A new strain of infectious salmon anemia has been discovered in Jonesport, and although it’s having no visible effect on the fish, it has left the scientific community bewildered.

In November, state and federal scientists working together to monitor ISA outbreaks at Washington County salmon farms discovered that fish in several pens at the Heritage Salmon farm near Jonesport were carrying a virus, but not exhibiting any symptoms of illness.

Four months and countless tests later, the Maine Department of Marine Resources announced Thursday that the fish are carrying a strain of ISA that seems to be genetically distinct from any strain previously discovered anywhere in the world.

No strain of ISA has ever been transmitted to humans.

All of the ISA outbreaks in Maine and New Brunswick have been caused by “the Canadian strain,” a flulike virus that spreads via bodily fluids and causes anemia, organ hemorrhage and eventually death, said Samantha Horn Olsen, aquaculture policy coordinator at the Department of Marine Resources.

After each of these outbreaks, state and federal scientists mandated quarantines and fish kills to keep the disease from spreading. These drastic measures have cost aquaculture companies tens of thousands of dollars.

But since the few infected fish and their pen mates remain healthy, state and federal scientists are recommending only prevention for this situation, which DMR is hesitant to term an “outbreak.”

“If there were any concerns about this farm, we would take action immediately, [but] the best course of action at this point is research,” Olsen said Thursday.

The three nearest inhabited pens have all been tested, and none of the fish have shown signs of the virus, she said.

Heritage has responded by limiting boat traffic and boosting its disinfecting efforts.

“We’ve stepped up the surveillance. … We’re testing every week. We’re doing everything we can,” Dave Morang, Heritage’s general manager in Maine, said Thursday.

At this point, it is not possible to say how the mysterious virus got to Jonesport. Because it is not something that has been seen at nearby farms, it is unlikely that the virus was spread by human means, such as boats, nets or boots, Olsen said.

And the genetic differences between the Canadian strain and this new variety of ISA are too profound for it to be a mutation, said Deborah Bouchard, whose Richmond company, Microtechnologies Inc., researches fish pathogens.

That leaves the possibility that the virus came from the environment – perhaps from a wild fish or from sea lice, a parasite that lives part of its life on salmon and is known to spread ISA.

In Norway and other parts of the world, scientists have identified strains of ISA that never cause disease. This virus could be similar, but until ongoing genetic studies are completed, scientists won’t be able to say for sure, Olsen said.

“We’re trying to get to the bottom of it,” Morang said. “We want to know, too.”