The viruses described in this chapter are transmitted by the fecal–oral route and enter the body via the enteric tract. Some, such as norovirus and rotavirus, cause diarrheal disease, whereas others, such as poliovirus, Coxsackie virus, echovirus, and enterovirus (EV) 68, 70, and 71 cause disease primarily outside the enteric tract. Polio, Coxsackie, and echoviruses are well-known causes of central nervous system disease, such as meningitis and encephalitis. Coxsackie virus also causes hand, foot, and mouth disease and myocarditis. EV D68 is suspected of causing acute flaccid myelitis.
Poliovirus, Coxsackie virus, echovirus, and EV 68, 70 and 71 are members of a group of viruses called Enteroviruses within the Picornavirus family. The term “Enterovirus” refers to the enteric tract as an important site of viral replication and to the feces as a common source of infection and a common specimen from which these viruses are isolated. Note, however, that Coxsackie virus, echovirus, and EV 68, 70 and 71 also replicate and cause disease symptoms in the upper respiratory tract and in the central nervous system.
All of the viruses described in this chapter are naked nucleocapsid viruses (i.e., they do not have an envelope). Viruses without an envelope are more stable in the environment, a feature that allows them to survive outside the body and to be transmitted by the fecal–oral route.
Note that other viruses also infect via the enteric tract such as hepatitis A virus and hepatitis E virus. These are discussed in Chapter 41 with the other hepatitis viruses. Note also that two viruses, Astrovirus and Sapporo virus, cause diarrhea. Because these are less common, they are described in Chapter 46 entitled Minor Viral Pathogens.
Additional information regarding the clinical aspects of infections caused by the viruses in this chapter is provided in Part IX entitled Infectious Diseases beginning on page 607.
VIRUSES THAT PRIMARILY CAUSE DIARRHEA
Norovirus is one of the most common causes of viral gastroenteritis in adults both in the United States and worldwide. Norovirus is also the most common cause of viral gastroenteritis in children in the United States because the rotavirus vaccine has lowered the incidence of disease caused by that virus. Norwalk virus is an important norovirus and is named for an outbreak of gastroenteritis in a school in Norwalk, Ohio, in 1969.
Norovirus is a member of the Calicivirus family. It is a nonenveloped virus with an icosahedral nucleocapsid and has a nonsegmented, single-stranded, positive-polarity RNA genome (Table 40–1). There is no polymerase within the virion.
Table 40–1Properties of Viruses Commonly Infecting the Intestinal Tract