SCOTTISH shellfish growers have called for improved water quality standards as a new study found more than three-quarters of British-grown oysters contain the highly contagious vomiting bug, norovirus.

The study, conducted on behalf of the Food Standards Agency (FSA), discovered that 76% of oysters, tested from oyster beds across the UK including Scotland, had traces of the infectious bug.

Low levels of the virus, which causes symptoms such as vomiting and diarrhoea, were found in 52% of the positive samples, according to the data.

The FSA said it was difficult to assess the potential health impact of the findings, as researchers were unable to differentiate between infectious and non-infectious norovirus material in the shellfish.

However, it said the results of the study would be used as part of a review by the European Food Safety Authority, which is to advise the European Commission on what a legal safe level for norovirus in oysters should be.

The Association of Scottish Shellfish Growers (ASSG) said industry members had been using a method of quality testing which they believe has been proven to protect the consumer, in the absence of any EU or UK standards.

Walter Speirs, chairman of the ASSG, said: "If we have standards set for levels in oysters, we must have equivalent standards in place for the waters in which they are grown, ensuring that the problem is dealt with 'at source', as is the policy of the European Food Standards Agency for infections attributed to food.

"The study focuses on oysters, but it should be considered in context, as person-to-person transmission is by far the most common method of contracting norovirus.

"ASSG looks forward to working further with FSA (Scotland) and the Scottish Environment Protection Agency, in an attempt to tackle the issues this report may highlight."

As part of the study, scientists from the Centre for Environment, Fisheries and Aquaculture Science (Cefas) took samples from 39 oyster harvesting areas across the UK.

Between 600,000 and one million people in the UK catch norovirus every year. In 2009, the Michelin-starred Fat Duck restaurant of top British chef Heston Blumenthal was forced to close after more than 500 people fell ill with norovirus. Raw oysters and clams were later identified as the main source.

Oysters filter large volumes of water to get their food, and any bacteria and viruses in the water can build up within them. They are subject to controls before and after harvesting which help remove these.

Oysters, which are grown in lochs on Scotland's west coast are worth almost £1.6 million a year to the Scottish economy and the industry employs almost 350 workers.

Pacific oysters are the type most commonly grown in Scotland and are harvested year-round. The Strathclyde region produces about 84% of Scotland's farmed Pacific oysters. Last year 241 tonnes of Pacific oysters were grown in Scotland, just over three million shells – a rise of 4% on the previous year.

Meanwhile, an outbreak of vomiting and diarrhoea at a hospital is being investigated.

NHS Lanarkshire said the norovirus infection, which has affected patients in seven wards at Monklands Hospital, Airdrie, is being closely monitored by an outbreak control team.

The health board said the wards have been closed to new admissions, and no new patients will be brought in until the infection has been eradicated.

The winter vomiting virus usually lasts for 24 to 48 hours, but is infectious for two days after the symptoms stop.