Common fish diseases: causes, symptoms and treatments

Fish farming has become increasingly important as it meets the demand for high quality animal protein. In addition, the consumption of fish offers consumers very specific nutrients that are fundamental to the human body, such as essential fatty acids.

Brazil has great potential for fish farming, since it has around 6.5 million hectares of reservoirs for the production of around 700,000 tons of fish per year.

The new fish breeding techniques have led to high density of animals, favoring the proliferation of parasitic outbreaks and bacterial diseases.

Some conditions may lead to an increase in this type of pathology in tropical fish, such as inappropriate management procedures, lack of systematic adoption of prophylactic measures, and inadequate water for the species to be created.

In addition, inadequate nutrition due to a lack of knowledge about the nutrients that should be consumed by a particular species can also lead to diseases.

It is important to know more about the most common fish diseases to avoid that the profitability of the business is impaired.

In this blog post we will learn more about ichthyophytariasis, hydropsy and holes in the head. We will discuss the causes, the main symptoms and the possible treatments and preventions for each of these diseases.

Ichthyophytariasis

Also known as ictus or white spot disease, it usually occurs when fish suffer sudden drops in temperature. It is caused by the protozoan Ichthyophthyrius multifiliis.

This parasite needs to penetrate into the epithelial tissue of the fish to complete its life cycle. Penetration usually occurs between the scales, fins, cornea, oral cavity and gill tissue.

From that point on, the parasite undergoes structural changes that lead to the change of life phase, where it feeds on secretions, tissue fragments and inflammatory cells of the host. The development of this parasite is delayed at very low temperatures (<50 ºF) and very high (> 82,4 ºF).

The main form of transmission of Ichthyophthyrius multifiliis happens when a parasitized fish becomes the source of infestation for the rest of the batch. In addition, the water in the tank is also one of the sources of transmission.

The presence of white spots along the body is the main indication of this disease. As the parasite has an irritating action, it is also common to observe fish exhibiting friction movements against the walls, bottom or screens of the tanks.

In more advanced stages of the disease, the fish becomes apathetic and lacks energy. There is also a change of coloration in the skin, respiratory difficulty and production of mucus.

Treatment of ichthyophytariasis should not be carried out at breeding sites, since the medicinal products administered cannot reach the places where the parasites are found.

The best way is to prevent transmission, taking advantage of the phase of life where the parasite has free swimming. In environments with temperature control, it is possible to increase it to 30 °C, provided that the species of fish created will resist to this condition.

Some prophylactic measures can be taken to avoid mortality outbreaks caused by this parasite:

• To use biological filters with aquatic macrophytes in the water. Thus, they are used as substrate for the parasite, which is removed by mechanical or UV filters;
• To disinfect tools used in fish farming and strengthen the hygiene of employees who have direct contact with the animals;
• To do quarantine whenever a new lot of fish arrives at the property, until the sanitary conditions of the tank are tested;
• To monitor water quality.

Hydropsy

It is mainly caused by bacteria of the genus Aeromonas. The presence of these bacteria in the body of the fish leads to the production and accumulation in the abdominal cavity of a pink or greenish liquid, rich in lymphocytes.

Therefore, the fish’s belly expands, which makes this disease also known as water’s belly.

In addition, these bacteria also cause severe intestinal and liver inflammation, kidney damage and hypertrophy of other organs, which can cause fish death within a few days.

One of the major factors that lead to the development of this disease is the ingestion of frozen foods that have suffered breaks in the cold chain. These breaks lead to the multiplication of the bacteria of the genus Aeromonas, which are the cause of this condition.

This problem can also occur in dry foods that are stored in places of high relative humidity

Another factor is water quality. Polluted waters and high temperatures (above 26 °C) are also favorable to the multiplication of this strain of bacteria.

This is especially true if there is too much feeding or inadequate feeding that will not be ingested by the fish. The presence of this organic matter in water is the substrate for bacterial multiplication.

In this way, it was already possible to understand that feeding is linked to the risk factors that lead to the development of hydropsy.

As a way to eliminate this risk factor, it is necessary to invest in high-quality fish feed that has complete nutrients that will be promptly ingested and absorbed by the animal’s organism, avoiding the accumulation of leftovers and excess excrement.

A good option to be used in fish farming, are the feeds produced based on chicken protein hydrolysate. This ingredient is produced by enzymatic hydrolysis, generating bioactive peptides that play beneficial roles in the body of fish.

In addition, PHF has a high content of proteins that have a balanced profile of amino acids and high digestibility, helping in the growth and development of the fish. Because it is so nutritious and as it has low ash content, PHF feeds reduce water pollution, helping to prevent the diseases discussed here.

In order to treat hydropsy, it is possible to use bacteriostatic treatments, such as Acriflavina or Malachite Green. It is also possible to add antibiotics to fish food.

Holes in the head

This disease mainly affects fish of the family Ciclidae, that have as representatives species like angelfish, oscar, peacock bass, discs, parrots and FH.

The main aspect of this disease is the erosion of the sensory pores located in the head and the side lines of the fish, like holes, hence the name of the disease. These holes gradually increase in size and can reach the bone, causing deformities in the fish.

Poor water quality, unbalanced feed and high nitrate levels are some of the factors that can lead to the development of this disease. However, the main cause is the presence of the protozoan Hexamita intestinalis.

Infection with this protozoan is fast, and within 24 hours it is able to reproduce inside the fish at high rates.

The presence of this parasite in fish is even more problematic for captive-bred animals. In nature, the eggs of the protozoan are released into the environment, infecting other fish, which attenuates the disease so that often the symptoms are not even visible.

However, when in captivity, the eggs of the protozoan can even infect the same fish again, creating a very high parasite load that leads to the various problems mentioned. If left untreated, the disease leads to fish mortality.

The main symptoms of the disease are the presence of gelatinous feces in the tank, anorexia, anemia and the characteristic holes in the head.

The control and cure of this disease are not simple. Hexamite is extremely resistant to salt concentration and UV filters. In addition, it spreads easily through the environment, also infecting other fish, often without apparent signs of infection.

Some farmers, however, say that metronidazole in water at 8 mg/L, with 5 repeated doses every 48 h and with 20% TPA before each new dose, is an effective treatment.

In addition, the addition of magnesium sulfate or bitter salt, 0.1% mixed with metronidazole in 1% food is also considered an effective treatment.

Conclusion

Ichthyophytariasis, hydropsy and holes in the head are some of the diseases that affect fishes the most. The losses that they can cause to the fish farming are enormous, being able to get to the loss of several batches of fish, damaging the profitability of the business.

One of the best ways to prevent the spread of these diseases is to work on prevention. For this, it is essential to maintain a good quality of the water where the fish are raised.

Quality food should be provided, so it does not generate leftover of feed and leftover excess in the water, eliminating one of the major risk factors for the development of such diseases.