Vibrio spp. (Larval Vibriosis) of Scallops
Category 4 (Negligible Regulatory Significance in Canada)
Common, generally accepted names of the organism or disease agent
Larval vibriosis, Bacillary necrosis.
Scientific name or taxonomic affiliation
Vibrio splendidus, Vibrio alginolyticus, Vibrio pectenicida, Vibrio lentus, Vibrio anguillurum-related larval pathogen, and Vibrio sp. Various species of bacteria including Aeromonas hydrophila and species in the genera Pseudomonas, and Moraxella may also be involved in diseases of larval scallops (Riquelme et al. 1995, 1996a).
In all marine waters where bivalve hatchery culture is practised. Specific problems have been reported from scallop hatcheries located on the northeast coast of the U.S.; British Columbia, Canada; Chile; Venezuela; and Brittany, France.
Argopecten irradians, Argopecten purpuratus, Patinopecten yessoensis, Pecten maximus, Euvola (=Pecten) ziczac and other cultured bivalve larvae including oysters, clams and abalone. However, some species of bivalves may be more resistant to the pathogenic effects of these bacteria than other species (Nicolas et al. 1996).
Impact on the host
Systemic infection of the soft-tissues of the larvae, resulting in tissue necrosis (due to production of exotoxin by the bacteria) and death. The course of disease is usually swift and dramatic. In a heavily infected larval culture, mortality often reached 100% within 18 hours (McGladdery et al. 2006). Cytoplasmic extracts of V. pectenicida were pathogenic to P. maximus haemocytes (Paillard et al. 2004). Ciliated protozoa may occur as secondary invaders of larvae with vibriosis. In general, adult bivalves do not suffer high mortalities when experimentally challenged with larval pathogens (Paillard et al. 2004).
Note: Definitive diagnosis of the disease as vibriosis or one resulting from other bacteria requires identification of the specific species or strain involved by appropriate biochemical, immunodiagnostic, or molecular methods. However, consistent isolation of numerically dominant bacteria (Gram negative rods) from tissues with characteristic lesions provides a strong presumptive diagnosis.
Wet Mount: Examination by light microscopy should reveal massive invasion by bacteria throughout the larval tissues.
Histology: Indications of tissue necrosis and the presence of rod shaped bacteria (usually slightly curved) within the tissues.
Culture: Isolation and culture, in media such as TCBS (thiosulfate citrate bile salts sucrose) and Zobell bacterial culture agar, colonies of Vibrio spp. from the tissues of diseased scallop larvae.
Molecular characteristics: Genomic analysis using polymerase chain reaction (PCR) and the nucleotide sequences of various genes (e.g., 16S ribosomal DNA (16S rDNA), small subunit ribosomal DNA (SSU rDNA), rpoA, recA, pyrH and gyrase B subunit (gyrB) genes) are being employed to enable the differentiation between species of some Vibrio spp. (Nicolas et al. 1996, Lambert et al. 1998, Le Roux et al. 2004, Paillard et al. 2004, Thompson et al. 2005).
Bioassay: The pathogenicity of bacteral isolates was assessed by inoculating 24-hr suspensions of the test organism into 15-ml cell culture chambers containing 30 to 45 early-stage healthy scallop larvae (Riquelme et al. 1996).
Methods of control
Vibrio bacteria are ubiquitous, hence eradication of the aetiologic agent is impossible. Vibriosis appears to be directly related to poor husbandry. Sources of infection include broodstock, algal cultures, incoming seawater and hatchery system surfaces. Determine the source of infection by culturing bacteria from these candidates. DiSalvo (1994) indicated that chronic infections of potentially pathogenic bacteria, especially Vibrio spp. in broodstock may be a source of larval infections and recommended screening of broodstock scallops prior to spawning. Riquelme et al. (1995) reported that the use of filtered seawater decreased the bacterial concentration in reproductive organs of scallop broodstock and suggested that broodstock conditioning was an effective procedure to reduce the risk of vertical transmission of bacterial pathogens. Batches containing infected individuals should be destroyed in an approved manner; disinfect all containers and equipment in contact with the infected stock.
Antimicrobial agents to reduce bacterial populations in and around the bivalve molluscs for the control and treatment of disease in mollusc hatcheries have been assessed and employed (Tubiash et al. 1965, Le Pennec and Prieur 1977, Nicolas et al. 1996). However, the use of inhibitory compounds may lead to the rapid development of pathogen populations resistant to antibiotics (Riquelme et al. 1996a), the elimination of beneficial organisms and the emergence of other microbial pathogens of the bivalves.
The use of probiotics (naturally occurring disease control agents that produce inhibitory substances or prevent pathogenic colonization of the host) is a promising method to control and prevent bacterial infections during the culture of scallop larvae in hatcheries systems (Riquelme et al. 2000). Riquelme et al. (1996b) reported that Alteromonas haloplanktis isolated from the gonad of A. purpuratus had inhibitory effects against pathogenic Vibrio spp. including strains of V. alginolyticus and V. anguillurum that cause severe mortalities in cultured scallop larvae. Riquelme et al. (1997) found that a strain of Vibrio sp., when used as a pre-treatment, protected the scallop larvae against subsequent experimental infection with a Vibrio anguillurum-related larval pathogen. However, as indicated by Riquelme et al. (1997), the evaluation of potentially probiotic strains of bacteria requires the study of numerous variables, no less complicated than the historical development of medical antibiotic technology. A great deal of study will be necessary in the future to determine optimal ways for evaluating probiotic bacteria, and their practical value in hatchery culture of scallops.
DiSalvo, L. 1994. Chronic infection of broodstock as a potential source of substandard gametes and larval infection in Chilean scallop hatcheries. In: Bourne, N.F., B.L. Bunting, L.D. Townsend (eds.) Proceedings of the 9th International Pectinid Workshop, Nanaimo, B.C., Canada, April 22-27, 1993, Vol. 1. Canadian Technical Report of Fisheries and Aquatic Sciences 1994 pp. 107-111.
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Le Roux, F., M. Gay, C. Lambert, J.L. Nicolas, M. Gouy and F. Berthe. 2004. Phylogenetic study and identification of Vibrio splendidus-related strains based on gyrB gene sequences. Diseases of Aquatic Organisms 58: 143–150.
McGladdery, S.E., S.M. Bower and R.G. Getchell. 2006. Diseases and parasites of scallops. In: Shumway, S.E., G.J. Parsons (eds.). Scallops: Biology, Ecology and Aquaculture, Second ed. Elsevier B. V., Amsterdam. pp. 595-650.
Mortensen, S., I. Arzul, L. Miossec, C. Paillard, S. Feist, G. Stentiford, T. Renault, D. Saulnier and A. Gregory. 2007. Molluscs and crustaceans, 5.3.6 Vibriosis caused by Vibrio spp. / V. splendidus-related strains. In: Raynard, R., T. Wahli, I. Vatsos, S. Mortensen (eds.) Review of disease interactions and pathogen exchange between farmed and wild finfish and shellfish in Europe. VESO on behalf of DIPNET, Oslo. pp. 334-340. (For electronic publication see www.dipnet.info under "Documents", subgroup "Reports and project deliverables").
Nicolas, J.L., S. Corre, R. Robert and D. Ansquer. 1995. Why do scallop (Pecten maximus) larvae die, when they are reared without antibiotic? In: Book of Abstracts, 10th International Pectinid Workshop, Cork, Ireland, April 27 - May 2, 1995.
Nicolas, J.L., S. Corre, G. Gauthier, R. Robert and D. Ansquer. 1996. Bacterial problems associated with scallop Pecten maximus larval culture. Diseases of Aquatic Organisms 27: 67-76.
Paillard, C., F. Le Roux and J.J. Borrego. 2004. Bacterial disease in marine bivalves, a review of recent studies: trends and evolution. Aquatic Living Resources 17: 477-498.
Riquelme, C., G. Hayashida, N. Vergara, A. Vasquez, Y. Morales and P. Chavez. 1995. Bacteriology of the scallop Argopecten purpuratus (Lamarck, 1819) cultured in Chile. Aquaculture 138: 49-60.
Riquelme, C., A.E. Toranzo, J.L. Barja, N. Vergara and R. Araya. 1996a. Association of Aeromonas hydrophila and Vibrio alginolyticus with larval mortalities of scallop (Argopecten purpuratus). Journal of Invertebrate Pathology 67: 213–218.
Riquelme, C., G. Hayashida, R. Araya, A. Uchida, M. Satomi and Y. Ishida. 1996b. Isolation of a native bacterial strain from the scallop Argopecten purpuratus with inhibitory effects against pathogenic vibrios. Journal of Shellfish Research 15: 369-374.
Riquelme, C., R. Araya, N. Vergara, A. Rojas, M. Guaita and M. Candia. 1997. Potential probiotic strains in the culture of the Chilean scallop Argopecten purpuratus (Lamarck,1819) Aquaculture 154: 17-26.
Riquelme, C., R. Araya and R. Escribano. 2000. Selective incorporation of bacteria by Argopecten purpuratus larvae: implications for the use of probiotics in culturing systems of the Chilean scallop. Aquaculture 181: 25–36.
Thompson, F.L., D. Gevers, C.C. Thompson, P. Dawyndt, S. Naser, B. Hoste, C.B. Munn and J. Swings. 2005. Phylogeny and molecular identification of vibrios on the basis of multilocus sequence analysis. Applied and Environmental Microbiology 71: 5107–5115.
Tubiash, H.S., P.E. Chanley and E. Leifson. 1965. Bacillary necrosis, disease of larval and juvenile bivalve mollusks. Journal of Bacteriology 90: 1036-1044.
Bower, S.M. (2009): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Vibrio spp. (Larval Vibriosis) of Scallops.
Date last revised: December 2009
Comments to Susan Bower
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