Paul W. Reno

Factors Involved in the Dissemination of Disease in Fish Populations

Abstract Infectious diseases have been observed in both human and animal populations for millenia. Unlike diseases of “higher” animals, the dispersal of disease in fish populations rarely has been studied quantitatively. However, the principles that govern the spread of diseases of humans and other mammals should, with modification, be applicable to the study of infectious diseases in fishes. Disease in populations is a dynamic phenomenon; fluctuations in prevalence and impact are dependent on the interactions among host, pathogen, and environment. Models of the dynamics of infectious diseases in salmon and other fishes can be constructed and refined to reflect the characteristics of diseases by integrating the most important factors in the process. Among the factors that have been shown to be important in other systems are the “contagiousness” of the pathogen (transmission coefficient, β), duration of infection, host population density, development of immunity, and efficacy of therapeutants.

Conditioning improves performance of juvenile chinook salmon, Oncorhynchus tshawytscha, to transportation stress☆

Abstract A positive conditioning approach was used, where a dewatering Stressor was associated with feeding, to help moderate the severity of the response of juvenile chinook salmon, Oncorhynchus tshawytscha, to transportation. Not only did this procedure attenuate the physiological response to transportation stress, as evidenced by more rapid recovery of plasma cortisol, glucose and lactate concentrations following the stressful experience, but it also enhanced survival during transport, elevated resistance to the pathogen Aeromonas salmonicida, increased seawater adaptability, and helped the animals to cope with other Stressors relative to control groups. Fish given prior exposures to Stressors not associated with feeding performed intermediately between positively conditioned fish and controls in most tests.

Survey of Pathogens in Juvenile Salmon Oncorhynchus Spp. Migrating through Pacific Northwest Estuaries

Abstract Although the adverse impact of pathogens on salmon populations in the Pacific Northwest is often discussed and recognized, little is currently known regarding the incidence and corresponding significance of delayed disease-induced mortalities. In the study reported herein, we surveyed the presence and prevalence of selected micro- and macroparasites in out-migrant juvenile coho salmon Oncorhynchus kisutch and Chinook salmon O. tshawytscha from 12 coastal estuaries in the Pacific Northwest over a 6-year period (1996–2001). The major finding of this study was the widespread occurrence of pathogens in wild salmon from Pacific Northwest estuaries. The six most prevalent pathogens infecting both juvenile Chinook and coho salmon were Renibacterium salmoninarum, Nanophyetus salmincola, an erythrocytic cytoplasmic virus (erythrocytic inclusion body syndrome or erythrocytic necrosis virus), and three gram-negative bacteria (Listonella anguillarum, Yersinia ruckeri, and Aeromonas salmonicida). The most pre...

Virulence comparison of three Buhl-subtype isolates of infectious pancreatic necrosis virus in brook trout fry.

Abstract Infectious pancreatic necrosis virus (IPNV) is an important aquatic pathogen that can cause high mortality in populations of young salmonids. To determine the molecular basis of virulence, the fry of brook trout Salvelinus fontinalis were experimentally infected with three different A1-serotype, Buhl-subtype isolates of IPNV. The three isolates were selected on the basis of results of previously completed virulence assays, which indicated that the isolates had substantially differing virulence levels. To confirm this, mortalities from each treatment were recorded for the duration of the experiment (62 d), along with observation of any clinical disease signs. Mortalities began on day 5 postexposure, peaked on day 7, and then rapidly decreased for all three isolates tested. Diseased fry exhibited whirling, ascites, abdominal hemorrhaging, and prostration on the bottom of the tank. Daily virus titers from live fish were determined for 10 d postexposure (dpe), as well as at 28 and 62 dpe. Viral titer...

Inactivation of Vibrio parahaemolyticus and Vibrio vulnificus in phosphate-buffered saline and in inoculated whole oysters by high-pressure processing

Inactivation studies for Vibrio parahaemolyticus TX-2103 (serotype O3:K6) and Vibrio vulnificus MO-624 (clinical isolate) were conducted in phosphate-buffered saline (PBS) and in inoculated oysters under high-pressure processing conditions. V. parahaemolyticus was more resistant than V. vulnificus in PBS at all pressures and times. A 6-log reduction of V. parahaemolyticus and V. vulnificus in PBS at 241 MPa required 11 and 5 min, respectively, which included a 3-min pressure come-up time. A 4.5-log reduction of V. parahaemolyticus in oysters at 345 MPa required 7.7 min, which included a 6.7-min pressure come-up time. More than a 5.4-log reduction of V. vulnificus in oysters at 345 MPa occurred during the 6-min pressure come-up time. Both V. parahaemolyticus and V. vulnificus in PBS and in oysters were reduced to nondetectable numbers at 586 MPa during the 8- and 7-min pressure come-up times, respectively.

Effect of fasting on feed intake, growth and mortality of chinook salmon, Oncorhynchus tshawytscha, during an induced Aeromonas salmonicida epizootic

Anorexia is a common sign of bacterial and viral diseases and is thought to be a negative consequence of the disease process. However, this inappetence may be an active portion of the host defence system. We tested this idea by withdrawing food for 32 days from juvenile chinook salmon (Oncorhynchus tshawytscha) during an Aeromonas salmonicida epizootic, induced by cohabitation. Disease-specific mortality was low (5.0% and 12.5% in fed and fasted groups, respectively); there was no mortality in uninfected control fish. While only very few fish had detectable A. salmonicida in the kidney, at the termination of the experiment, an average of 18.5% and 65.0% of the fish in fed and fasted groups, respectively, had this bacterium in or on mucus, but these mean values were not statistically different because of high variation between replicates. Feed intake was measured by X-radiography at days 16 (fed groups) and 32 (all groups). Feed intake as well as growth were unaffected by exposure to bacteria. However, food consumption was greater when fasted fish exposed to A. salmonicida were offered a meal than in those infected individuals that had been eating. This result may be relevant for application of medicated diet, as it seems possible that fasting of sick fish before administration of medicated ration could increase the probability that sick individuals would also eat.

Prevalence of Furunculosis in Chinook Salmon Depends on Density of the Host Exposed by Cohabitation

Abstract Chinook salmon Oncorhynchus tshawytscha were experimentally infected with Aeromonas salmonicida to determine the dependence of initiation and spread of a furunculosis epizootic on host density. Groups of 30 and 60 recipient Chinook salmon (1.7 ± 0.14 g) were held in various volumes of water at a wide range of densities (15.52, 7.76, 1.23, 0.61, 0.32, 0.15, and 0.047 g fish/L) for 23–33 d, and each were exposed to a single infectious individual by cohabitation. Significant relationships (r 2 = 0.55) were found between host density (g fish/L) and survival from furunculosis. No pathogen transmission occurred at the lowest density. Acute and chronic patterns of furunculosis were obtained by adjusting densities. It took longer to observe disease-specific mortality among recipient fish at lower densities compared with higher densities even when donors were similarly infectious. The results have important implications for stocking salmonids regarding natural route of transmission and in understanding th...

Early Kinetics of Infectious Hematopoietic Necrosis Virus (IHNV) Infection in Rainbow Trout

Abstract A series of experiments was carried out with infectious hematopoietic necrosis virus (IHNV; 193-110 isolate) in rainbow trout Oncorhynchus mykiss (weight, ∼1.2 g) to determine the duration of the patent period and the timing of onset of the infectious periods. We first attempted to transmit IHNV to recipient fish from infected rainbow trout 2–3 d after they had been exposed. No infection transfer occurred despite high titers (104.79 to 104.91 plaque-forming units 5–8 d postexposure (dpe). To determine the number of secondary cases produced by one infectious individual, we exposed approximately 50 rainbow trout (weight, ∼1.5 g) in each of seven replicate tanks to a donor fish that had been infected with virus by bath exposure 3 d earlier. The prevalence of infection in recipient fish rose from 0.84% at 2 dpe to 7.9% at 6 dpe. Maximum incidence (22 cases) occurred between 2 and 4 dpe. No disease-specific mortalities occurred in recipient fish during the experiment. The titer of virus in both recipi...

Prevalence of Myxobolus cerebralis at Juvenile Salmonid Acclimation Sites in Northeastern Oregon

Abstract Fry of rainbow trout Oncorhynchus mykiss held as sentinel fish became infected with Myxobolus cerebralis when held at all juvenile acclimation sites for spring Chinook salmon O. tshawytscha and steelhead O. mykiss (anadromous rainbow trout) and thus expand the known range of M. cerebralis in northeastern Oregon. In spring 2001, replicate cages of rainbow trout fry were placed at the intake site of each facility and sampled at approximately 14, 28, and 50 d. Infection prevalence, as determined by the presence of M. cerebralis DNA following polymerase chain reaction (PCR) amplification, was relatively low (<17.5%) at most acclimation sites. However, a significantly higher prevalence of infection in sentinel fry held at the Wallowa facility (65%) identified this site as presenting a higher risk for juvenile steelhead to M. cerebralis exposure than other sites. Further, PCR analysis of ossified cranial elements from steelhead juveniles held at the Wallowa facility for 6 weeks demonstrated a comparabl...

The Effect of in Vitro Passage of Infectious Pancreatic Necrosis Virus (IPNV) on Virulence and Sensitivity of the Virus to Rainbow Trout Serum

Abstract Three infectious pancreatic necrosis virus (IPNV) isolates (A1 serotype, Buhl subtype) were passaged five times in rainbow trout gonad (RTG)-2 cells with either minimum essential medium supplemented with 10% fetal bovine serum (MEM-10) or MEM-10 supplemented with 1% rainbow trout Oncorhynchus mykiss serum (MEM−RTS) to determine the effect of passage of the virus on its virulence and sensitivity to RTS. Mortalities in brook trout Salvelinus fontinalis fry were highly variable during viral passages; however, in general, most of the IPNV isolates were virulent under all conditions. The IPNV mortalities ranged between 30% and 89%. Maintenance of virulence during the five passages was dependent on the IPNV isolate and culture conditions. Rainbow trout serum did not always facilitate maintenance of viral virulence during the passages. Isolate mortalities after multiple passages were virulence dependent: the highly virulent isolate showed peak mortality from 4 to 11 d postexposure, while the low-virulen...