Pathological changes in juvenile Atlantic halibut Hippoglossus
hippoglossus persistently infected with nodavirus
R.
Johansen, T. Ranheim, M.K. Hansen, T. Taksdal, G.K. Totland-2002
Diseases
of Aquatic Organisms, 50: 161-169
Abstract:
This
is the first description of a persistent subclinical nodavirus infection in
the Atlantic halibut Hippoglossus hippoglossus. Juvenile fish (1 to 5
g) were sampled at 4, 5 and 8 mo of age at a fish farm in Norway during and
after weaning. None showed clinical signs of viral encephalopathy and
retinopathy (VER) or other disease. Pathological changes and/or nodavirus
were detected by light microscopy, immunohistochemistry, reverse
transcriptase polymerase chain reaction (RT-PCR) and transmission electron
microscopy in all fish examined. High numbers of virus particles were found
in macrophage-like cells in the central nervous system, including brain and
retina (CNS). The virus particles displayed the icosahedral shape and size
(approximately 25 nm) characteristic of nodaviruses. The virus-infected
cells formed focal cell aggregates and were seen in all regions of the brain
and all nuclear cell layers of the retina. The cytoplasm of the infected
cells was filled with membrane-enclosed inclusions packed with virus
particles. Some virus particles lay along membranes and formed
membrane-bound necklace-like arrangements. The virus-infected cells of the
retina also contained pigment granula located generally inside virus
inclusions and sometimes forming a coating around the virus particles. All
frontal parts with the eyes and brain and 50% of the mid-parts, which
included the abdominal organs, were found positive for nodavirus with
RT-PCR. Pathological changes in these persistently nodavirus-infected fish
differ from earlier descriptions in Atlantic halibut during outbreaks of
VER. Vertical transmission from infected spawners is believed to be a major
route for nodavirus infection. Detection of nodavirus in subclinical
infected fish and a better understanding of its pathogenesis are important
in order to prevent the spread of nodavirus in the fish-farming industry.
(National
Veterinary Institute, PO Box 8156 Dep., 0033 Oslo, Norway, e-mail: renate.johansen@vetinst.no)