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ELLSWORTH — An
apparently new strain of Infectious Salmon Anemia
(ISA) virus detected on a Heritage salmon farm near Jonesport in November
seems to be fading away as mysteriously as it
appeared.
Deborah Bouchard
who operates Micro Technologies Inc., a private
fish health laboratory in Richmond, said recent
tests from the lease site indicate a decrease in
the occurrence of the virus.
The presence of
the salmon virus has been mysterious from the
outset. First of all, there were no outward signs
of infection.
“We have not seen
clinical disease symptoms at the site,” said
Samantha Horn-Olsen, aquaculture policy
coordinator for the Department of Marine
Resources. She said the indicators of the anemia
showed up in routine testing but resisted attempts
to confirm it as ISA.
“Testing has not
actually reached the threshold for removal of the
fish,” she said, “so there’s not clinical disease.
There’s not mortality.”
But the mere
suggestion of the presence of a virus, which led
to the destruction of millions of farmed salmon in
Cobscook
Bay in 2002, got the
department’s attention. Horn-Olsen said
Commissioner George Lapointe immediately sent the
test results to the Fish Health Technical
Committee, a group of veterinarians and scientists
from federal and state agencies, and academia that
advises on fish health matters.
“It was suggested
that the best use of the situation would be to do
extensive testing and research and learn as much
as we could because this is such an unusual
event,” Horn-Olsen said.
Although a number
of ISA outbreaks in the Cobscook
Bay area have been met with
swift eradication orders since farms there were
restocked, the fish in the Jonesport farm have
stayed in the water.
“There was no
trigger for automatic removal,” Horn-Olsen said of
the Jonesport site. That meant any removal order
would have had to come from a special decision by
the commissioner.
“We don’t have ISA. We have the detected presence of genetic material of a virus that’s not
causing a disease,” she said.
“The advice we
were getting from scientists and vets said this
didn’t pose a threat,” Horn-Olsen said.
So the fish
remained in the water under increased scrutiny and
biosecurity measures.
Testing of
sluggish fish in pens was stepped up to a weekly
schedule from the usual monthly routine. The
initial positive tests kept coming back at about
the same rate.
“Prevalence
doesn’t seem to be changing,” Horn-Olsen said.
Bouchard said her
results showed what seemed to be an increase in
frequency. But recently, it has been falling off.
Bouchard said the
initial tests performed on all sampled fish simply
identify the presence of the virus in general.
After that, the sample is genetically sequenced to
specifically identify it. Then it can be compared
with other known strains. She said it was at this
point that the virus was shown to be “very
different from the North American strain” and
“more similar to European.”
While that
determination leads to some questions as to where
the virus came from, Bouchard said the appearance
of a strain of the virus that doesn’t lead to
wide-scale infection isn’t new to European
scientists.
“In Norway there
are both pathogenic and apathogenic strains, and
they manage for them. Whenever they get ISA positive, they do genetic sequencing to determine what strain they are
dealing with before they decide whether to remove
or not,” said Horn-Olsen.
Bouchard said that
although the Jonesport virus was similar to
European strains, it wasn’t similar enough to
determine whether it would be pathogenic or not.
She explained the variations in infection rates
and virulence of different ISA virus strains as similar to human viruses, which don’t infect everyone
who comes into contact with them.
“Environment, host
and pathogen all play an important role,” she
explained.
She echoed
Horn-Olsen’s urgent call for scientific study
saying, “the more research that’s done, the more
strain types identified, the more confident we can
be in making similar predictions.”
Horn-Olsen said,
“It’s just now getting to the point where I think
people are really starting to be satisfied that
this is a new strain of the virus.”
She said the virus
was announced by the department last week because
“we wanted to make sure it was public in a way
that people could really collaborate and
understand what’s happening.”
Monitoring of the
farm that shows the presence of the virus and
neighboring sites will continue, according to
Horn-Olsen. She said there haven’t been any signs
of the virus on those sites. |
