Graham L. Bullock

Observations on the Occurrence of Bacterial Gill Disease and Amoeba Gill Infestation in Rainbow Trout Cultured in a Water Recirculation System

Abstract Spontaneous outbreaks of bacterial gill disease (BGD) occurred in 17-44-g rainbow trout Oncorhynchus mykiss after they were placed in a water recirculation system. In each of five groups stocked from September 1991 through July 1992, BGD occurred within 6–8 d after stocking. In each instance, BGD was followed by a secondary amoeba infestation. The spontaneous BGD outbreaks did not occur among previously stocked groups that had recovered from earlier BGD disease outbreaks. Examination of gill tissue by Gram stain and indirect fluorescent antibody technique showed increased numbers of filamentous bacteria associated with BGD after the rainbow trout were stocked into the system. Bacterial numbers decreased after a 1-h treatment with chloramine-T at concentrations of 9–15 mg/L but increased within 2 d after treatment. Although the chloramine-T treatments controlled mortality related to BGD, the amoeba infestation persisted. Histological examination of gills showed some focal hyperplasia before the ra...

Recurrent Amoebic Gill Infestation in Rainbow Trout Cultured in a Semiclosed Water Recirculation System

Abstract Five lots of commercially purchased juvenile rainbow trout Oncorhynchus mykiss (17–44 g) stocked in a continuous-production water recirculation system became infested with gill amoebae. The amoebae were introduced into the recirculation system, as evidenced by their presence on gills of fish held in quarantine tanks. Based on their morphology, as seen in histological sections and by electron microscopy, the amoebae appeared to be more closely related to the family Cochliopodiidae than to other taxa of free-living amoebae. Attempts to culture the amoebae in different media, at different temperatures of incubation, and in fish cell culture were not successful. Initial treatment of the recirculation system with formalin at 167 parts per million (ppm) for 1 h eliminated amoebae from the gills. Subsequent treatments of the entire system with formalin at 50–167 ppm reduced the intensity of further infestations.

Nature of Aeromonas salmonicida Carriage on Asymptomatic Rainbow Trout Maintained in a Culture System with Recirculating Water and Fluidized Sand Biofilters

Abstract An asymptomatic carrier population of rainbow trout Oncorhynchus mykiss was examined for Aeromonas salmonicida by primary dilution counts on Coomassie Brilliant Blue agar and also by streaking on bacteriological media after a 24–48-h pre-enrichment in tryptic soy broth. The pathogen was detected by primary dilution plate counts in 1 spleen, 15 gill, and 19 mucus samples of the 100 trout examined. Aerornonas samonicida was detected only after 48-h preenrichment in the one spleen that had already tested positive via primary dilution counts. The pathogen was not detected in kidney, liver, and intestinal samples. The occurrence of the pathogen in mucus and gills suggests a predominantly external nature of asymptomatic carriage of A. salmonicida within this population of fish. Repeated examination of fluidized biofilters and tank water showed that A. salmonicida did not become established in the recirculation system.

Vaccination against Furunculosis in Arctic Char: Efficacy of a Commercial Vaccine

Arctic char Salvelinus alpinus are susceptible to furunculosis, a serious bacterial disease that affects the productivity of salmonid farms around the world. The objective of this laboratory study was to evaluate the efficacy of a commercially available vaccine against furunculosis in two strains of Arctic char. Labrador and Nauyuk char were injected with Aqua Health Furogen 2 or 0.9% NaCl. At 87 and 108 d postvaccination, fish were challenged with Aeromonas salmonicida by immersion. In both challenges, the Aqua Health vaccine provided significant protection against furunculosis; percent cumulative mortality was significantly (P = 0.005) greater for the fish vaccinated with 0.9% NaCl than for those vaccinated with Furogen 2. Strain (P = 0.30), days since vaccination (P = 0.38), and the interaction between vaccine and strain (P = 0.40) were not significant. The point estimate for the preventive fraction (PF) was greater than 0.90 for the Labrador fish vaccinated with Furogen 2; for the Nauyuk fish, the PF was 0.89 and 0.38 at 87 and 108 d, respectively. Relative proportion survival values were also estimated and were usually similar to, but always lower than, the PF estimates.