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DISEASE
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VIBRIOSIS
(Vibrio anguillarum)
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Pathogen (name, taxonomy, description):
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Vibrio anguillarum (mainly serotype I). Family: Vibrionaceae.
Gram negative bacterium, appearing
as a slightly curved rod with round edges and bipolar staining. Motile in fresh suspensions in
sterile saline 0.9%
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Economic Implications:
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Severe
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Frequency of occurrence:
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Frequent (once or twice a year)
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Farmed fish species affected:
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Sea bass (Dicentrarchus labrax).
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Age/size of fish mostly
susceptible:
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All
age classes are susceptible. Mortality is higher among the young fry.
Obviously, the economic damage is greater when larger growing bass are lost.
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Seasonal occurrence:
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Vibriosis
outbreaks may occur any time, usually after handling stress or adverse
weather conditions (e.g. stormy weather with lightning), but most often
during early Spring and late Autumn when seawater temperatures are unstable.
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Regional pertinence:
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Sites
in areas with high farming activity/pressure are more prone to suffer.
However, the probability of occurrence is high at any site.
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Predisposing factors:
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Overfeeding,
overcrowding, fouling of cage nets, recent occurrence of the disease,
environmental stress of any kind, in particular, daily water temperature
instability.
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Main lesions:
[photo archive]
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Vibriosis presents itself as a haemorrhagic
septicaemia. The bass "turn red" due to extensive skin haemorrhages
mainly around the head, the belly and the inflamed anus as well as the base
of the fins. Bilateral exophthalmus and distension of the abdomen are common.
The gills are pale with excessive mucous secretions, but usually not
necrotic. The liver is pale, often with petechiae, the spleen is enlarged,
the kidney is congested and the intestine is distended full of transparent
fluid (catarrh), whereas petechiae are found on the
peritoneum hymens. The swim bladder is frequently distended, thus, most
moribund fish are found lethargic close to the surface. The young fry may
show none of the above clinical symptoms other than widespread lethargy
(large numbers of fry swimming sluggishly close to the surface and the cage
nets) and darker colouration.
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Diagnosis (field,
laboratory):
[photoarchive]
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History, clinical symptoms,
necropsy findings, isolation of the bacterium on agar plates (usually TSA or
TCBS) and identification either biochemically (Biomerieux API system) or
serologically by means of rapid agglutination test kits.
On TSA medium, incubated at room temperature
(about 25°C), pale round colonies of about 1-1.5mm in diameter, develop within 24-36 hours.
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Consequences
(mortality, growth reduction, extra labour):
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All
age classes may be affected. Young fish are mostly susceptible. Mortality is
high, usually 15% but up to 25% on aggregate (35% in young fry). The effects
on growth have not been quantified but are expected to be serious due to the
prolonged loss of appetite and the long and drastic reduction of feeding rate
as a management response. Extra costs comprise labour for the daily removal,
transportation and the sanitary disposition of the dead fish. Extra labour
costs and time are also associated with the preparation of medicated feed on
a daily basis. There is also a significant unquantifiable psychological
burden on the fish farmers.
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Prevention:
[photoarchive]
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Vaccination by immersion (dip)
and/or injection is usually successful to prevent the disease. There are
several licensed vaccines, which have proved successful in the field (RPS
>80%). It is important to apply the appropriate vaccination protocol
according to the requirements of each site. In-feed/oral vaccination has not
yet proved its worth in the field (inconsistency). In practice, vaccination,
although its effectiveness is not in doubt, may not be acceptable always.
This is due to the associated extra costs and labour/time, in particular as
regards vaccine administration by intraperitoneal injection.
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Treatment:
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Daily
administration of antibiotics, mixed in the feed, for 10 days is usually
effective to eliminate mortalities (oxytetracycline at 100mg/kg biomass per day,
flumequine at 80mg/kg biomass, oxolinic acid at 60mg/kg biomass) and
potentiated sulphonamides (trimethoprim +
sulfadiazine) at 70mg/kg biomass per day. However, the disease often
reappears in about 2-3 weeks post a seemingly successful therapy.
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Management advice:
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Prevent
vibriosis by a proper vaccination scheme covering all age classes of the sea
bass on farm. In case of an outbreak, promptly seek veterinary advice to
confirm diagnosis and suggest proper treatment measures. Reduce feeding rate
even starve the fish for several days. 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.
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Environmental issues:
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None
studied. For example, the significance of the fish farms as amplifiers for
the disease in the sea is unknown. However, it is evident that neighbouring
farms are in danger and usually contract the disease and that some wild fish
species around the cages suffer vibriosis during a farm outbreak (e.g. Mugil
cephalus, Liza saliens). In addition, the quantity and potential effects
of the drug residues or their metabolites, which are unavoidably released in
the water, have not been studied.
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Regulations:
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Currently no regulations
are in place.
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