Economically Important Pathologies of the
Marine Fish Cultured in Greece and the Aegean Sea

Based on the presentation by Dr. Panos Varvarigos at the 10th National Conference of the Italian Society of Fish Pathologists 9-11 October 2003

Copyright (c) Dr. Panos Varvarigos.


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The major marine fish grown in the Eastern Mediterranean basin are sea bass (Dicentrarchus labrax, family Serranidae) and sea bream (Sparus auratus, family Sparidae). Additional species enter intensive farming at an increasing rate. These belong mainly to the family Sparidae (breams), such as sharp snout bream (Diplodus puntazzo), white bream (Diplodus sargus), striped bream (Lithognathus mormyrus), red porgy (Pagrus pagrus), pandora (Pagellus erythrinus) as well as dentex (Dentex dentex).

More recently, Dover sole (Solea solea) and meagre (Argyrosomus regius) are successfully hatched and grown.

The contribution of all these alternative species to the total farmed output, as compared to sea bass and sea bream production, remains marginal, but is expected to increase in line with the accumulation of research experience, nutrition and stock management know-how.

The production challenges of the industry relate to stock management and welfare, application of new technologies, environment protection, fish nutrition, disease prevention and control (fish health management).

Bacterial, viral and parasitic diseases are caused often by relatively poorly studied pathogens that are common across the warm Mediterranean waters. The severity of infection and hence mortality and the associated treatment cost is usually fish size, farm site (and management) and water temperature (season) dependent. Nevertheless, despite the intensification of production, the prevailing damaging pathologies are still few.


The most frequent, widespread, deadly and economically important bacterial fish pathogens are Vibrio anguillarum causing vibriosis to sea bass, and Photobacterium damsela subsp. piscicida (Pasteurella piscicida) causing pasteurellosis (or pseudotuberculosis) to both sea bass and bream (plus many of the new entrants to aquaculture).

Other bacterial diseases of frequent occurrence but of moderate economic bearing comprise those septicaemias caused by several strains of vibrio (Vibrio vulnificus, V. alginolyticus), cytophaga like filamentous bacteria (Flexibacter spp., Tenacibaculum spp., Flavobacterium spp.), as well as strains of motile aeromonads (Aeromonas hydrophila, A. sobria), photobacteria (Photobacterium damselae) and staphylococci. Most of these bacterial infections occur more frequently in hatcheries after omissions in hygiene or larval nutrition.



Among the viral diseases, viral encephalopathy and retinopathy (VER) or viral nervous necrosis (VNN) of sea bass (piscine beta Nodavirus) causes excessive damage to all ages of bass. Lympocystis of sea bream, caused by lympocystis Iridoviruses should not be underestimated, but often the losses associated with it are not severe. The lymphocystis viruses affect young bream and have rather indirect consequences by weakening the fish and predisposing to other pathologies. Nonetheless, lymphocystis produces appalling skin lesions and is economically damaging to fry suppliers.

Viral Encephalopathy & Retinopathy


Most endo-parasitic diseases are caused by parasites belonging to the class Myxosporea. Myxosporidioses (sphaerosporosis, polysporoplasmosis, ceratomyxosis). produce non-quantified, but rather moderate damage to fish health and growth and are not considered of critical economic importance with the exception of enteromyxosis. The latter is a myxosporidiosis of sharp snout bream as well as sea bream caused by Enteromyxum leei, affecting the gastrointestinal tract. Another endoparasitic myxosporean, Polysporoplasma sparis, is gaining importance affecting the kidney and causing polysporoplasmosis to sea bream.

Mild but not precisely studied is the consequence of the gut endo-parasitism by protozoa of the subclass Coccidia causing coccidiosis (Goussia spp., Eimeria spp.).



Among the ecto-parasites, the cymothoid isopod Ceratothoa oestroides usually infests sea bass and bream reared in net cages. Apart from fry mortality, the isopods provoke growth retardation and devalue the end product (isopodosis).

Usual ecto-parasites commonly found on the gills of all farmed fish species, comprise the common metazoan flatworms of the order Monogenea (either Monopisthocotylea or Polyopisthocotylea). The most detrimental among these gill monogenetic trematodes (flukes) are the haematophagus species Sparicotyle chrysophrii infesting sea bream gills and Sciaenocotyle panceri found attached on the gills of meagre. Intense haematophagy results in chronic anaemia, growth stunning, skin discolouration and ulceration, liver and myocardium damage, ascites and eventually death of the host.

Gill cryptocaryoniasis and amyloodiniasis are rare in the region and are diagnosed occasionally in brood-stock fish.



Non-infectious diseases comprise skeletal and fin deformities with poorly understood aetiology. Although it is debatable whether anatomic disorders are due to malnutrition or of man-made adverse environment, the fact remains that discarded, deformed fish comprise a serious cost element of hatchery production.

Winter (or Spring) syndrome manifests itself on sea bream after exceptionally cold winters with variable losses among bream. It is believed to result from metabolic imbalance and immuno-suppression caused at sea temperatures below 14°C.

Chronic copper toxicity results from the prolonged exposure to elevated copper and copper compound levels in sea water. Copper oxides comprise the active compound contained in net antifoulants.

Anatomic deformities

Treating infectious diseases is not always feasible, or economically acceptable in practice, nor always environmentally compatible. For example, there are no suitable drugs against endo-parasites, whereas knowledge gaps exist in the environmental compatibility of most antiparasitic bath treatments (formalin baths, antilouse baths with pyrethroids and organophosphates). Licensed antibiotics against bacterial pathogens are only few. Vaccines against vibriosis and pasteurellosis have been licensed, but protection against pasteurellosis is limited and short-lived. There is no vaccine against VNN as yet. On the other hand, simple common sense management measures, such as regular disinfection, clean net maintenance, avoiding over-crowding and over-feeding of the fish, combined with deep waters with sufficient currents ensure stock welfare and enhance natural resistance to pathogens.


Author: Dr. Panos Varvarigos
Freelance Veterinarian - Fish Pathologist, Athens, Greece.


Reproduction of this website (or parts of it) is illegal and strictly forbidden.
No rights can be derived from this website.


Every effort has been made to ensure that the information is accurate until the date of last editing. It is based upon the accumulated personal experience of applied veterinary work. The author cannot take responsibility for incorrect interpretation or any resulting consequences. The contents may be used as an educational guide and are definitely not meant to become a stand-alone diagnostic tool or operations manual.


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