Between 1988-1992, the most recent period for which data have been issued, Hepatitis A virus became the fourth leading cause of foodborne illness and gastroenteritis viruses became the ninth, Cliver stated.
"Hepatitis A, which is notoriously under reported in the United States, is the only foodborne viral disease for which official reporting is mandatory for all diagnosed cases. Thus, records of the incidence of the other viral diseases are certain to be less accurate," he said.
Hepatitis A infects human liver cells, causing fever, tiredness, anorexia, nausea, and abdominal discomfort, which is often followed by jaundice in adults. It is one of the most severe foodborne illnesses; a few weeks of debility is common, and permanent impairment of some liver functions is possible. By contrast, gastroenteritis viruses, causing nausea, vomiting, and/or diarrhea, typically last one or two days.
Viruses are transmitted to people via foods by direct or indirect contamination of the foods with human feces; with the exception of the virus responsible for tick-borne encephalitis, all foodborne viruses are transmitted by the fecal-oral route (enterically), Cliver said.
Unlike bacteria, viruses cannot multiply outside of their hosts, he added. Despite this, viruses transmitted by food can survive the low pH of the stomach and upper small intestine as well as conditions designed to preserve food quality, such as refrigeration and freezing.
Food contamination may come from the environment (e.g., polluted shellfish beds, fruit or vegetable growing fields), poor processing or preparation, or unsanitary handling by workers.
Because detection methods for foodborne viruses are very difficult and costly, they are not used routinely. Hepatitis A is particularly difficult to detect in foods because it has an incubation period of 15-50 days (28 mean); pertinent food samples are unlikely to be available after the virus has been diagnosed in an individual.
However, some fooods are more susceptible to contamination than others. Clams, oysters, mussels, and cockles, for example, are especially prone to transmitting viruses because of their filter feeding activity; shellfish collect viruses in their digestive tracts where they are harbored for days or weeks, Cliver explained. Unlike many other seafoods, shellfish are usually eaten with their digestive tracts in place, often raw or lightly cooked.
Viruses in foods can usually be inactivated by adequate heating; viruses in water and on exposed surfaces can be inactivated with ultraviolet light or strong oxidizing agents such as chlorine or ozone.
The best way to prevent virus transmission in foods is through good personal hygiene practices and high standards of food protection and sanitation procedures among food handlers. Cliver emphasizes the importance of frequent and efficient hand washing, using friction action and a nail brush.
"Gloves may be of some value in preventing hand contact, but availability of hand washing facilities and management's emphasis on proper hand washing techniques are the most important precautions to prevent direct human contamination of foods with viruses," he said.
Other measures to prevent virus transmission via food include monitoring the environments in which food is grown or harvested and thoroughly cooking foods at risk for contamination. Also, the U.S. Centers for Disease Control and Prevention recommends immunizing children against Hepatitis A with routine vaccination.
Dean O. Cliver, Ph.D., is a professor in the Department of Population Health and Reproduction at the School of Veterinary Medicine, University of California at Davis.
Founded in 1939, the Institute of Food Technologists is a non-profit scientific society of 28,000 members working in food science, technology and related professions in industry, academia, and government. As the society for food science and technology, IFT brings sound science to the public discussions of food issues.