• What's the vibriosis?
• Outbreaks of disease
• Pathogenicity
• The charactericst of Vibrio spp
• Diagnosis
• Chemotherapy
• Vaccines
• Bibliography

   Vibriosis

Vibriosis has become the economically most important disease in marine fish culture, affecting a large number of species.  It is also an important disease of many wild fish populations.
The term vibriosis is mainly used to describe infections associated with Vibrio anguillarum.  V. anguillarum is an important pathogen of marine and estuarine fish, causing the hemorrhagic septicemia vibriosis.  However, V. ordalii and other Vibrio species may cause similar clinical disease signs in wide and farmed fish from many parts of the world.  Once referred to as V. anguillarum biotypes 1 and 2, biotype 2 has recently been classified as a seperate species V. ordalii.  Closely related to V. anguillarum, V. ordalii is associated with vibriosis outbreaks in Japan and on the Pacific coast of North America and although in most respects the disease it causes is similar, studies on Pacific salmon affected with this organism have suggested that it is responsible for more localized lesions, in heart and body muscle, rather than the generalized septicaemia generally fod with V. anguillarum.  Although these vibriosis appear to infect all teleosis, vibriosis has been of great concern to intensive fish culture industries and consequently the need for a Vibrio vaccine became apparent.

Other Vibrio species reported to be pathogenic to fish are as follows.
V. alginolyticus which has been implicated in vibriosis of sea bream in Israel and of sea mullet.
V. carchariae isolated from diseased sharks.
V. cholerae (Non-O1) isolated from diseased ayu in Japan.
V. damsela isolated damselfish in southern California
V. vulnificus isolated from eels in Japan and the United Kingdom.
 

The occurrence of a fish disease depends on the balances among three factors, environment, pathogenic agent, and host.
Temperature and salinity are two important environmental factors that affect the infection rates and the adhension process of fish pathogenic Vibrio species.  High water temperature, rapid changes in water temperature or salinity, over-crowding, buffeting by wave action and poor water quality including low oxyzen levels and high suspended solids can be effective.  There also appears to be a seasonal effect on the disease, outbreaks occurring mainly in spring and autumn, though this may simply reflect rapid changes of temperature and salinity.
Many species of marine fish are susceptible to vibriosis and many fish can act as Vibrio carries.  Vibriosis generally occur at temperatures above 10°C, particually when the surface of the fish is damaged, there is a high stocking density or there are other stressrelated conditions present.  The bacteria are considered to be part of the environmental flora, but may also be isolated as part of the normal gut microflora.  Infection can occur by ingesting infected materal from other fish or via skin wounds and loss of scales after grading or transport, etc predisposes to vibriosis.  In addition, there is a possibility that Vibrio can survive for a considerable time in the slime of uncleaned tanks and fouled nets, thus acting as a reservoir of infection.
In practice Vibrio epizootics can very often be linked to obvious stress such as overstocking, excessive handling, or transferring smolts to sea in poor conditions.  Prompt treatment with antibiotics such as oxytetracycline or oxolinic acid via the feed usually saves those fish which are still feeding.  Previded dead fish are promptly removed and husbandry standards are maintained, there need not to be subsequent recurring outbreaks as with recycling furunculosis.

 Pathogenicity

The external pathology produced by V. anguillarum includes haemorrhaging at the base of the fins, around the vent and inside the mouth.  Petechiae, necrotic lesions and diffuse haemorrhages can appear on the body surfaces.  Internally, the intestine is often inflamed with petechiae present on the viscera and musculature.  The intestine may be distended and filled with clear viscous fluid.  This histopathology produced by V. anguillarum and V. ordalii in salmonides is somewhat different.  Experimentally induced infection by water-born exposure demonstrated that both species entered the fish by penetrating the descending intestine and rectum while V. ordalii could also enter penetrating the skin.
 

For examples...

In diseased rainbow trout (Salmo gairdneri) vibriosis results in muscle necrosis, accompanied by interfibrillar hemorrhages, congestion of interfibrillar vessels, and an absence of leucocytic response. In winter flounders, muscle necrosis and focal interstitial and tubular necrosis of the kidneys were reported.

In contrast with the generalized septicemia of Pacific salmonids caused by V. anguillarum, V. ordalii preferentially attacks skeletal and cardiac muscle, the gills, and the gastrointestinal tract. Some investigators have observed a severe leucopenia in salmonids infected with V. ordalii, thus suggesting a leucocytic factor.

In sharks, V. carchariae produces vasculitis in organs of the reticuloendothelial system and in tropical aquarium species, V. damsela characteristically causes skin ulcers. In eels, V. vulnificus biogroup 2 infections are associated with red patches or swollen lesions on the trunk or tail; in the late stages of infections, histopathologic changes develop in the gills and internal organs.

Salmon infected with V. salmonicida show no external pathology but severe anemia develops internally, hemorrhaging occurs in the swim bladder and rectum, and petechiae occur in the caecum and abdominal wall.
 

Virulence Factors

Different strains of vibrio have been shown to have one or more mechanisms for expressing virulence in the fish host.

1. High virulence strains of V. anguillarum contained a large plasmid that enabled the bacterium to obtain iron necessary for its metabolism, even though the host produced factors that bind iron.
2. High virulence strains of V. anguillarum resisted the bactericidal effects of normal serum and agglutinated trout erythrocytes.
3. Hemolysins, cytolysins, proteases, and other extracellular toxic substances have been demonstrated among some vibrios.
4. The production of collagenase by V. vulnificus and V. alginolyticus.

 The characteristics of Vibrio

The genus Vibrio consist of Gram negative, straight or slightly curved rods which are motile by polar flagella.  Colony morphology, biochemical tests and the use of diagnostic keys will confirm the family Vibrionaceae.  Both catalase and cytochrome oxidase are produced by V. anguillarum which can be isolated from the kidney, haemorrhagic muscle or other tissues on TSA supplemented with NaCl.  Colonies on a non-selective media appear white, entire, smooth and convex after 48 hours growth at 20°C.
 

Charcateristics of fish pathogenic Vibrio

  Diagnosis

The disgnosis of vibriosis is confirmed through clinical observations and biochemical or serological characterization of the isolated bacteria on TSA with added sodium chloride.  However, the characterization of V. anguillarum by biochemical tests usually takes longer than the time available for identification when outbreaks of vibriosis occur in fish population and must be quickly brought under control to avoid mortalities.  Yhe serological techniques using polyclonal antibodies can give false-positive results because of cross-reaction among the members of the genus Vibrio, leading to improve the identification V. anguillarum in environmental samples.  The development of highly specific molecular techniques may be use to present methods.

• ELISA and EITC methods by using monoclonal antibodies

• 16S rRNA analysis

  Chemotheraphy

Prior to the introduction of immunization, the most common method employed to control vibriosis was treatment of diseased fish with antibiotics and antimicrobial chemicals.  Commonly used substances are tetracycline, sulphonamides, nitrofuran derivatives, trimethoprim and quinolines, and antibiotic therapy for the control of vibriosis has met with variable success.

For examples...
A 10 day treatment with either sulfamerazine at the rate of 200 mg/kg of fish, or oxyteracycline at 5075 mg/kg, has been used to control vibriosis outbreaks.  Other antibacterials that have been used inclued trimetoprin at 30 mg/kg, piromidic acid at 1040 mg/kg, furanace at 25 mg/L, and the substituted quinoline halquinol at 75 mg/kg.

The most significant difficulty caused by the continued and indiscrimanated use of antibiotics has been the development of serious drug resistance problems with V. anguillarum.  Random V. anguillarum isolates from vibriosis epizootics in Japan carried transferable drug resistance factors (R plasmid).  The most common type of plasmid-determined resistance is to chloramphenicol, tetracycline and sulphamonomethoxine.
With the incresing expansion of commercial aquaculture for the production of food fish there was an requirement for more effective methods for the control of vibriosis.  In many cases, control of the disease by management practices has not proven practical and problems presented by drug resistance have resulted in attempts to control vibriosis by immunization and the development of commercial vaccines.

  Vaccines

The use of antibiotics in the control of vibriosis is discouraged owing to the increased occurrence of drug resistant bacteria, hence the awuaculture industry is increasingly relying on vaccines for disease control.  Vibriosis vaccines are currently the most successful and show the most promise of any vaccines against the major diseases of fish.  Commercial vaccines are well-established and well-researched.  Vibrio vaccines have the double advantage of being simple to manufacture and simple to use.  With the enormous expansion of the marine farming of trout and salmon in the last few years, vaccination has proven itself to be a successful and highly cost-effective tool in management of the disease.

• Nature of Vibrio vaccines

Current commercial Vibrio vaccines contain mixtures of the most commonly encountered species which cause disease problems, V. anguillarum and V. ordalii.  The majority of these vaccines are simple inactivated cultures containing mixtures of whole cells and extracellular products.  The lipopolysaccharide (LPS) moleculors are considered to be the major immunogens of the Vibrio cell wall and fish vaccinated with heat-inactivated, formalin-treated, or phenol extracts of V. anguillarum are protected against subsequent injections by these pathogens.  LPS antigens from V. ordalii and the serotype O2 strains of V. anguillarum were found vaccines which give a high degree of protection against vibriosis are now available.  Several methods of administering commercial Vibrio vaccines are currently employed.  These are injection, oral, hyperosmotic immersion, immersion or dip, bath vaccination, spray or shower and automated immersion.  They must be given by individual injection or on a mass vaccination basis by bath or spray.