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, and echovirus, 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.
Poliovirus, Coxsackie virus, and echovirus 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 and echovirus also replicate and cause disease symptoms in the upper respiratory tract.
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.
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 has a nonsegmented, single-stranded, positive-polarity RNA genome (Table 40–1). It is a nonenveloped virus with an icosahedral nucleocapsid. There is no polymerase within the virion. In the electron microscope, 10 prominent spikes and 32 cup-shaped depressions can be seen. There are two or more serotypes; the exact number is uncertain. Six genogroups have been identified. Most human infections are caused by members of genogroup II.
TABLE 40–1Properties of Viruses Commonly Infecting the Intestinal Tract |Favorite Table|Download (.pdf) TABLE 40–1 Properties of Viruses Commonly Infecting the Intestinal Tract
|Property ||Norovirus ||Rotavirus ||Poliovirus ||Coxsackie Virus ||Echovirus |
|Virus family ||Caliciviruses ||Reoviruses ||Picornavirus ||Picornavirus ||Picornavirus |
|Genome ||Single-stranded RNA; positive polarity ||Double-stranded RNA; 11 segments ||Single-stranded RNA; positive polarity ||Single-stranded RNA; positive polarity ||Single-stranded RNA; positive polarity |
|Virion RNA polymerase ||No ||Yes ||No ||No ||No |
|Nucleocapsid ||Icosahedral ||Icosahedral ||Icosahedral ||Icosahedral ||Icosahedral |
|Envelope ||No ||No ||No ||No ||No |
|Number of serotypes ||Two or ...|