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DISEASE
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ENTEROMYXOSIS
(formerly
MYXIDIOSIS)
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Pathogen (name, taxonomy, description):
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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.
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Economic Implications:
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Severe
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Frequency of occurrence:
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Frequent
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Farmed fish species affected:
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Sea
bream (Sparus auratus),
sharp snout sea bream (Diplodus puntazzo), striped sea bream (Lithognathus
mormyrus), sea bass (Dicentrarchus
labrax).
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Age/size of fish mostly
susceptible:
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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).
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Seasonal occurrence:
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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.
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Regional pertinence:
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Sites
in areas with high farming activity/pressure are more prone to suffer. The probability of occurrence is high at
any site.
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Predisposing factors and
mode of infection:
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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.
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Main lesions:
[photo archive]
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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.
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Diagnosis (field,
laboratory):
[photoarchive]
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Clinical symptoms necropsy
findings, microscopic examination of bile, gut content or scrapings of the
intestinal mucosa.
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Consequences
(mortality, growth reduction, extra labour):
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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.
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Treatment:
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Plant
extracts (nutraceuticals) have been tried, which
condition the gut epithelium and seem to block the development of mature
spores. Further evaluation is necessary.
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Management advice (prevention):
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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.
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Environmental issues:
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The
significant role of the fish farms as amplifiers for spreading the parasite
in the sea is expected, but has not been studied.
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Regulations:
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Currently
no regulations are in place.
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