FOOD SAFETY & QUALITY
Many people believe that most foodborne illness is caused by bacteria, but the truth is that the most common cause of foodborne-disease outbreaks in the United States is a virus. Human norovirus causes about 21 million illnesses, 70,000 hospitalizations, and 800 deaths each year, according to the Centers for Disease Control and Prevention (CDC) (www.cdc.gov/norovirus).
A Major Challenge
Norovirus is the official genus name for a group of viruses that cause acute gastroenteritis. The most common symptoms are diarrhea, vomiting, nausea, and stomach pain. The illness typically lasts one to two days, is usually not life-threatening, and does not generally cause long-term effects. However, because there are many different types of noroviruses, being infected with one type may not provide protection against other types.
Represented by the prototype strain Norwalk virus, named for Norwalk, Ohio, where the virus was first identified in 1972, the virus is highly contagious. People with the illness can shed billions of virus particles in their feces and vomit, and fewer than 100 virus particles are needed to cause illness in susceptible individuals. It is most contagious during the illness and for three days after recovery. The virus can be present in feces even before a person starts feeling sick and can stay in stool for two or more weeks after the person feels better. It can persist on surfaces for up to two weeks, is resistant to most disinfectants used at manufacturer-recommended concentrations, and may survive temperatures as high as 140°F. There is no drug to treat the illness (antibiotics do not work on viruses), and there is no vaccine to prevent it, although research to develop vaccines is being done or supported by the National Institute of Allergy and Infectious Diseases (www.niaid.nih.gov) and some commercial sources.
People become infected with norovirus by consuming food or liquids that are contaminated with norovirus; touching surfaces contaminated with norovirus, then touching their mouth and subsequently ingesting the virus; or having contact with someone who is infected. The CDC emphasizes that the best way to help prevent norovirus illness is to practice proper hand-washing before eating, preparing, or handling food. Leafy vegetables, fruits, nuts, and molluscan shellfish (oysters, clams, and mussels) are the most common food sources involved in norovirus outbreaks, but any food handled extensively after being cooked is at increased risk of becoming contaminated.
More than 90% of foodborne illness outbreaks on cruise ships are caused by norovirus. Outbreaks can also occur in other settings, such as schools, nursing homes, child care centers, prisons, and military camps, but most of these outbreaks are not associated with food.
Detection Methods Lacking
Genetically, human noroviruses are divided into five genogroups. Genogroups I and II cause virtually all of the human disease, with genogroup II being the most prevalent. Of the many genogroup II strains, genogroup II.4 is responsible for more than 85% of all outbreaks. The viruses are difficult to study because they cannot be cultivated outside of the human body, few commercial diagnostic tests are available in the United States, and only a few scientists are trained specifically in food virology.
Diagnostic methods focus on detecting viral RNA or antigen, according to the CDC. Most public health laboratories test for norovirus using real-time reverse-transcription polymerase chain reaction (RT-qPCR) assays, which can detect as few as 10–100 norovirus genome copies per reaction. The laboratories use different primers to differentiate between genogroup I and genogroup II noroviruses. The assays are also semi-quantitative and can provide estimates of viral load.
Conventional RT-PCR followed by sequence analysis of the RT-PCR products is used for norovirus genotyping. CaliciNet (www.cdc.gov/norovirus/php/reporting.html), a national laboratory surveillance network, was established by the CDC in 2009 to track norovirus outbreaks. Public health and food regulatory laboratories submit norovirus sequences identified from outbreaks to a national database to link norovirus outbreaks that may be caused by common sources (such as specific foods), monitor trends, and identify emerging norovirus strains.
Rapid commercial assays, such as enzyme immunoassays (EIAs), that detect the norovirus antigen have been developed but have poor sensitivity, the CDC said, and are not recommended for diagnosing norovirus infection in sporadic cases of gastroenteritis. Among the tests that are commercially available are the RidaScreen® Norovirus 3rd Generation EIA, the Rida®Gene Norovirus I & II real-time RT-PCR, and the RidaQuick Norovirus qualitative lateral-flow immunochromatographic test for determining genogroup I and II noroviruses from R-Biopharm (www.r-biopharm.com) and the norovirusGI@ceeramTools™ and norovirusGII@ceeramTools™ detection kits for clinical, food, and environmental samples from Ceeram S.A.S. (www.ceeram.com). However, the only test approved for use in the United States is the RidaScreen Norovirus 3rd Generation EIA. The U.S. Food and Drug Administration (www.fda.gov) approved it last year for preliminary identification of genogroups I and II noroviruses when testing multiple specimens during outbreaks but added that samples testing negative should be confirmed by a second technique, such as RT-qPCR (i.e., that EIA kits should not replace molecular methods during outbreak investigations).