Foodborne Viruses

Between 1886 and 1898 viruses were first recognised as organisms different from other disease-causing microbes. In 1886, Adolf Mayer was studying tobacco mosaic disease, but tests and attempts to isolate and culture a bacterial agent failed. In 1892, Dmitri Ivanovski demonstrated through an experiment that the disease was either caused by a toxin or by something much smaller than any previously described organism. In 1898, Martinus Beijerinck replicated Ivanovski's experiment and showed that the disease was caused by an infectious life form of some type. This organism became known as tobacco mosaic virus.

In 1914, food was first recognised as a vehicle for viruses when it was identified as the source of an outbreak of raw-milk-associated poliomyelitis. Norovirus was identified in 1968 following an outbreak in a primary school in USA (Norwalk, Ohio), which affected both children and adults. Since the introduction of molecular detection methods in the 1990s, alongside advancements in epidemiology, viruses have been recognised as the leading cause of human gastroenteritis in developed countries.

Image courtesy of the Centers for Disease Control and Prevention

Noroviruses are a highly diverse group of non-enveloped viruses within the Caliciviridae family and are among the leading causes of sporadic infections and epidemic outbreaks of gastroenteritis in humans. The diversity within noroviruses is reflected in their classification into ten genogroups, each containing multiple genotypes. Genogroups GI and GII are primarily responsible for human infections, with GII accounting for a significant portion of norovirus-related illnesses worldwide. Other genogroups (GIII through GX), have host ranges that include pigs, bovines, canines, felines, bats, mice and macaques³. Each genogroup contains several genotypes that exhibit genetic variability. Notably, the GII genogroup includes the most prevalent genotype, GII.4, currently responsible for numerous epidemic outbreaks worldwide. Noroviruses exhibit significant genetic evolution, often undergoing rapid mutations, allowing them to adapt and evade immune responses, contributing to their role as recurrent pathogens. The virus’s single-stranded, positive-sense RNA genome contains three open reading frames (ORFs) encoding various proteins that are crucial for viral replication and assembly. The disease is self-limiting, typically 12-48 hours to 3 days for the majority of people, usually mild, and characterised by nausea, vomiting, diarrhoea, myalgias and abdominal pain. Prolonged virus shedding of up to 8 weeks may occur in asymptomatic people and immunosuppressed individuals. Headache and low-grade fever may occur and there is anecdotal evidence that there may be other diseases caused by norovirus including infant necrotising enterocolitis. The primary route of transmission is person-to-person transmission through the faecal-oral and vomit-oral routes and indirectly through food (ready-to-eat including leafy vegetables and herbs, berries and foods handled after cooking), water and environment. In Europe, outbreaks in healthcare facilities are common.

HEV belongs to the genus Orthohepevirus within the family Hepeviridae and has been formally given the taxonomical name Paslahepevirus balayani. It is a positive-stranded RNA virus approximately 7.5 kb in length. The virus is icosahedral in shape, with a diameter of 27−34 nm. The virion is composed entirely of viral protein and RNA. As of 2024, there are eight recognised genotypes (gt) of HEV, each containing several sub genotypes or strains. Genotypes 1 and 2 exclusively infect humans. Genotypes 3 and 4, however, are zoonotic - capable of infecting humans as well as various wild and farmed animals such as pigs, wild boar, rabbits, and deer. Genotypes 5 and 6 are known to infect wild boar. Genotype 7, while primarily associated with camels, has also been recorded in a human infection. Finally, genotype 8 is known to infect camels. The number of confirmed non-travel related hepatitis E cases in the UK has increased particularly since 2010, with similar trends observed in several European countries such as Germany, Netherlands, Spain and the Czech Republic. Several reports have implicated the consumption of specific foodstuffs in the UK as being correlated with HEV infection, but to date, no food has been directly associated with infection in the UK ^{5, 6, 7}.

From the intestinal tract, the virus reaches the liver through an unknown route and mechanism. HEV appears to replicate primarily in liver and gallbladder cells but has been noted in the small intestine, lymph nodes, colon and salivary glands. The incubation period following exposure can range from 3 to 8 weeks, with a mean of 6 weeks.  The human infectious dose is currently unknown. The disease is usually mild, asymptomatic and self-resolves in 2 weeks; it is usually seen in age groups 15−40 and can be asymptomatic in children. Symptoms include jaundice, malaise, anorexia, enlarged tender liver, abdominal pain, arthralgia, hepatomegaly, vomiting and fever. Chronic hepatitis has been reported in organ transplant recipients and in patients with active HIV infections. Extended faecal shedding when present occurs for approximately 2 weeks after jaundice develops. Fulminant hepatic failure has been observed especially in pregnant women where mortality rates rise from less than 1% to 25%. In immunocompromised patients, hepatitis E can be persistent and also associated with increased mortality and morbidity in people with progressed liver disease.

The viral form that is transmitted from cell to cell and from one host to another is called a particle. If the virus particle’s outer layer contacts a homologous receptor on a susceptible cell’s plasma membrane, the virus’s protein coat, or lipid envelope if present, attaches and infection ensues. To be food or water-borne, a virus must be capable of infectivity upon ingestion by the host in a cell type that is accessible from the digestive tract.

Enteric viruses are usually resistant to environmental stresses such as heat and acid. The majority are also resistant to freezing and drying, are stable in contact with lipid solvents and may be resistant to ultrahigh hydrostatic pressure. These properties enable foodborne viruses to survive in pickled, marinated and acidic foods. Enteric viruses are capable of surviving and retaining infectivity in marine, estuarine and freshwater for several weeks at 4°C, and survival may be increased by attachment to sediment or particulate matter.

• NoV - noroviruses are resistant to drying and can survive on almost any hard surface (including glass, door handles and railings, for example) for up to 12 hours. Norovirus can survive for at least 56 days on stainless steel and 15 days on carpet. The virus is relatively resistant to high levels of chlorine (up to 10ppm free chlorine) and to varying temperatures. In chilled and frozen environments NoV may be able to survive for months or even years. NoV remains infective after being subjected to pH 2.7 for 3 hours at ambient temperature. While the virus is inactivated by boiling, it can remain infectious at 60°C for 30 minutes and can survive some pasteurisation and steaming processes. It should be noted that since there is no routine and robust infectivity assay yet available for NoV, survival studies have been performed mainly by using surrogate viruses which mimic the characteristics of NoV, or molecular methods (e.g. the reduction of NoV RT-PCR units), so this information is yet to be verified.
• HAV - readily inactivated by heating foods to 85°C for 1 minute but is not subject to thermal denaturation at lower temperatures (70°C for up to 10 min). HAV can survive chilled and frozen temperatures for prolonged periods e.g. several months. Disinfection with a 1:100 dilution of household bleach in water (0.5mg free chlorine for 30 minutes), or cleaning solutions containing quaternary ammonium and/or HCl, are also effective in inactivating HAV, although the virus is resistant to disinfection by some organic solvents and by a pH as low as 3, acid treatment (e.g. pH 1 for 2 h at room temperature) is effective. HAV can survive in the environment for at least 12 weeks at 25°C when excreted in human faeces and can remain infectious after 1 month on environmental surfaces at ambient temperatures, and three to four hours in faecal matter on a person's hand. It has been found to survive for several days in experimentally contaminated freshwater, seawater, wastewater, soils, marine sediment, live oysters, and creme-filled cookies. HAV is more resistant to heat and drying than other enteroviruses.
• HEV - in February 2016, the Food Standards Agency (FSA) and the European Food Safety Authority (EFSA) jointly organised a workshop on foodborne viruses, including HEV. Subsequently, in 2017, EFSA published a scientific opinion on the public health risks associated with HEV as a foodborne pathogen. These findings highlighted significant evidence gaps and identified research priorities, such as developing an assay to evaluate HEV infectivity in food and investigating its thermal stability. Current evidence suggests that heating impacts HEV stability, but there is uncertainty regarding the virus's survival at relevant temperatures and in different food matrices. This complexity is furthered by HEV's ability to exist in various states, with or without a viral envelope, or with a partial envelope, all of which can be present simultaneously in a product. The UK FSA funded additional research, providing further insights. The recommended time/temperature range for short-time cooking is between 65°C and 75°C, but data remain inconsistent. It has been suggested that assays directly evaluating detection or infectivity, such as cell culture or ICC (integrated cell culture)-PCR, are necessary to determine the precise conditions for eliminating HEV⁸.

Detecting viruses in foods is more difficult than identifying foodborne bacteria. Enteric viruses, such as HAV and NoV, are exceptionally small (approximately 100 times smaller than bacteria) and are usually found in low numbers on foods. Due to their low infectious dose, only minimal quantities of these viruses are needed to pose a health risk when present in food. They do not alter the appearance, taste, or smell of the food, making their presence undetectable through sensory evaluation. Additionally, they cannot proliferate in food, and unlike foodborne bacteria, their numbers cannot be increased through enrichment in nutrient media in the laboratory. Analysts must, therefore, employ complex methods to determine if a food sample is contaminated with viruses. NoV and HAV can be detected in foods using an international standard method (ISO15216 parts 1 & 2^{9, 10}). This method is suitable for use on shellfish, fresh and frozen produce such as leafy greens and soft berry fruits, bottled water and food contact surfaces. Testing is based on the detection of viral nucleic acids by real-time PCR and therefore does not provide any indication of infectivity. Although there are several methods to detect HEV already published¹¹, there is currently no standardised method available; efforts to develop such a method are underway by the working group ISO/TC 34/SC 9/WG 31 within the International Organization for Standardization (ISO). Although detection of foodborne viruses has been performed for many years, it is still not considered a routine method, due to the lack of regulation or criteria for limits of viruses in foods. In the UK there are very few laboratories with either the expertise or capability to detect viruses in foods.