the materials necessary to support life. Many common health problems can be prevented or alleviated with a healthy diet....

 will help symptoms most effectively (Blasiola 1992). Key vitamins in preventing or curing HLLE seem to be Vitamins A, C, and D. Adding these vitamins to the diet of affected fish usually leads to improvement, however, deficiencies in any of these vitamins do not always lead to HLLE, so nutrition is also questionable as a cause.

It has also been suggested that HLLE is an autoimmune disorder

Autoimmunity

Autoimmunity is the failure of an organism to recognize its own constituent parts as self, which allows an immune response against its own cells and tissues. Any disease that results from such an aberrant immune response is termed an autoimmune disease...

that is triggered by stress. The disease does not appear to occur to fish in the wild, only those in captivity, supporting the idea that stress and unnatural living conditions are to blame. Anything that reduces stress appears to help in the prevention and recovery from this disease.