Viral hemorrhagic septicemia (V.H.S.) is an invasive virus that causes internal bleeding and organ failure of most of the sport and commercial fish in the Great Lakes. It has already killed tens of thousands of fish in the eastern Great Lakes, and is now spreading through the Great Lakes. It is likely to indirectly change the Great Lakes’ already unstable ecological structure. A New York Times article Fish-Killing Virus Spreading in the Great Lakes and the Toronto Star article Pathogen stalks fish report on the spread of the virus:

One of Dr. Casey’s colleagues researching the virus, Dr. Paul Bowser, a professor of aquatic animal medicine, added, “This is a new pathogen and for the first number of years — 4, 5 or 10 years — things are going to be pretty rough, then the animals will become more immune and resistant and the mortalities will decline.”

No one is sure where the virus came from or how it got to the Great Lakes. In the late 1980s, scientists saw a version of V.H.S. in salmon in the Pacific Northwest, which was the first sighting anywhere in North America. V.H.S. is also present in the Atlantic Ocean. But the genesis of a new, highly aggressive mutated strain concentrating on the Great Lakes is a biological mystery.

“We really don’t know how it got there,” said Jill Roland, a fish pathologist and assistant director for aquaculture at the U.S. Department of Agriculture. “People’s awareness of V.H.S. in the lakes was unknown until 2005. But archived samples showed the virus was there as early as 2003.”

Scientists pointed to likely suspects, mainly oceangoing vessels that dump ballast water from around the world into the Great Lakes. (Ships carry ballast water to help provide stability, but it is often contaminated and provides a home for foreign species. The water is loaded and discharged as needed for balance.)

Fish migrate naturally, but also move with people as they cast nets for sport, for instance, or move contaminated water on pleasure boats from lake to lake.

The United States Department of Agriculture issued an emergency order in October to prohibit the movement of live fish that are susceptible to the virus out of the Great Lakes or bordering states. The order was later amended to allow limited movement of fish that tested negative for the virus.

“Getting rid of it is extremely hard to foresee,” said Henry Henderson, director of the Natural Resources Defense Council’s Midwest office in Chicago. “These species spread, and reproduce. It is a living pollution.”

From the Toronto Star:

The deaths to date are just a small fraction of the millions of fish in the lakes. Even so, governments around the lakes are worried enough to try unprecedented steps to contain the virus.

VHS is suspected to be the latest on a growing list of destructive species – including zebra mussels and round gobies – brought into the lakes from Europe and Asia, usually in the ballast water of ocean-going ships.

The potential impact on fish isn’t the only concern. VHS doesn’t harm humans, but that doesn’t mean others that follow will be so benign, says Jennifer Nalbone, of Great Lakes United, a cross-border advocacy group based in Buffalo that for years has demanded strict controls on ballast.

“It’s a wake-up call that the lakes are vulnerable to any pathogen getting in here. We need to try to slow the spread but also to close the door.”