Vibriosis, caused by the bacterium Vibrioanguillarum which occurs in both fresh- andsalt-water, has become one of the most serious diseases of cultivated marine species of fish and invertebrates. Vibrio infections are reported to have been responsible for the greatest financial loss in salmonid culture in countries like
Norway. Rainbow trout, pink salmon and char can be attacked at any size or age, but Atlantic salmon are normally vulnerable only in the parr and smolt stages of development, and perhaps as they approach maturity. Sea-pen culture of Pacific salmon, particularly of coho salmon, was threatened by vibriosis until methods of control by antibiotics and immunization were developed.
Vibriosis in salmon normally occurs in smolt units, which pump water from the sea, and in sea units soon after the smolts are stocked. The vibrio usually enters the fish through surface wounds and acts mostly on the skin, where lesions are formed. The ulcers can extend deep into the muscles and internal haemorrhage, kidney damage and a swollen spleen are sometimes found in dying fish. Vibriosis is essentially a disease of spring and summer and the growth of the bacterium appears to be accelerated at higher temperatures. The bacterial septicaemia in shrimps appears to be mainly caused by vibrios, even though Pseudomonas sp. and Aeromonas sp. are generally associated withthe disease. Disoriented swimming and increasing opaqueness of abdominal muscle in juve-niles and adults are common signs of the infection. Infection occurs in larvae, juveniles and adult shrimps. Complete mortality of the stock can occur.
Recommended preventive measures for salmonids are reduction of stocking density in warm waters and disinfection of eggs brought into the hatchery. Immunization by injection of juveniles or by hyperosmotic spray and bath immunization has been found to be useful. Oral immunization has been effective in some experiments, but is not recommended for use in commercial aquaculture. The appearance of new strains or serotypes of V. anguillarum has made it necessary to incorporate them also into the immunization protocol. Some cross-protection is provided by various vaccines but not enough, and not consistently (Sindermann, 1986).
Treatment of vibrios is by addition of drugs to the food, but since in the advanced stages of infection the fish stops feeding, treatment has to be started early. The suggested dosages are 0.2 g of sulphamerazine per kg fish per day for three days first of all, then after an interval of a day or two another treatment of 0.2 g per kg fish for a further three days. An alternative treatment is oxytetracycline or furazolidone at the rate of 50–75 mg per kg fish for up to 10 days. The suggested treatment for shrimps is oxytetracycline at the rate of 40 mg per kg body weight per day with feed or 1 ppm baths for the larval stages (zoeae and mysis) every 48 hours. Alternatively, furanace can be used at the rate of 100 mg per feed or 1 ppm baths for the larval stages every 48 hours.
The brown spot disease of shrimps also appears to be primarily caused by vibrios, but as in the case of bacterial septicemia, other organisms like Pseudomonas sp. and Beneckea are implicated. The infection affects adult and juvenile shrimps. Brownish, eroded areas can be seen on the exoskeleton. The lesion generally results from a break on the exoskeleton, as a result of physical stress. The eroded areas may become portals of entry for secondary pathogens, causing mortality. The infection may be eliminated at moulting, except when under-lying tissues are affected. Suggested treatment is administration of oxytetracycline at the rate of 450 mg per kg feed, or external treatment with 0.05–1 ppm malachite green or 20–75 ppm formalin.