A bloody killer is on the rampage among wild fish stocks of the Great Lakes of North America, prompting concern among fishers, fish farmers, anglers and scientists around the world of a potential pandemic.

Viral Haemorrhagic Septicaemia (VHS) was not seen in the Great Lakes before 2005 and, apart from freshwater fish species in continental Europe, it was believed to be restricted to marine fish in the Atlantic and Pacific Oceans.

Then fish with bulging eyes and blood-mottled skins began floating to the surface in Lakes Ontario and St Clair in huge numbers.  By 2006 the disease had spread from American into Canadian waters in Lake Erie, infecting more than a dozen species and causing mass deaths. It was clearly highly infectious and – disturbingly – scientists found the virus (VHSV) in both healthy and ailing fish. A search through frozen samples indicates it probably entered the Lakes in 2003.

They concluded VHS had two forms – an acute one causing rapid death and a chronic form from which fish gradually recover but continue to carry the virus, possibly for the rest of their lives.

Where VHS came from is not clear. Reports of disease, indicative of VHS, in rainbow trout from Europe go back to the 1930s. The virus was first visualised in the 1960s. Until the 1980s it was thought to be a problem only among intensively farmed European rainbow trout. But since then the number of known fish species susceptible to infection has increased dramatically, says Dr Mark Crane of CSIRO’s Australian Animal Health Laboratory, which is keeping a sharp eye on the global outbreaks.

Then it appeared among marine fish species off the coast of North America, and there were fears it had somehow been transported from Europe. However, the North American virus proved to be a slightly different strain.  Canadian scientists say it is a relative of this strain that has now invaded freshwater fish populations of the Great Lakes, and speculate that either ship’s ballast water or wild fish movements are to blame.

VHS is caused by a rhabdovirus which is now known to infect more than 50 species of fin fish worldwide, including trout, salmon, perch, herring, pilchards and flounders. It can cause up to 100% mortality among fry and 80-90 per cent mortality in farmed fish populations.  It does not affect shellfish or crustaceans.

In its acute form, fish become lethargic, dark and anaemic, with bulging eyes, congested kidneys, mottled liver and with haemorrhage in the eyes, skin, gills, fin bases, skeletal muscle and viscera. Mortality is very high and the disease is short-lived.

In the chronic form of the disease, mortality is low and the symptoms are similar to the acute form, except that haemorrhage is less common; instead, the liver, spleen and kidneys accumulate fluid so that the body becomes bloated and the liver and kidneys become very light in colour. Survivors can be carriers of the virus for the rest of their lives. It is probably spread mainly in fish urine.

Scientists say that, like most other fish diseases, VHS has no impact on human health.  Fish which are infected by VHSV but still healthy are safe to handle and eat. However, no-one should consume fish that are sick or that have died from disease.

“It’s a huge worry,” says Dr Crane.  “It’s spreading through wild fish populations with consequences for those resources and there are huge implications for the aquaculture industry if it gets into farmed fish, where it can cause 80-90 per cent mortality. If it got into a hatchery, for example, you’d have to close it completely, de-stock and disinfect. Tracing the source and preventing the further spread of the infection would be a huge operation.”

VHS is exotic to Australia and has never been seen here, but the fact that it seems to be extending both its physical range and the range of species it infects make it grounds for serious concern, he says.

In enclosed fish farms good health management of stock and strict quarantine should keep VHS at bay, but sea-cage aquaculture is exposed to wild fish populations such as pilchards which, overseas, are known to carry the virus, and thus face an unquantifiable risk. The main defence will consist of maintaining low stress and good animal welfare in farmed stock, Dr Crane says.

“In Europe it is thought VHSV was originally a virus of marine fish species that were used as feed in the early days of intensive rainbow trout farming – and that was how it began there. It is not clear why now a marine virus has managed to invade freshwater fish in the Great Lakes,” he adds.

One difficulty is posed by the fact that infected fish can look perfectly healthy and the initial signs may be hard to spot. However he is confident that the aquaculture industry’s existing veterinary surveillance systems combined with the diagnostic capability at AAHL and strict border protection by AQIS will maintain a good lookout for it.

Julian Cribb is adjunct professor of science communication at the University of Technology, Sydney and edits R&D Review and ScienceAlert.