Human and Animal Viruses in Food (Including Taxonomy of Enteric Viruses)

In recent years, there has been an increase in the incidence of food-borne diseases worldwide, with viruses now recognized as a major cause of these illnesses.The viruses implicated in food-borne disease are the enteric viruses, which are found in the human gut, excreted in human feces, and transmitted by the fecal-oral route. Many different viruses are found in the gut, but not all are recognized as food-borne pathogens. The enteric viral pathogens found in human feces include noroviruses (previously known as Norwalklike viruses), enteroviruses, adenoviruses, hepatitis A virus (HAV), hepatitis E virus (HEV), rotaviruses, and astroviruses, most of which have been associated with food-borne disease outbreaks. Noroviruses are the major group identified in food-borne outbreaks of gastroenteritis, but other humanderived and possibly animal-derived viruses can also be transmitted via food. The diseases caused by enteric viruses fall into three main types: gastroenteritis, enterically transmitted hepatitis, and illnesses that can affect other parts of the body such as the eye, the respiratory system, and the central nervous system including conjunctivitis, poliomyelitis, meningitis, and encephalitis. Four of the enteric viruses—noroviruses, HAV, rotaviruses, and astroviruses—are included in the thirteen major food-borne pathogens identified by the Centers for Disease Control and Prevention (CDC) (Mead et al., 1999). These four viruses are reported to comprise 80% of all foodborne illnesses in the United States, with noroviruses by far the greatest contributor at an estimated 23 million cases per year (Mead et al., 1999). All enteric viruses except the adenoviruses contain RNA rather than DNA, have a protein capsid protecting the nucleic acid, and are nonenveloped. In the environment and in food, the enteric viruses are inert particles and do not replicate or metabolize because, like all viruses, they are obligate pathogens and require living cells to multiply. Many of the enteric viruses such as astroviruses, enteric adenoviruses, HAV, and rotaviruses are fastidious in their in vitro growth requirements but can still be grown in cell cultures. Noroviruses, on the other hand, do not grow in vitro, and no animal model exists for the human noroviruses yet. For many years, the lack of a culture system limited investigations focusing on the role of noroviruses in food-borne disease, although progress is now being made after the in vitro culture of a mouse norovirus (Wobus et al., 2004). Cell cultures are