A virus that has already killed tens of thousands of fish in the eastern Great Lakes is spreading, scientists said, and now threatens almost two dozen aquatic species over a wide swath of the lakes and nearby waterways.

The virus, a mutated pathogen not native to North America that causes hemorrhaging and organ failure, is not harmful to humans, even if they eat contaminated fish. But it is devastating to the ecosystem and so unfamiliar, experts said, that its full biological impact might not be clear for years. It is also having a significant impact on the lakes’ $4 billion fishing industry.

There is no known treatment for the virus. As a result, scientists are focusing on managing its spread to uncontaminated water — quite a challenge since the Great Lakes are linked and fall under the jurisdiction of several states and provinces in Canada.