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

AquaHealthTM
Laboratory.

 

DISEASE

PASTEURELLOSIS (Pseudotuberculosis)

Pathogen (name, taxonomy, description):

Photobacterium damselae subsp. piscicida (former name: Pasteurella piscicida). Family: Vibrionaceae.

Gram negative bacterium, appearing as a very short ellipsoid rod with bipolar staining. Immobile in fresh suspensions in sterile saline 0.9%.

Economic Implications:

Severe

Frequency of occurrence:

Frequent (almost every summer)

Farmed fish species affected:

Sea bass (Dicentrarchus labrax). Sea bream (Sparus auratus). Red porgy (Pagrus pagrus).

Age/size of fish mostly susceptible:

Sea bream is susceptible when young; even larvae prior to weaning or juveniles are often devastated in the hatcheries. The bream remains very sensitive until the size of 6g. Then onwards its sensitivity gradually decreases, so that breams larger than 20g are rarely found sick or die in numbers from pasteurellosis. Thus, for this species, pasteurellosis is mostly a problem in the hatchery/nursery and during the first months in the grow-out facilities.

On the contrary, sea bass is mostly susceptible to pasteurellosis between the sizes of 1g and 60g. The disease provokes the highest mortalities in caged bass between 5g and 40g. Thus, for bass, pasteurellosis is mostly a problem during the first summer in the grow-out facilities (see below temperature dependency of the disease).

Seasonal occurrence:

In grow-out facilities, pasteurellosis outbreaks occur from late spring or early summer until mid-autumn, when sea-water temperature rises above 21°C. In hatcheries where warm borehole water is used (>18°C), pasteurellosis constitutes a major threat all year round. It is a temperature dependent disease.

Regional pertinence:

Sites in areas with high farming activity/pressure and regions with warmer waters are more prone to suffer. The probability of occurrence during the summer months is high at any site.

Predisposing factors:

Cage sites that accept most of their fry consignments during late spring or in the summer. High stocking densities, fouling of cage nets, recent occurrence of the disease, prolonged periods of warm waters (long summers).

Main lesions:
[photoarchive]

Pasteurellosis presents itself in the hatcheries as a hyper-acute or acute septicaemia. Bream larvae, juveniles or fry are often found dead in great numbers on the tank bottom with only a few darker fish swimming sluggishly and imbalanced near the surface. Usually there are no alarming signs. The fish behave normally and feed well hours before the disease strikes. The dead fish carry amounts of feed in their stomach and gut. When ill-symptoms develop on either bass or bream fry, these comprise lip and lower jaw inflammation and necrotic skin patches on the body flanks, dorsal area and tail. The fins, mainly pelvic, dorsal and caudal may be eroded. Overall, there is no haemorrhagic appearance. Skin and fin erosions are covered with mucus, thus the lesions appear in the water as white patches.

The gills are inflamed with excessive mucous secretions and most often show areas of necrotic tissue next to congested areas. The liver is most often congested; the spleen is grossly enlarged (splenomegaly) and the kidney pale and oedematous. In the more chronic form of the disease typical pseudotuberculi form in the spleen and/or kidney parenchyma. The intestine carries moderate quantity of transparent fluid. The swim bladder is not distended, thus, the majority of dead and moribund fish are found sunk to the bottom of the tank or cage.

Diagnosis (field, laboratory):
[photo
archive]

History, clinical symptoms, necropsy findings, isolation of the bacterium on agar plates (usually TSA or BHI agar) and identification either biochemically (Biomerieux API system) or serologically by means of rapid agglutination test kits.

Colonies with characteristic morphology develop on TSA medium after 24 to 36 hours of incubation at room temperature (about 25°C). They are smaller than 0.5mm in diameter, whitish (semi-translucent with irregular margins if observed under the light magnification of a stereoscope), somewhat viscous and adhere well to the substrate.

Consequences
(mortality, growth reduction, extra labour):

Hatcheries may lose entire batches of bream juveniles and fall short of their production plans. Extra cost for additional disinfection may be added to the damage. On cage farms, the seriousness of fish losses depends on the age of the fish and the time of the disease onset. It is more problematic when pasteurellosis occurs early in the warm season. Then, it may trouble the farm with a series of repetitive outbreaks, despite therapy, for several months until late autumn, or until the fish grow beyond the sensitive size. If pasteurellosis onsets early and is combined with the presence of the most susceptible age classes of bass (5-35g) and bream (1.5-6g) on farm, the losses are maximised. Losing 25% of the young bream and 35% of young bass is not uncommon. The effects on overall growth have not been quantified but are expected to be serious. Extra costs comprise labour for the daily removal, transport and sanitary disposition of the dead fish. Extra labour cost and time is also required to prepare the medicated feed on a daily basis. There is also the significant but unquantifiable psychological burden on the fish farmers.

Prevention:

There are few licensed vaccines, but vaccination by immersion (dip) has shown poor results in preventing pasteurellosis in the field (RPS <45%). In practice, vaccination should prevent the disease from as early as possible in the hatcheries and for the initial few months at sea -during the first summer. Thus, vaccines should be administered to juveniles and fry by immersion or long bath, but there are practical problems and strong stress associated with vaccination of such very young fish and the expected result is rather poor. Improvement of the vaccine formulations is necessary. Injection vaccination of small fish is practically difficult. Oral administration of antigens should be researched further.

Treatment:

Daily administration of antibiotics, mixed in the feed, for 10 days is usually effective to eliminate mortalities temporarily (oxytetracycline at 100mg/kg biomass per day, flumequine at 80mg/kg biomass, oxolinic acid at 60mg/kg biomass) or potentiated sulphonamides (trimethoprime + sulfadiazine) at 70mg/kg biomass per day. However, the disease often reappears in about a week post a seemingly successful therapy. Thus several therapeutic cycles may be necessary using alternative medications as shown by a sensitivity test of the pathogen (antibiogramme). In case of juveniles prior to weaning stage, antibiotics may be added to the enrichment medium of live prey albeit with vague outcome.

Management advice:

Each tank in the hatchery should have its own utensils (siphons, nets) and hatchery water should be treated with UV light or ozone prior to entering the production areas. Daily disinfections and footbaths are a must. In case pasteurellosis is diagnosed in a hatchery, it would be wise to stamp out the larvae or juveniles in those tanks and carry out thorough disinfections. If not, it is certain that it will spread to most other tanks and destroy the on-going production.

Prevent pasteurellosis from entering a cage site by ensuring that the incoming fry batches are checked free from the disease. If practically possible, avoid new fry entries during the warm months. In case of a suspected outbreak, promptly seek veterinary advice to confirm diagnosis and suggest proper treatment measures. 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. Neighbouring farms are in danger and usually contract the disease. Wild fish species around the cages suffer pasteurellosis during a farm outbreak (e.g. Mugil cephalus, Liza saliens) spreading the pathogen. 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.
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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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