Tens of thousands of fish have been bleeding to death from an aggressive Ebola-like virus in North America's Great Lakes. Officials fear that the plague will spread to devastate waters across the continent.

The epidemic - caused by what one US government scientist calls "the most important and dangerous fish virus known worldwide" - is believed to have been brought into the lakes by ocean-going ships.

It is focussing attention on the scores of alien species already spreading uncontrolled in the lakes, which contain a fifth of the world's fresh water. State governments are belatedly trying to introduce measures to control this "living pollution", only to be sued by shipping companies for "placing an undue burden" on them.

The killer - called viral haemorrhagic septicemia (VHS) - has already affected some 37 species of fish in the lakes, including salmon, trout, perch and white bass. Almost every species caught commercially or for sport in the lakes' $4bn (£2bn) fishing business has been hit.

Victims bloat up and bleed from bulging eyes before dying. The extraordinarily virulent disease is sweeping rapidly through the lakes and is expected to start going on full rampage again within the next couple of weeks when the water warms up to its favoured temperature.

When first discovered, just two years ago, the virus was affecting only two species in a few patches of water, but it quickly spread to Lakes Ontario and Erie, and Lake St Clair, on which Detroit stands. It has recently been confirmed in Lake Huron, and is expected to reach Lake Michigan soon.

"Updates over the winter suggest that it has spread further than we thought, even last year", says John Dettmers, a fisheries biologist for the Great Lakes Fisheries Commission. "As much as I'd like to say we know exactly what is going on, we don't. We're all sitting on the edge of our chairs waiting to see how bad it's going to be this year."

Genetic tests show that the virus probably originated in the Atlantic off New Brunswick, Canada, right next to the start of the St Lawrence Seaway, the shipping route which leads to the lakes. That points the finger squarely at the 220 ocean-going ships that enter the lakes each year as the source of the plague.

Ships take in ballast water to steady them when they are sailing without a cargo, only to discharge it when they reach the ports where they load up. But the water is inevitably populated by species of fish, plants and viruses which are then given free rides around the globe. Some flourish vigorously in their new surroundings, creating plagues.

In all, 183 such alien species have been found in the Great Lakes and a new one is being discovered every six months. Just one - the zebra mussel, originally from the Caspian Sea and imported in ballast water in 1988 - has already caused $3.1bn of damage, clogging up drains and vents and damaging boats. Others include the colourfully-named bloody red shrimp, quagga mussels, round gobies, sea lampreys, and spiny and fishhook water fleas.

The invasion - and the spread of the virus - is fuelling an ecological xenophobia over "creepy crawly things from foreign lands."

"How would you like to invite a bunch of strangers from other countries to come over and go to the bathroom in your bathtub?", asked a writer on Detroit's Metro Times earlier this year. "Sound sick and disgusting? Sure does."

The disease does not threaten people; cooking kills the virus. But environmentalists fear that it is only a matter of time before one arrives that poses a danger to human health. "The lakes are vulnerable to any pathogen getting in here", says Jennifer Nalbone of the Great Lakes United conservation pressure group.

Scientists fear that the virus will spread throughout the continent's rivers and lakes. "Once a species has been introduced and establishes itself, it will continue to grow and spread, not only in the Great Lakes, but across the waterways of North America", says Professor David Lodge, the director of the Centre for Aquatic Conservation at the University of Notre Dame, Indiana.

And Dr James Winton, the chief of fish health at the US Geological Survey in Seattle - who calls VHS "the most important and dangerous fish virus known worldwide" - adds; "Its discovery in our freshwater is disturbing and potentially catastrophic."

Last autumn, the US Department of Agriculture banned the transport of live fish from Great Lakes states unless they could be shown to be free of the virus.

The State of Michigan has banned ocean going ships - known as "salties" as opposed to the "lakers" that ply their trade within the Great Lakes system - from discharging ballast water unless it its first sterilised. Wisconsin, Minnesota, Indiana and other states are considering introducing similar rules.

But nine shipping groups representing the salties are suing Michigan saying that the law is unconstitutional, unjust and unfair. They say that it "places unreasonable burdens on interstate commerce and is clearly excessive in relation to the putative local benefits gained."

But Bob McCann of the state's Department of Environmental Quality replies that the law is "Michigan's way of saying enough is enough."

The biggest freshwater lake system on Earth

* The Great Lakes - Superior, Michigan, Huron, Erie and Ontario - were formed by glacial meltwater at the end of the last ice age

* They are the largest surface freshwater system on Earth, covering 243,000 square km

* They hold one-fifth of the world's surface freshwater supply

* They contain 35,000 islands - including Manitoulin in Lake Huron, the largest in any inland body of water

* 183 invasive "alien" species have been found in them, and a new one is discovered every six months

* Spread out evenly, their water would cover the 48 contiguous US states to a depth of 9.5ft