Bull. Eur. Ass. Fish Pathol., 22(5) 2002, 311
Herring (Clupea harengus): A ho t for infectious
salmon anemia virus (ISAV).
Nylund, A., Devold, M., Mullins, J. and Plarre, H.
Department of Fisheries and Marine Biology, University of Bergen, N-5020 Bergen, Norway
Abstract
The emergence of infectious salmon anaemia virus (ISAV) in several countries outside Norway
and frequent new outbreaks of the disease within Norway, strongly suggests that there are natu-
ral reservoirs for the virus, probably in fish occurring in the coastal waters. Both in Norway and
Canada fish farmers have claimed that there could be a possible connection between wild her-
ring (Clupea harengus) mi rating through fish farms and an outbreak of ISA in the same farms. It
has also been claimed that wet feed made from herring could contain the ISA virus and, hence,
transmit the disease to salmon (S lmo sal r). Both these claims are “mythical” in that respect that
they are not based on any scientific study or verification that the ISA virus may propagate in
herring. Hence, the aim of this study was to challenge herring with the ISA virus, check for virus
replication and see if the virus could be transmitted from challenged herring to disease-free
Atlantic salmon. With the help of RT-PCR it was shown that the herring became infected with
the ISA virus after bath challenge. A drop in haematocrit towards day 20 followed the infection.
One salmon that was challenged with filtered homogenate made from ISA challenged herring
died. However, most of the salmon survived, but they were positive in the RT-PCR test. It is
concluded that the ISA virus is able to propagate in herring and that the herring may be an
asymptomatic carrier of the virus.
Introduction
The emergence of infectious salmon anaemia
virus (ISAV) in Canada (Mullins et al. 1998),
Scotland (Rodger et al. 1998, Rowley et al.
1999), Faeroe Islands, Chile (Kibenge et al.
2001a) and USA (Bouchard et al. 2001) and fre-
quent new outbreaks of the disease in Nor-
way, strongly suggests that there are natural
reservoirs for the virus, probably in fish oc-
curring in the coastal waters. The ISA virus
has been shown to replicate in Atlantic salmon
(Salmo salar), trout (S. rutta) and Oncorhynchus
spp., and it can not be excluded that the virus
may also replicate in other species (Thorud &
Djupvik 1988, Hovland et al. 1994, Nylund et
al. 1994, 1995abc, 1997, Nylund & Jakobsen
1995, Kibenge et al. 2001a, Raynard et al. 2001,
Snow et al. 2001). Both in Norway and Canada
fish farmers have claimed that there could be
a possible co n ct on between wild herring
(Clupea harengus) mi rating through fish
farms and outbreak of ISA in the same farms.
It has also b en claimed that wet feed made
from herring could contain the ISA virus and,
hence, transmit the disease to new farms. Both
these claims are “mythical” in that respect that
they are not based on any scientific study or
v ification that the ISA virus may propagate
in herring. The aim of this study was there-
fore to challenge herring with the ISA virus,
heck for virus replication and see if the vi-
ru could be transmitted from challenged her-
ring to disease-free Atlantic salmon. Cell cul-
tures, monocl nal antibodies and polymerase
chain reaction (RT-PCR using primers against
a sequence from the ISA virus) were used as

Bull. Eur. Ass. Fish Pathol., 22(5) 2002, 312
diagnostic tools for testing the presence of the
ISA virus (cf. Devold et al. 2000).
Materials and methods
The herring (Clupea harengus) were hatched
and cultured at our laboratory using
Rhodomonas, Isochrysis, Brachionus plicatilis and
commercial pellet-food (Kvenseth et al. 1997,
Balbuena et al. 2000). The smolts (Salmo salar)
were supplied by a smolt hatchery close to
Bergen with no history of ISA. The fish were
kept at 10 °C in running seawater. The salmon
were acclimatised for 8 days to particle (20
µ m) and UV-filtered (Katadyn J1/P, effect: 50
mWs cm
-2
) seawater (34‰), kept at 10 °C in
0.15 m
3
tanks (flow rate = 5 l min
-1
; 60 speci-
mens in each), and fed commercial pellets
twice a day.
First challenge
The herring used in the first experiment were
0+ with a mean weight and length of 7.0 g
and 10.0 cm, respectively. The salmon smolt
weight and length were 123 gram and 22 cm,
respectively. The fish, five salmon (1S) and 77
herring (1H), in one tank (0.15 m
3
) were chal-
leng d for 15 minut s in a bath containing 250
ml sonicated blood, collected from Atlantic
salmon during a natural outbreak of ISA (iso-
late ISA8, cf Devold et al. 2001), diluted in 50
l of sea water (Table 1). After challenge the
fish were transferred to running seawater. A
control group of 200 herring (1HC) were kept
in a separate tank. After 6 days 20 herring
(CH) from the group challenged with ISA vi-
rus, were transferred to a tank containing 20
disease-free salmon (CS). The experimental
period lasted from the 4
th
N vember 1997 to
the 6
th
April 1998 (154 days).
Second challenge
The weight and length of the herring (0+) and
salmon used in the second experiment were
42 g and 16 cm and 314 g and 31 cm, respec-
tively. The fish came from the same cultured
stocks as in the first experiment. The experi-
mental conditions were the same as in the first
experiment, but the co-habitation between
challenged herring and naïve salmon was not
included (Table 1). The bath challenge was re-
peated with 43 herring (2H) and 5 salmon (2S)
in one tank. The ISAV used in this second chal-
puorG
N
egnellahC
gnirrehnomlas
1tnemirepxE
S1-H 75 htabVASI1knaT
CH 002-SSBH2knaT
SC-H 02gnirrehdegnellahcVAS 3knaT
2tnemirepxE
S2-H 345tabVASI1knaT
CH 5-SSBH2knaT
ST-03
VASImorfetanegomoH
gnirrehdegnellahc
3knaT
Table 1. An overview of the different experimental groups. H = herring, S = salmon, HC = control herring,
CH = ISAV-challenged herring cohabiting with ISAV-free salmon, CS = salmon cohabiting with ISAV-
challenged herring, TS = salmon challenged with intraperitoneal injection of filtered homogenate from bath-
challenged herring. HBSS = Hanks balanced salt solution. ISAV = infectious salmon anaemia virus.