Foodborne viruses of primary importance are norovirus (NoV) and hepatitis A virus (HAV). Norovirus foodborne outbreaks occur frequently, whereas HAV outbreaks occur incidentally. Other viruses that may be involved in food-borne virus infections include rotaviruses, enteroviruses, and hepatitis E virus. Almost all food-borne viruses replicate in the intestinal tract.
Electron microscopy presentation Calicivirus
Food-borne viral disease
By far the most important clinical feature is gastro-enteritis, several food-borne viruses cause hepatitis and a third group of important clinical symptoms is neurologically based.
Food-borne NoV infections normally become manifest in 1-3 days after eating a contaminated food item. Most prominent symptoms are diarrhoea and vomiting, often together with abdominal pain, nausea and low grade fever. Symptoms last for a median of five days and decrease with age from six days in children aged < 1 year to three days in patients aged over 11 years. The symptoms caused by the enterically transmitted hepatitis viruses are fever, headache, fatigue, nausea and abdominal discomfort, followed some days later by hepatitis specific signs like jaundice and elevated liver enzymes. The acute phase lasts approximately two weeks but recovery can take several months. The incubation period varies from 2-7 weeks for HAV and 2-8 weeks for HEV. In young children HAV as well as HEV infections often are asymptomatic, and most serious symptoms are seen in young adults and elderly people. A remarkable clinical feature of HEV is the high case fatality rate (10-20%) due to unexplained fulminant disease that has been reported for pregnant women, especially in the third trimester.
Foods of concern
Viruses are strict intracellular parasites and do not replicate in food or water. Therefore virus contamination of food firstly depends on the degree of virus contamination and secondly on the survival of the virus in the food item. The foods that have been implicated in food-borne viral disease are largely those that are consumed without virus inactivating heat treatment (e.g. fresh produce, bread rolls) or are not cooked after handling (e.g. deli foods). High numbers of viral particles are shed in the stools and vomits from infected persons and food-borne viruses typically are very stable outside the host and are acid resistant. In fact all food that needs manual handling and is not properly heated before consumption is a potential risk food. NoVs can occur on all kinds of fresh produce, including lettuce, tomatoes and strawberries. Produce can be contaminated with viruses from the human reservoir during pre-harvest as well as post harvest practices. Other foods can become contaminated directly or indirectly by an infected person or a carrier. Seafood species harvested from virus contaminated areas may have accumulated pathogenic viruses. The filter feeding bivalve molluscan shellfish have repeated proven to be an effective vehicle for the transmission of NoV and HAV.
Sources of contamination
Norovirus as well as HAVs are quite stable in the water environment. Water therefore can be an important source of contamination for these viruses. Surface waters can easily become contaminated with HAV and NoV of human origin through raw sewage, but also by effluent from sewage treatment plants. Especially after heavy rainfall, untreated sewage may enter harvesting areas of bivalve molluscs and thus contaminate this product. Another route of water borne virus contamination can be the use of surface water for irrigation. In this way, crops can be contaminated with viruses,
Food-borne NoV outbreaks can result from contamination by an infected food handler. Norovirus infected food handlers can shed virus not only during clinical disease but also up to at least 3 weeks after recovery. Apart from that, a-symptomatic infections are common for NoV as well as HAV and HEV. Food handlers may also transmit viruses from their infected housemates and relatives, especially from sick children.
The majority of enteric viruses are quite host specific. Variants of NoVs are found in cattle and pigs but direct animal to human transmission of NoV has not been reported. Hepatitis E virus genotype 3 from pigs can infect humans. Hepatitis E virus therefore can be designated as a zoonotic agent, and pigs may be an important animal reservoir. Recently, several meat borne HEV infections have been reported from Japan Hepatitis A virus is often detected in shellfish, but since evidence of an active infection of this virus in animals has never been reported, HAV is not regarded a real zoonosis.
Virus detection in foods
Until quite recently, methods for detecting human enteric viruses in foods were based on the inoculation of suitable food extracts into cell cultures. However since several important food-borne viruses are very hard to grow in cell culture, detection of these enteric viruses is now generally done using the molecular technique of reverse transcription-PCR (RT-PCR), which involves amplifying the conserved regions of the microorganisms’ genomes. This technique is extremely sensitive and specific, but unable to distinguish between infectious and non-infectious virus particles Also, the amplification of genetic material can often be inhibited by substances that are present in the foods, and therefore the removal or inactivation of potential inhibitors is a very important step in this methodology Thus, most RT-PCR methods are designed to concentrate and purify viruses and remove inhibitors before performing RT-PCR. This usually involves multistep elution and extraction procedures using a combination of several reagents.
To prevent contamination of bivalve molluscs a strict control of harvesting areas is indicated. For this purpose guidelines on virus contamination and monitoring of virus contamination of harvesting areas will be needed. Vegetables and fruit should not be grown or washed in faecally contaminated water.
To prevent contamination of foods during harvest, processing and preparation, personal hygiene is very important. This includes hygiene during harvesting like hand washing and toilet facilities. Frequent hand washing is important along the whole production chain and wearing gloves can be an extra preventive measurement. Food processors and food handlers experiencing or recovering of gastro-intestinal disease must be dismissed from work. Cooking of foods appears to be an effective control measure for NoVs and hepatitis A.