What is the hepatitis A virus?
The hepatitis A virus (HAV) is a single-stranded RNA virus belonging to the Picornaviridae family and the genus Hepatovirus. It is an enteric virus and causes a liver disease in humans now known as hepatitis A (previously known by other names including infectious jaundice, viral hepatitis and infectious hepatitis). There are a number of different hepatitis viruses but only the HAV, and possibly the hepatitis E virus, can cause foodborne disease.
What foods can be contaminated?
The HAV can only be present in foods as the result of faecal contamination. Although this means that any food handled under poor hygienic practices could potentially be contaminated, it is bivalve molluscan shellfish, such as oysters, cockles and mussels, which are the most common source of foodborne viruses. These shellfish concentrate any virus particles in their tissues during filter feeding in contaminated water.
In recent years, fresh produce, such as salads, fresh fruits and vegetables, has increasingly been implicated in foodborne outbreaks of hepatitis A. These products are likely to be consumed raw or lightly cooked, and can become contaminated with faecal matter at almost any point during growing, harvesting, transport and packing.
How does it affect human health?
The infective dose for the HAV is uncertain, but it may be that as few as 10 – 100 virus particles could cause disease. The incubation time before symptoms appear is on average about 4 weeks, but it can vary from 2 – 6 weeks. This long incubation time can mean that it may be difficult to trace the exact source of the infection, and that large numbers of individuals are affected before it becomes evident that there is viral contamination in the food chain.
Many cases of HAV infection are asymptomatic, particularly in children. When disease is evident, hepatitis A infection is usually a mild illness. Initial symptoms include headache, fatigue, fever, poor appetite, abdominal discomfort, nausea and vomiting. After a week or so, viraemia (where virus can be detected in the blood stream) and liver disease in the form of jaundice, or liver enzyme elevation, occurs. Hepatitis A is usually a self-limiting disease lasting up to 2 months, but in a few cases the virus can cause long or recurring illness lasting up to 6 months. Infection can be fatal, particularly in people over 50 years old. In the USA, this age group has a mortality rate reported as 1.8 %
During infection individuals can excrete high numbers of virus particles (more than 1,000,000 particles/g of faeces). The shedding of particles can start in the last 2 weeks of the incubation period and in some individuals can continue for up to 5 months after infection.
How common is illness?
In many developing countries the disease is endemic and exposure during early childhood because of poor hygiene is common. Early childhood infections are usually asymptomatic and confer lifelong immunity.
Outbreaks of hepatitis A are more likely to occur in developed nations, or amongst travellers from developed countries to the developing world, because exposure to the virus during early childhood in individuals from developed regions is low. Countries where the adult population has no immunity are at risk of large hepatitis A outbreaks when food or water supplies are contaminated with the virus.
In 2007 there were an estimated 25,000 new hepatitis A infections in the USA, although only a small percentage (less than 5%) of these are likely to have been food or waterborne. In the EU in 2008 there were just over 17,000 reported cases of hepatitis A reported by 29 countries. In England and Wales the incidence of hepatitis A has decreased from just over 1,800 in 1997 to around 400 in recent years.
Contaminated water and bivalve shellfish such as oysters, cockles and mussels, are often associated with hepatitis A infections. The largest recorded foodborne outbreak, involving 290,000 cases, was in Shanghai, China in 1988 and was caused by clams harvested from waters polluted by raw sewage.
Fresh fruits, such as strawberries, blueberries and raspberries harvested by infected pickers, and associated products such as orange juice, have caused outbreaks in the UK, New Zealand and the USA, respectively. Imported lettuce, and in 2003 imported raw/undercooked green onions (601 cases with 3 deaths), have also caused large outbreaks in the USA.
Other foods linked to outbreaks include bakery products, sandwiches, iced beverages, milk and milk products, semi-dried tomatoes, raw beef, beer and soft drinks.
Where does it come from?
The human intestine is the main reservoir for the HAV and asymptomatic infected individuals, especially children, are an important source of the virus.
Transmission can occur via the faecal-oral route by direct person-to-person contact, or from the ingestion of contaminated food or water. It has been reported that transmission of the virus can occur as the result of using contaminated drinking glasses. Infected food handlers with poor hygiene are a potential source of the virus in food.
How is it affected by environmental factors?
Viruses, including the HAV, are unable to multiply outside of the host. Although the HAV cannot grow in food or water, it can survive in many environments for some time. When excreted in human faeces the virus can survive in water or soil for at least 12 weeks at 25°C. It has a high resistance to many chemicals and solvents and is more resistant to heat and drying than other enteric viruses. It can survive refrigeration and freezing for up to two years and it is resistant to acid (pH 1 for 2 hours at room temperature).
The HAV is resistant to low levels of free chlorine (0.5 – 1 mg free chlorine/l for 30 minutes). It is also resistant to perchloroacetic acid (300 mg/l) and chloramines (1g/l) for 15 minutes at 20°C. The virus can be inactivated on surfaces with a 1:100 solution of sodium hypochlorite, or household bleach in tap water.
The HAV is relatively heat resistant, although thorough cooking at higher temperatures will usually inactivate the virus. It is resistant at 70°C for up to 10 minutes but is inactivated at temperatures of 85°C for 1 minute. In the UK it has been recommended that cockles are heated to an internal temperature of 85 – 90°C for 1.5 minutes to inactivate HAV and data from the World Health Organization suggests that shellfish from HAV-contaminated areas should be heated to 90°C for 4 min or steamed for 90 sec.
How can it be controlled?
Strategies to reduce the risk of foodborne outbreaks of hepatitis A should focus on preventing foods from becoming contaminated. In developing countries young children should be kept away from areas where fresh produce is grown and harvested, and clean water should be used for the irrigation, washing and processing of foods. Shellfish harvesting areas should be monitored for sewage contamination.
For food processors
Food handlers should wash hands frequently and wear gloves, particularly at points in the food chain where foods that will receive no further cooking are handled. Those suffering from symptoms of hepatitis A should be removed from the food production area until they have a medical release.
For caterers and consumers
Caterers should be careful to purchase shellfish and fresh produce from sources where there is little risk of contamination.
If food could be contaminated with the HAV, consumers should be advised only to eat thoroughly cooked foods from known sources and not to eat uncooked fruits or vegetables that they have not peeled or prepared themselves.
Are there rules and regulations?
There is no specific legislation in the EU or in the USA regarding levels of enteric viruses, such as HAV, in foods. However, EFSA has recommended the development of microbiological criteria for viruses in bivalve molluscs unless they are labelled: “to be cooked before consumption.”
Where can I learn more?