People preparing food are advised to wash their hands to prevent the spread of disease, yet an unseen threat may well lie on their kitchen knife, a new study says.

Researchers at the Center for Food Safety at the University of Georgia used new knives and graters on six types of fruit and vegetables that had been smeared with minute doses of hepatitis A virus and a “vomiting” bug called norovirus.

More than half of the utensils became contaminated by this contact — and in turn often handed on virus to non-contaminated produce.

The experiment was conducted on cucumbers, strawberries, tomatoes, carrots and cantaloupe and honeydew melons.

The type of knife, whether serrated, sharp or dull, did not matter, according to the paper, published in the latest issue of the journal Food and Environmental Virology.

Co-author Marilyn Erickson told AFP the findings could be important for people with a weak immune system.

As for recommending what to do, she acknowledged that it could be impractical to wash a knife or grater between each vegetable.

However, “it would be wise to wash (the utensil) carefully between meals rather than leaving it on the counter, thinking it’s not that dirty,” using a dishwasher or at dishwater-type temperature, she said.

Previous research into contamination of kitchen utensils and surfaces has focussed on bacteria, not viruses.

A 2011 study found that the norovirus is the leading cause of foodborne illnesses in the United States. Ingesting as few as between one and 100 norovirus particles can cause vomiting or diarrhoea.