Globally, tilapia is among the most important species produced in aquaculture that grows a 7% annually 7% [1] and the production of tilapia worldwide has reached the 6 million tons in 2020.

Production of tilapia

Top tilapia producers are China, Indonesia, Egypt, Philippines and Bangladesh and the main species are Oreochromis niloticus, O. mossambiculs, O. aureus and hybrids.

The success of tilapia production is due to its characteristics, as it easily adapts to different culture conditions (salinity, temperature), and has high reproductive and growth capacities. It can also get used to commercial diets and tilapia’s meat is appreciated by consumers due to its price.

Traditionally, it was considered resistant to stress and diseases, but the conditions in intensive farming proved this consideration to be wrong.

Predisposing factors

Industrial production of tilapia favors the presence of predisposing factors that lead to disease outbreaks, particularly those related to stress. These factors can be environmental, related to the animal or to the pathogen and impair the functioning of the immune system, a situation used by microorganisms present in the environment or the animal to cause a disease outbreak.

In some cases, paying attention to environmental conditions, such as temperature, or the production stage in which diseases appear can help to reach a diagnosis.

Main diseases of tilapia

Viral

The most relevant viral diseases that affect tilapia are characterized by sudden deaths and high mortality rates. Despite some vaccines are available, they are hardly effective, and no treatment has been proved useful.

• Tilapia-Lake Disease virus (TiLVD)

This disease was first described in Israel in 2009 [2] and is caused by a virus of the Orthomyxoviridae that is already present in several parts of the world.

It usually affects tilapias during the first production stages [3]. The most common sign, apart from acute mortality that can affect up to 90% of the tilapias, is skin discoloration that starts in the tip of tail and fins.

Eye lesions (cataracts, exophthalmia) and loss of the ocular function can be observed in affected animals, as well as dermal erosions and multifocal ulcers, necrotic areas in the liver and hemorrhages in the brain [2], [4], [5].

Diagnosis should be based on observation of clinical signs and RT-qPCR in affected liver, brain, spleen and/or kidneys. If the PCR is positive, histopathology or isolation of the virus in a cell culture will confirm the diagnosis [2].

• Infectious Spleen and Kidney Necrosis virus (ISKNV)

Emerging disease (FAO, 2018) caused by a megalocytivirus (DNA virus) that mainly affects fries and fingerlings. Despite it was described in the 90s, it was not until 2016 when it was isolated in the Middle East, the United States, Thailand, among other countries.

It produces notorious signs such as ascites, exophthalmia, lethargy and erratic swimming, apart from gill paleness and tail necrosis (rottenness) [8]. Mortality can affect all stages and be above 50%.

Diagnosis should be based on clinical signs and viral identification with PCR or isothermal amplification (LAMP) in the affected tissues.

• Lymphocystis Disease virus (LCDV)

Chronic disease caused by a DNA virus of the Alphairidovirinae subfamily that causes nodular lesions in the skin, muscle, intestines and other internal organs because of its development inside the fibroblasts [9].

Mortality is typically low, differently from other viral diseases. Despite this, since it worsens productive performance and causes sales losses (the affected tilapia cannot be put in the market), it has an important economic impact.

Diagnosis should be based on histopathology: affected cells form clusters with inclusion bodies and vacuoles in their cytoplasm. [8].

Bacterial

More than 30 bacterial pathogens are considered capable of infecting tilapias [10].  This article describes the most important ones.

• Streptococcus spp.

It is one of the bigger threats for tilapia production that produces a granulomatous septicaemia. The main species involved are S. agalactiae, S. iniae and S. dysgalactiae; which are usually present in the environment and take advantage of predisposing factors to cause disease.

They can affect tilapias during all the cycle, though it has been reported that, outbreaks typically appear during the latter stages in tilapias vaccinated during early in the production cycle.

The most characteristic signs are granulomatous lesions in the internal organs. Other signs include eye problems, such as exophthalmia, skin hemorrhages, ascites, meningoencephalitis, pericarditis, skin darkening and erratic or circular swimming [11]. Mortality can be up to 75%.

Diagnosis is usually based on the identification of the pathogen with microscopy or PCR in samples of affected tissues. It is a zoonosis that can affect manipulators by infecting the wounds in their hands.

• Francisella spp.

This disease caused by Francisella noatunensis supesp. orientalis can affect tilapias during all the productive cycle, despite it is more frequent in the initial stages and is particularly related to cold stress (20-25°C). The temperature is the differential factor for the granulomatous lesions caused by Edwarsiella.

The most characteristic lesions are granulomas in the gills, spleen, and kidneys. Affected tilapia are lethargic, show erratic swimming, anorexia, and even fusion of the gill laminae. Mortality be up to 40-50%.

For the final diagnosis,  the microorganisms can be isolated from samples of affected tissues [12].

• Edwarsiellosis

It is a zoonosis and produces granulomatous septicemia in tilapias. The main species involved are E. tarda and E. ictaluri. It often affects tilapias during the first month of production, after the transportation of larvae to the grow-out facilities in spring and summer (warmer temperatures).

Gill and muscle paleness, ascites, whitish nodules in the spleen, kidneys and liver are the most characteristic signs and mortality can be variable, from 10 to 80%.

To confirm the diagnosis, isolate the microorganism the affected tissues or conduct specific PCR-tests.

• Flavobacteriosis

This disease is caused by Flavobacterium columnare and particularly related with sudden temperature changes. It is considered more pathogenic when temperatures are above 20°C. It is an ubiquitous microorganism that takes advantage of immunosuppression or skin and gill lesions to cause an outbreak [13].

The most representative lesions are rottenness of the gills, fins and tail that starts from the tip and advances cranially, erosions in the skin that can affect the muscle, and necrosis of the muscular tissue. Tilapias cannot exchange gas properly due to gill lesions and gasp in the surface. Mortality rates are between 60 and 90%.

Diagnosis should be based on observation of the mentioned signs, which are very characteristic of flavobacteriosis. Additionally, the bacterium can be observed with phase contrast microscopy at 400x.

• Motile Aeromonas Septicemia (MAS)

Aeromonas hydrophila causes hemorrhagic septicemia and is one of the most common bacteria in fresh water. Outbreaks typically occur in warmer months. It produces lesions in the surface of tilapias, such as hemorrhages, ulcers, abscesses and, in some cases, exophthalmia. Presentation is usually acute due to the septicemia and mortality up to 80%.

Isolation of the bacteria from affected tissues and histopathology can be useful to reach a diagnosis. Treatment is difficult since many resistances have been described against tetracyclines and other antibiotics.

• Vibriosis

Zoonotic disease that produces hemorrhagic septicemia in tilapias caused by several species of Vibrio, like V. vulnificus, V. anginolyticus and V. anguillarum. Vibrio is a ubiquitous bacterium in tilapia production; some species are present in freshwater and others in saltwater.

Clinical signs are lethargy, skin darkening and hemorrhages in the body surface that can also affect the internal organs such as the liver, the spleen or the kidneys [15]. In fact, vibriosis is the most probable cause of severe hemorrhages in intensive production systems. If hemorrhages are milder, like ecchymosis, MAS could also be the cause.

Vibriosis can cause up to 80% mortality. The bacterium can be isolated from the heart and other affected tissues and be identified in the microscope or via biochemical tests.

• Tuberculosis

Mycobacterium fortuitum causes mycobacteriosis or tuberculosis, a chronic granulomatous disease important because the clinical signs do not appear until tilapia are at the end of the production cycle.

Parasitical

Unlike viral disease and many bacterial infections, parasitosis use to be chronic diseases that act as predisposing factors for other infections.

Parasites damage the gills and the body surface, so the most common signs are gasping in the water surface, irregular breathing, and skin lesions.

The main parasites affecting tilapias are Trichodina spp., Ichthyophthirius multifiliis (enfermedad del punto blanco) and Gyrodactilus spp. The latter is related to destruction of the gill laminae, which causes gill paleness and the presence of abundant mucus.

Diagnosis is based on the microscopic observation of high amounts of these parasites, although, very often, only secondary infections are diagnosed because they are more severe.

Fungal

Saprolegnia parasitica is one of the most important fungi in aquaculture. It is more common in winter and spring and usually grows in the skin and gills, producing cotton-like greyish lumps (mycelia) and mycotic filaments (hyphae) [19]. They also grow in epidermic cells, causing focal edemas and sloughing of the epidermis. Mortality tends to be high in juvenile and the eggs since fungal formations impair gas exchange.

Mycotoxicosis

Mycotoxins are toxic metabolites produced by different species of fungi that can reach tilapias through contaminated feed. They are highly resistant to heat treatments such as pelletization and extrusion, and it is not possible to completely avoid their presence in the feed but they can be attached to mycotoxin binders.

Their effects depend on the concentration of each mycotoxin in the feed and the age of tilapias. The losses caused by mycotoxins are estimated to be about 5% in feed conversion, and they also worsen the growth rate [21]. Besides, mycotoxins can also cause reproductive failure and, in chronic contaminations or high doses, mortality.

Another consequence of importance is immunosuppression, which leads to an increase of the prevalence of infections by the opportunistic microorganisms described in this article.

For example, aflatoxin B1 at 1 ppm can reduce growth rate in tilapias and, at 100 ppm, it causes external hemorrhages and necrosis, tumors, and liver cancer in chronically affected fish. Fumonisin in juveniles can also affect growth rate from 10 ppm.

The lack of knowledge about mycotoxins and the wrong conclusions of analyses because of overestimation or minimization of the problem make diagnosis complicated. To avoid errors, use quantitative analysis techniques and evaluate the results looking at the mycotoxins groups, not at the individual mycotoxins: mycotoxins can be classified depending on their common chemical structures into 5 groups, which are related too their toxic effects, too (see the table below).

Due to the fact that most grain samples (around 100%) contain at least one mycotoxin, and more than half of the samples contain two or more mycotoxins, the continuous addition of effective mycotoxin binders in the diet is highly recommended. Particularly those that are highly effective to bind the different mycotoxin groups to prevent their negative effects, such as Silicoglycidol, a patented molecule that has great binding properties thanks to its optimal adsorption surface.

Pronutrient deficiencies

Pronutrients are plant-based molecules and derivatives of the shikimic acid. Their mechanism of action is based on the stimulation of certain genes in the DNA of the target cells related to specific cell functions [23], [24]. This mechanism was described in genetic, molecular, and in vivo trials conducted by Biovet S.A. in collaboration with universities and experimental centers from all over the world.

When these active molecules are not in sufficient amounts in the diet, tilapias suffer pronutrient deficiencies, that means, the organs and systems (including the immune system) do not function optimally, and tilapia obtain worse performance results and become more sensitive to infections.

Therefore, including pronutrients in tilapia diets improves productive performance, like the feed conversion and growth rates [25], [26], optimizes reproductive results thanks to having healthier animals and reduces mortality and morbidity in a physiological, natural way.

For example, immunobooster pronutrients in diets for tilapia were proved to improve their immune status, so that these animals were more resistant to infections, particularly those related to immunosuppression caused by stress; and had an enhanced vaccine response (charts 2 and 3)

Conclusions

Intensive production of tilapia in high densities that require handling and manipulation of the animals has led to the arise of infections caused by microorganisms that take advantage of several predisposing factors to cause diseases, particularly of stress-related immunodeficiencies.

The lack of effective treatments makes prevention the best tool to achieve good performance results and low mortality rates. Prevention should be based on avoiding stressful conditions, such as sudden temperature changes or inadequate pH values; the inclusion of mycotoxin binders in the diets, such as Silicoglycidol, and the administration of a balanced diet with the pronutrients necessary for a proper functioning of the organs.

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