Foodborne viral infections - Part 1

Part 1 — Types of foodborne viruses

Also: Part 2 — Sources and detection of foodborne viruses
          Part 3 — Foodborne viruses in the food factory

All foodborne viruses originate from the human intestine and contamination of food occurs either by contamination from an infected food handler during preparation or by contact with sewage, sewage sludge or polluted water. Control measures mainly depend on staff education and good factory and kitchen hygiene; food handlers suffering from symptoms should be excluded from work immediately and all staff made aware of the ease with which viral contamination is transmitted. The use of clean water for irrigation of crops that are likely to be eaten raw and cultivation of molluscan shellfish in sewage-free seawater are also essential to prevent viral contamination of food.

Foodborne viruses are a common, and probably the most under-recognised, cause of outbreaks of gastroenteritis. Human infection can occur following consumption of contaminated food, person-to-person body contact or release of aerosols. Food may be contaminated by infected food handlers or by contact with water contaminated by treated or untreated sewage. Outbreaks of viral foodborne illness have been associated with the consumption of shellfish that have been harvested from sewage-polluted waters. The greatest risk of foodborne illness occurs with catering operations preparing ready-to-eat foods, although foodborne spread is difficult to prove.

The types of foodborne virus fall into two main groups:

• Norovirus (formerly named Norwalk-like viruses (NVLs) or small round structured viruses (SRSVs)) which cause gastroenteritis;
• Hepatitis A virus, which causes hepatitis.

Norovirus

Illness caused by norovirus is usually sudden in onset and characterised by vomiting, diarrhoea and abdominal pain. Vomiting frequently occurs without warning and may be projectile and uncontrollable, while diarrhoea may be explosive. The incubation period is usually 24–36 hours after eating an implicated food but may be from 15–72 hours, depending on the number of virus particles consumed. The duration of the symptoms varies from 12–72 hours but, after the initial uncontrollable onset, the symptoms may be relatively mild. Despite this, sufferers may feel debilitated for 2–3 weeks.

Very few virus particles are needed to cause illness, so the attack rate in an outbreak can be very high, with the majority of people who ate the contaminated food becoming ill. Because the viruses multiply in the gut, a very large number of virus particles are excreted during the illness (often more than 10 million per gram of faeces or vomitus). As a result of the uncontrollable nature of the symptoms, food can easily become contaminated by infected food handlers and secondary person-to-person spread is also common.

Hepatitis A

Viral hepatitis has a long incubation period of 3–6 weeks, with symptoms developing gradually. Symptoms include loss of appetite, malaise, fever and vomiting, followed by jaundice. Illness usually lasts a few weeks but may last several months, and is usually more severe in adults than in children. Death may occur, particularly in the elderly, but is very rare. Large numbers of virus particles can be shed in the urine and faeces during the latter part of the incubation period, before jaundice is apparent, but they are usually absent one week after the onset of jaundice.

Other foodborne viral infections

Norovirus is not the only virus that can cause diarrhoeal illness. Although many of these viruses are transmitted via person to person, especially during childhood, they can also be transmitted via food handlers as well as by sewage contamination of drinking water, shellfish, fruit, vegetables and salad products. These viruses include aichivirus, rotavirus, sapovirus, parvovirus and astrovirus. As with norovirus, there is, to varying degrees, evidence for environmental robustness which allows survival on dry surfaces, in food matrices and in water (including seawater). However, all are killed by boiling.

Hepatitis A virus is not the only cause of foodborne hepatitis, which can also be caused by Hepatitis E virus. The clinical features of infection are similar to Hepatitis A, although the incubation period for infection is often longer (mean 40 days, range 15–60 days) and the disease is more severe. The case fatality rate is 0.1–1%. Hepatitis E can cause life-threatening infections in women in the later stages of pregnancy, with case fatality rates approaching 20%. The incidence of Hepatitis E is highest in developing countries in tropical or subtropical areas of the world with inadequate environmental sanitation. Most disease presents as endemic or sporadic cases although major epidemics do occur affecting tens of thousands of patients.

The primary source of Hepatitis E infection appears to be faecal contamination of water and epidemic outbreaks are often preceded by periods of heavy rain which lead to contamination of water supplies with untreated sewage. Foodborne transmission occurs uncommonly and person-to-person transmission is rare. Hepatitis E infections do occur in developed countries and, although usually associated with travel to endemic regions, recent studies have identified cases with no history of recent foreign travel (termed autochthonous), often with links to either contact with pigs or the consumption of pig meat. Certainly, Hepatitis E is endemic in British pigs and several human cases have been shown to be infected with virus strains similar to those found in pigs.

Please see also Parts 2 and 3 of this article.

The Institute of Food Science & Technology authorised this Information Statement dated April 2008, which replaces the version issued in March 2002.