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





Pathogen (name, taxonomy, description):

Enteromyxum leei.

Phylum: Myxozoa, class: Myxosporea (pluricellular protozoa), order: Bivalvulida, suborder: Variisporina, family: Myxidiidae, genus: Enteromyxum.

A myxosporidium endoparasite. Spores and sporoblasts are found in the bile ducts, gall bladder and the alimentary tract of fish (coelozoic). Extrasporogenic histozoic phases take place in subcutaneous tissue, gills, gall bladder and intestinal epithelia.

Economic Implications:


Frequency of occurrence:


Farmed fish species affected:

Sea bream (Sparus auratus), sharp snout sea bream (Diplodus puntazzo), striped sea bream (Lithognathus mormyrus), sea bass (Dicentrarchus labrax).

Age/size of fish mostly susceptible:

All age classes are susceptible with high mortality among the severely infected fish. The damage is greater among the sharp snout sea bream (see below the "consequences" section).

Seasonal occurrence:

Enteromyxosis outbreaks are evident mainly from spring to late summer (rising water temperature), but this does not mean that infection occurs during this time only. Handling stress usually provokes symptoms and mortality of latent heavy infections.

Regional pertinence:

Sites in areas with high farming activity/pressure are more prone to suffer. The probability of occurrence is high at any site.

Predisposing factors and mode of infection:

Growing sharp snout bream (the most vulnerable species) in large numbers and stocking densities. Infection is believed to be direct from fish to fish by the oral route. Hence, overcrowding and fouling of cage nets allow better contact with the pathogen and increase the possibility of ingestion of spores released by infested fish.

Main lesions:

The initial symptoms of enteromyxosis are seen as focal or extensive skin damage and lack of appetite. Discoloration, scale loss and ulceration are common, mainly on the dorsal area and on the body flanks. The skin lesions are not haemorrhagic. (These superficial lesions may be due to sporoplasms amoebulae/trophozoites migration in the host tissues during the extrasporogonic cycle of the parasite.) Distension of the abdominal wall is rare. In chronic cases muscle emaciation may be severe.

The intestine and pyloric cecae are filled with a creamy mucous content and the intestinal mucosa is inflamed and moderately haemorrhagic. The liver is inflamed and congested or degenerate and the gall bladder is grossly distended, usually full with dark brown bile. Bile stagnation in the liver is suspected due to the frequent greenish colour of the organ in gross examination. The parasites may cause disturbance to the drainage of bile.

The symptoms and necropsy findings may be obscured if the condition is perplexed by bacterial infections leading to further skin erosion and septicaemia due to the weakening of the fish defence mechanisms.

Diagnosis (field, laboratory):

Clinical symptoms necropsy findings, microscopic examination of bile, gut content or scrapings of the intestinal mucosa.

(mortality, growth reduction, extra labour):

All age classes may be affected. Sharp snout bream between 20g and 200g are mostly susceptible with very high mortality, usually 30% but up to 80% on aggregate. Sea bream and striped bream are rather less vulnerable with losses about 10-20% while sea bass suffers less mortality and only when in close contact with heavily infected breams. The effects on growth have not been quantified but are expected to be very serious (destruction of the digestive epithelium and vital organs for the metabolism, such as the liver). Enteromyxosis is also frequent in brood-stock fish and may destroy valuable stocks. Extra costs comprise labour for the daily removal, transport and the sanitary disposition of the dead fish. There is also a significant unquantifiable psychological burden on the fish farmers who are unable to treat the disease.


Plant extracts (nutraceuticals) have been tried, which condition the gut epithelium and seem to block the development of mature spores. Further evaluation is necessary.

Management advice (prevention):

Avoid overfeeding and select fish feeds with the highest protein quality. Move cages to areas exposed to sea currents. Micro-filtration of water (mesh size <5μm) supplied to the brood stock tanks. If enteromyxosis is diagnosed, avoid stresses, such as handling. Remove daily and dispose off mortalities away from the farm in a proper sanitary way, approved by the local authorities. Never reject dead or moribund fish in the sea. Remove and destroy infected brood-fish.

Environmental issues:

The significant role of the fish farms as amplifiers for spreading the parasite in the sea is expected, but has not been studied.


Currently no regulations are in place.



Author: Dr. Panos Varvarigos

Freelance Veterinarian - Fish Pathologist, Athens, Greece.


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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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