Most food-borne viral diseases are caused by consumption of molluscan shellfish. During filter-feeding of seawater, molluscs concentrate viral particles, originating from human faeces, in their tissues. Many viruses are host-specific, so do not cause disease in both humans and animals.
Methods for detection of viruses in foods are generally lacking, although routine PCR-based methods may be developed in the future. DNA or RNA sequence-based typing methods are used, but are not suitable for routine use. The only virus for which standard methods for its detection in foods have been developed is Norovirus in shellfish. Contamination of other foods, which have been implicated in outbreaks of food-borne viral disease, probably occurs via faeces or vomit from infected food handlers either directly or in aerosols, or via faecally contaminated water.
Noroviruses
Norovirus (family Caliciviridae) are a group of related, ssRNA viruses.
Norovirus were previously described as Norwalk virus (the prototype), Norwalk-like viruses (NLV), Small Round Structured Viruses (SRSV) or calicivirus. Norovirus cause viral gastroenteritis, believed to be a very common cause of enteritis worldwide. Other members of Caliciviridae includeSapovirus – which can cause enteric symptoms in young children, VesivirusandLagovirus.
Norovirusare normally transmitted person-to-person by the faecal–oral route, but may also be water-borne. However, it has been detected in shellfish implicated in outbreaks, particularly oysters, clams and shrimps.
The incubation period is typically 1 to 3 days. Symptoms include nausea, projectile vomiting, diarrhoea (watery, voluminous) and abdominal pain. The disease is normally self-limiting. Elderly and IC individuals are most at risk.
Norovirusreplicates in the mucosa of the small intestine and is shed in large numbers in faeces.
The infectious dose is not known. Any foods that require extensive handling could be vehicles of infection. Foods most commonly involved are shellfish, but fruits and salads have also been associated with infection.
Measures to control Norovirusinclude:
1. Don’t consume raw shellfish.
2. Prevent faecal contamination of food, even pre-harvest.
3. Dispose of sewage in a sanitary manner.
4. Infected individuals should not handle food.
5. Prevent cross-contamination from shellfish to other foods.
Hepatitis A virus
Hepatitis A virus (HAV) is a ssRNA virus of the family Picornaviridae.
The incubation period is typically 2 to 9 weeks, so food may become contaminated with HAV before the onset of symptoms, but after faecal shedding has commenced. Early symptoms are anorexia, fever, malaise, nausea and abdominal discomfort. Vomiting and fever can occur. Sequelae include liver damage, seen in patients when jaundice develops. Hepatitis E virus (HEV) causes similar human disease, but belongs to the family Hepeviridae. While HEV has potential to be food-borne it is believed to have caused water-borne disease.
HAV is absorbed through the gastrointestinal mucosa and carried via the blood to the liver. HAV then binds to receptor sites on the hepatocyte surface and penetrates cells. Viral replication occurs, HAV is excreted in bile, and shed in faeces.
Food-borne transmission appears to have occurred in some HAV outbreaks, but the virus has not been detected in any foods. The long incubation period frequently means suspect foods are unavailable, and in- food detection methods have not been developed. HAV is transmitted person-to-person by the faecal–oral route. The infectious dose is unknown but likely to be low: perhaps ten to 100 virus particles. Implicated foods include water, shellfish, salads, fruits, cold meats, sandwiches, fruit juices, milk, milk products and iced drinks.
Measures to control HAV include:
1. Prevent faecal contamination of food even pre-harvest.
2. Infected individuals should not handle food.
3. Dispose of sewage in a sanitary manner.
4. Prevent overcrowded living conditions.
5. Use good personal hygiene measures.
Rotavirus
Rotavirus is a dsRNA virus, of the family Reoviridae. Rotavirus is transmitted person-to-person by the faecal–oral route, and causes viral gastroenteritis. Believed to have only rarely caused food-borne disease, but may be more commonly water-borne. Rotavirus has not been isolated from foods.
Rotavirus is the most common cause of viral gastroenteritis in children
< 2 years old, and most or all children contract this virus after birth.
The incubation period is typically 1 to 3 days, and vomiting can last from 4 to 8 days after symptoms commence. Symptoms include vomiting, watery diarrhoea and low-grade fever. Rotavirus is normally self-limiting but, in severe cases, rehydration of patients is necessary.
Rotavirus replicates in the mucosa of the jejunum or ileum and is shed in faeces in large numbers.
The infectious dose is unknown but probably is ten to 100 virus particles. Water is most commonly implicated, but any foods that require extensive handling could be vehicles of infection. Implicated foods include salad, cold foods, shepherd’s pie and school lunches. Poor personal hygiene is frequently a contributing factor.
Measures to control rotavirus include:
1. Infected individuals should not handle food.
2. Dispose of sewage in a sanitary manner.
3. Prevent faecal contamination of food.
1.3 Chemical Hazards in Foods
Chemicals can occur in the food chain due either to their existence in the environment through unintentional contamination of food, or to their intentional use somewhere along the food production chain (Table 1.2).
Generally, industrial pollutants are unintentional contaminants of foods, so, even if regulated, may be difficult to control. Agricultural chemicals are deliberately applied to land or crops during production, so their use can be both regulated and controlled. Some toxic chemical compounds can occur naturally in foods and in the environment.
The rate of ingestion of chemical hazards by food animals can be either higher or lower than the rate of their excretion. In the former case, accumulation of chemicals occurs. In the latter case, animals have a
‘decontaminating’ effect from the public health perspective. Hazards that accumulate can be a greater public health risk than those which do not accumulate, because if animals are exposed even only to low levels of accumulating hazards but over extended time, their tissues can finally contain levels that pose a risk to consumers. With chemical hazards that accumulate, older animals are a higher risk than younger animals.