Stewart Hawthorn, Grieg's managing director, said preliminary positive results were found during routine monitoring.
"While we are disappointed about this result and will manage it proactively, it's important to note that this early detection and action is evidence of the effectiveness of our monitoring system and our responsible approach to fish-farm management," Hawthorn said.
The virus, which is not harmful to humans, is found in Pacific salmon but does not harm them.
However, it can lead to rotting flesh and organ failure in Atlantic salmon, which have no immunity. Atlantic salmon are used in most B.C. fish farms.
Earlier this year, 570,000 Atlantic salmon from Mainstream's Dixon Bay Farm in Clayoquot Sound were destroyed after the virus was confirmed.
IHN has not been seen in B.C. since outbreaks between 2001 and 2003 devastated the industry and resulted in the culling or early harvest of 12 million fish.
While fish farmers believe IHN spreads to farm fish from wild stocks, environmental groups such as the David Suzuki Foundation and Friends of Clayoquot Sound and anti-fish farming activist Alexandra Morton say the infections show that open-net fish farms should not be allowed in areas where there are wild salmon populations.
"Eliminating interactions between wild and farmed salmon, by shifting to closed containment, protects the industry's investment in their fish and the environment we all rely on," said Jay Ritchlin of the David Suzuki Foundation.
In a letter to Fisheries and Oceans, Morton said the heavy load of viral particles shed by fish farms could also hurt young salmon.
"If the farm is shedding trillions of viral particles a day, the wild inbound adult sockeye are passing this virus over their gills just before they enter the nursery grounds where last year's sockeye run are rearing as fry," Morton wrote.
"These young fish are not only the most susceptible to IHN. They could also become carriers, spreading it through the North Pacific to other Canadian and U.S. runs."
Mary Ellen Walling, executive director of the B.C. Salmon Farmers Association, said testing in Washington state shows wild fish have a high viral load this year.
Fish farmers would like to see more testing of wild fish in Canada, she said.
"We think there should be increased monitoring of wild fish health and diseases so we, as farmers, have a better understanding of the effects on the health of farmed fish," Walling said.
A vaccine against IHN is available and is used in areas such as Campbell River, where more than one company is operating in the same area, Walling said.
"Obviously, companies will be re-evaluating whether they should look at using the vaccine," she said.
Some companies have been reluctant to use the vaccine as it has not yet been proven in real-life settings and because it is extremely expensive, Walling said.
