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Plankton may spread oyster herpes virus

Pacfic oysters being lifted from the water

The virus does not affect humans but can kill up to 100 per cent of infected Pacific oysters (Source: James Lepre/McAsh Oysters)


Veterinary scientist Dr Richard Whittington, of the University of Sydney, and colleagues, report their findings on Pacific Oyster Mortality Syndrome (POMS) in a recent issue of the journal Diseases of Aquatic Organisms.

"This is the first time that we've had the suspicion that it's attached to particles, probably plankton, which opens up doors for practical management," says Whittington.

Since 2008, POMS, caused by the ostreid herpesvirus (OsHV-1), has been killing Pacific oysters in countries ranging from France, and the UK to Spain and the US.

"It's a herpes virus but it's in a branch of the herpes viruses that is completely distant and removed from any that cause disease in humans or terrestrial animals," says Whittington.

The Pacific oysters, however is very susceptible to the virus, which causes a systemic infection often leading to death.

"The mortality rate can be up to 100 per cent," says Whittington, adding that outbreaks of infection occur in the summer months.

In 2010 the virus hit New Zealand and Australia, but to date no one has been able to work out how the virus spreads from one place to another.

"It's a really important international disease issue but nothing has been discovered about how it spreads," says Whittington.

"The paper presents a working hypothesis about the mechanism of transmission of the virus in water."

Whittington says POMS is present in Sydney's Botany Bay, Port Hacking and the Hawkesbury River.

"It's devastated commercial production of Pacific oysters in all those locations," he says.

Virus transmission

Knowledge about transmission of the virus responsible is essential for breaking the cycle of infection and re-establishing commercial production of oysters in these areas.

In a six-month study, Whittington and colleagues placed 8000 juvenile and adult oysters at different heights in three different locations in the Botany Bay-Georges River estuary.

Some oysters were put at normal growing height, which is in the intertidal zone, between the low water and high watermarks.

Others were placed 300 millimetres above this, and the rest were put in floating structures so they were constantly in the water.

"We looked at every oyster every week and what we discovered was a hitherto unobserved and unrecorded patterns of mortality," says Whittington.

They found the virus was not randomly or uniformly distributed throughout the water, but instead was behaving as if it was distributed in plankton,

"The patterns of mortality were consistent with the behaviour of plankton," says Whittington.

He says under some conditions, plankton can transfer tens of kilometres with oceanic currents and may explain the transmission of the disease from Botany Bay to the Hawkesbury River, which happened in January 2013.

Practical trials

While the exact component of plankton that is the "Trojan horse" for the virus has yet to be identified, Whittington says the findings are still proving useful to oyster farmers.

Based on the plankton theory, Whittington has been helping farmers to trial growing oysters at water heights that enable them to avoid any virus.

This research is published in a separate paper in the journal Aquaculture.

He says in the future it may also be possible to remove the virus by using filtration, disinfection techniques, ozone and ultraviolet light.

Kevin McAsh, chair of the New South Wales Farmers' Oyster Committee welcomes the research.

He says the virus arrived in Australia "out the blue" and it's been a real mystery as to what is spreading it.

Promising signs

Given that it will take some time to identify the exact component in plankton responsible for virus transmission, McAsh says the industry also welcomes the trials Whittington has been leading based on his findings to date.

"He's working on the science but he's also looking at practical solutions in the interim," says McAsh.

McAsh says growing the oysters at different heights in the water column is showing promising signs of reducing mortality.

"We are confident the work will produce results."

More information on POMS research can be found at Oyster Health Sydney.

Tags: rural, fishing-aquaculture, animals