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Vibriosis In Atlantic Cod - A Challenge In Intensive Culture

By Solveig Nygaard, Fish Health and Environment Inc, and published in Intervet's Aquatic Animal Health Newsletter - Vibriosis, as caused by the bacteria Listonella (Vibrio) anguillarum, was the first of the known bacterial fish diseases.

Intervet

The disease was described (in Italy) as early as 1718 (1). Listonella anguillarum is divided into several serotypes, where 01 is dominant in salmonids while 02 is usual found in Atlantic cod (1). Despite the long history of the disease, vibriosis still causes problems in cod farming, thereby increasing the usage of antibacterial remedies in coldwater aquaculture.

Characteristics of vibriosis in Atlantic cod (Gadus morhua)

 

  • Distribution: In the marine environment
  • Species affected: Cod, saith, salmonids, pike, eel, turbot, halibut, etc.
  • Symptoms on cod: Haemorrhages on the jaw and exophthalmia (“pop eye”) in small fish; haemorrhages and wounds on the skin and fins (also enlarged) in larger fish
  • Mortality: High in juveniles, . 50%; lower in larger fish
  • Contributing factors: High water temperature, stress
  • Prevention Vaccination (as of now, two water-based vaccines are available for immersion and injection vaccination in Norway )
  • Treatment Antibiotics - good effect from (for example) oxolinic acid as long as the bacteria have not developed resistance

Experiences from farming

Vibriosis outbreaks are often prevalent at temperatures > 14 °C. When cod juveniles are produced in lagoons, vibriosis has a tendency to occur shortly after dip vaccination, while catching the fish or in other situations which cause stress, especially when combined with high water temperature. Prevention of vibriosis in large cod has been possible by moving the pens to locations which are deeper and have stronger currents (personal experience).

Current production of juveniles in intensive cod farming

Status:

 

  • The juveniles are produced in tanks with higher densities now than in the earlier days of cod farming
  • More repeated outbreaks of vibriosis at lower temperatures than experienced earlier
  • The juveniles are infected by vibriosis before, immediately after and some months after vaccination
  • Repeated treatment with antibiotics is beginning to result in resistance (1)

The juvenile producers have spent a lot of money both in the building of production facilities and in the large-scale production of juveniles. They vaccinate according to the instructions from the vaccine producers but still experience problems caused by repeated outbreaks of vibriosis, both before and after vaccination.

Note: The effect of the vaccine will be dramatically reduced if fish already suffering from disease (including an outbreak of vibriosis) are vaccinated. It is important that the fish have gained the necessary degree days in order to achieve good protection. If not, this may be an explanation for why the fish repeatedly experience outbreaks of vibriosis after immersion with vibriosis vaccine.

What to do:

  • Improve vaccination conditions in order to reduce stress
  • Improve water quality in hatcheries – this may be the causal agent in repeated outbreaks
  • Improve production and documentation of effects and side-effects for water-based vaccines
  • Test oil-based vaccines with vibriosis components on cod fish. Is it possible to reduce the vaccine quantity so that the IP injection may be given to the base of the pectoral fin? Injection site at the pectoral fin will not leave vaccine residues in liver and most likely avoid migration of vaccine to the fillet.

Production of cod juveniles is still in its infancy and problems caused by vibriosis will surely be eliminated through increased improvements by both the juvenile producers and the vaccine producers.

(1) Fish Health and Fish Diseases, Universitetsforlaget 1999, Chapter on Vibrio bacteria; Jens Lauritz Larsen and Karl Pedersen

November 2006

the Fish Site Editor

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