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Diseases of Aquatic Organisms

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DAO 55:205-210 (2003)  -  doi:10.3354/dao055205

Age- and weight-dependent susceptibility of rainbow trout Oncorhynchus mykiss to isolates of infectious haematopoietic necrosis virus (IHNV) of varying virulence

Sven M. Bergmann1,*, Dieter Fichtner1, Helle Frank Skall2, Hans-Jürgen Schlotfeldt3, Niels Jørgen Olesen2

1Federal Research Centre for Virus Diseases of Animals, Boddenblick 5a, 17493 Greifswald - Insel Riems, Germany
2Danish Veterinary Institute, Hangovej 2, 8200 Århus, Denmark
3State Fish Epidemics Control and Fish Health Service of Lower Saxony, Eintrachtweg 17, 30173 Hannover, Germany

ABSTRACT: The virulence of 5 European and 1 North American isolate of infectious haematopoietic necrosis virus (IHNV) was compared by infecting female sibling rainbow trout (ŒIsle of Man¹ strain) of different weights and ages (2, 20 and 50 g). The fish were exposed to 104 TCID50 IHNV per ml of water by immersion, and the mortality was recorded for 28 d. Two new IHNV isolates from Germany were included in the investigation. One was isolated from European eels kept at 23°C (±2°C) and the other was not detectable by immunofluorescence with commercially available monoclonal antibodies recognising the viral G protein. The results showed that IHNV isolates of high or low virulence persisted in rainbow trout of all ages/weights for 28 d, with the exception of fish over 15 g in the eel IHNV (DF [diagnostic fish] 13/98)-infected groups from which the virus could not be reisolated on Day 28. The smallest fish were most susceptible to an infection with any of the IHNV isolates. The lowest cumulative mortality (18%) was observed in fingerlings infected with the North American isolate HAG (obtained from Hagerman Valley), and the highest mortality (100%) in DF 04/99 infected fish. The DF 04/99 and Ö-13/95 viruses caused mortality in fish independent of their weight or age. The isolates FR-32/87 and I-4008 were virulent in fish up to a weight of 20 g and caused no mortality in larger fish. In the IHNV HAG- and DF 13/98 (eel)-infected rainbow trout, no signs of disease were observed in fish weighing between 15 and 50 g. An age/weight related susceptibility of rainbow trout was demonstrated under the defined conditions for all IHNV isolates tested.

KEY WORDS: Susceptibility · Infectious haematopoietic necrosis · IHN · Infectious haematopoietic necrosis virus · IHNV · Virulence differences · Rainbow trout

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