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

AquaHealthTM
Laboratory.

 

DISEASE

VIBRIOSIS (Vibrio anguillarum)

Pathogen (name, taxonomy, description):

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%

Economic Implications:

Severe

Frequency of occurrence:

Frequent (once or twice a year)

Farmed fish species affected:

Sea bass (Dicentrarchus labrax).

Age/size of fish mostly susceptible:

All age classes are susceptible. Mortality is higher among the young fry. Obviously, the economic damage is greater when larger growing bass are lost.

Seasonal occurrence:

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.

Regional pertinence:

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

Predisposing factors:

Overfeeding, overcrowding, fouling of cage nets, recent occurrence of the disease, environmental stress of any kind, in particular, daily water temperature instability.

Main lesions:
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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.

Diagnosis (field, laboratory):
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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.

Consequences
(mortality, growth reduction, extra labour):

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.

Prevention:
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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.

Treatment:

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.

Management advice:

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.

Environmental issues:

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.

Regulations:

Currently no regulations are in place.

 

       


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

AquaHealthTM
Laboratory.

Reproduction of this website (or parts of it) is illegal and strictly forbidden.
No text or photographs may be reproduced without written consent by Dr. Panos Varvarigos
No rights can be derived from this website.

Disclaimer:

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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