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Norovirus in seafood, how to protect yourself

In summer, the consumption of seafood is more common. A danger, even if not particularly serious, could however be lurking. These are small viruses that can hide in the digestive system of bivalve molluscs (mussels, clams, oysters and many others), called Noroviruses.


Noroviruses belong to the family of Caliciviridae, in which there are numerous viral groups infecting animals and humans. They are single-stranded RNA viruses, small in size (27-30 nanometers in diameter), without a glycolipidic coat (the so-called "envelope”) and icosahedral in shape. The first to be discovered was called the Norwalk virus, responsible for a large outbreak of gastroenteritis in Norwalk, Ohio in 1968.

They are divided into seven groups genetic, of which the first, second and fourth (GI, GII and GIV) are capable of infecting humans. The other Norovirus genogroups infect dogs, cats, cattle, pigs, sheep and mice, some exclusively (GIII, GV, GVI and GVII), others in admixture with humans (GII and GIV).


Once ingested with food (or with undrinkable water), after about 48 hours the Noroviruses cause intestinal symptoms characterized by nausea, vomiting and diarrhea, sometimes accompanied by fever and headache. The symptoms are short-lived and usually not particularly severe.

Only in subjects at risk, such as very young children or very elderly and debilitated people, the consequences can be serious and even lead to death from dehydration.

between genogroups that can infect humans, symptoms of particular severity are associated with Norovirus GII, genotype 4.

Seafood and unwashed vegetables

Noroviruses they are excreted via the fecal route by humans and infected animals and this route can lead to the contamination of various foods, among which unwashed vegetables and undrinkable water stand out, as well as bivalve molluscs. Among the latter, the greatest danger of transmission to humans is represented by bivalves which are often eaten raw, such as mussels (Mytilus galloprovincialis) and oysters (Crassostrea edulis e crassostrea gigas).

The bivalve molluscsBy their nature, they filter the water in which they live to draw oxygen and nourishment from it, but in this way they can retain viral particles, bacteria, heavy metals or other chemical pollutants in their internal organs.

How to inactivate Noroviruses

The heat it is the only way to inactivate Noroviruses, protected as they are inside the hepatopancreas and therefore capable of surviving rapid cooking in a pan (a few tens of seconds, and exactly those necessary for the valves to "open").

The heat treatment, to be effective, should reach the 80 ° C and it shouldn't last less than two minutes. These precautions would avoid many infections.

The role of labels

Another caution important concerns the purchase of "safe shellfish", i.e. packaged and labelled, carefully avoiding shellfish sold in bulk.

Packaging and labelling of bivalves are in fact a guarantee of control, as they presuppose that

  • upstream a survey has been carried out on the microbiological characteristics of the waters in which the molluscs grow e
  • if necessary, the molluscs have been purified in special establishments or in clean water.

EU regulation no. 2019/627 in fact, establishes that official controls are carried out on the production and relaying areas of bivalve molluscs, divided into class A, B and C areas according to the increasing content of Escherichia coli contained in the molluscs that grow there.

Only molluscs from class A waters they can be destined for direct marketing, while in other cases a short (class B) or long (class C) purification period is required.

The resistance of viruses

The effectiveness of purificationHowever, it is less safe for viruses than for bacteria such as Salmonella ed Escherichia coli, for example, which have the same faecal origin as Noroviruses.

In any case, it is necessary to avoid the consumption of bivalve molluscs whose origin is unknown and on which the identification mark of the dispatch center and/or the purification center is not affixed on the label with which they are provided.

Only in this way the consumer is certain that one or more producers have assumed the responsibility of having put products on the market that are not harmful to human health.

Silvia Bonardi


Robilotti E, Deresinski S, Pinsky BA. 2015. Noroviruses. Clin Microbiol Rev 28:134 –164. doi:10.1128/CMR.00075-14.

Commission Implementing Regulation (EU) 2019/627 of 15 March 2019 establishing uniform practical arrangements for carrying out official controls on products of animal origin intended for human consumption in accordance with Regulation (EU) 2017/625 of the European Parliament and of the Council and amending Regulation (EC) No 2074/2005 of the Commission as regards official controls. Official Journal of the European Union of 17.05.2019, n. L 131.

Regulation (EC) no. 853/2004 of the European Parliament and of the Council of 29 April 2004 che establishes specific rules on hygiene for food of animal origin Doc. Official Journal of the European Union of 30, n. L 04.2004.

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Graduated in Veterinary Medicine and Specialist in Inspection of Food of Animal Origin and in Veterinary Public Health, she is Professor of Inspection and Control of Food of Animal Origin at the University of Parma. 

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