Picornaviruses are small (20–30 nm) nonenveloped viruses composed of an icosahedral nucleocapsid and a single-stranded RNA genome. The genome RNA has positive polarity (i.e., on entering the cell, it functions as the viral mRNA). There is no polymerase within the virion. Picornaviruses replicate in the cytoplasm of cells. They are not inactivated by lipid solvents, such as ether, because they do not have an envelope.
The picornavirus family includes two groups of medical importance: the enteroviruses and the rhinoviruses. Among the major enteroviruses are poliovirus, Coxsackie viruses, echoviruses, and hepatitis A virus (which is described in Chapter 41). Enteroviruses infect primarily the enteric tract, whereas rhinoviruses are found in the nose and throat (rhino = nose) (Table 40–1).
TABLE 40–1Picornaviruses of Medical Importance |Favorite Table|Download (.pdf) TABLE 40–1 Picornaviruses of Medical Importance
|Virus ||Initial Site of Infection ||Inactivated by Stomach Acid ||Optimal Temperature for Replication ||Diseases |
|Poliovirus ||Gastrointestinal (GI) tract ||No ||37°C ||Poliomyelitis |
|Coxsackie viruses ||GI tract ||No ||37°C ||Meningitis Myocarditis, hand-foot-and-mouth disease, etc. |
|Echoviruses ||GI tract ||No ||37°C ||Meningitis |
|Hepatitis A virus ||GI tract ||No ||37°C ||Hepatitis |
|Rhinoviruses ||Upper respiratory tract ||Yes ||33°C ||Common cold |
Enteroviruses replicate optimally at 37°C, whereas rhinoviruses grow better at 33°C, in accordance with the lower temperature of the nose. Enteroviruses are stable under acid conditions (pH 3–5), which enables them to survive exposure to gastric acid, whereas rhinoviruses are acid-labile. This explains why rhinovirus infections are restricted to the nose and throat.
Important features of viruses that commonly infect the intestinal tract are summarized in Table 40–2. These include the picornaviruses but also rotavirus and norovirus, which are described later in this chapter, and adenovirus, which is described in Chapter 38.
TABLE 40–2Features of Viruses Commonly Infecting the Intestinal Tract |Favorite Table|Download (.pdf) TABLE 40–2 Features of Viruses Commonly Infecting the Intestinal Tract
|Virus ||Nucleic Acid ||Disease ||Number of Serotypes ||Lifelong Immunity to Disease ||Vaccine Available ||Antiviral Therapy |
|Poliovirus ||RNA ||Poliomyelitis ||3 ||Yes (type-specific) ||+ ||– |
|Coxsackie viruses ||RNA ||Meningitis, carditis, etc. ||Many ||No ||– ||– |
|Echoviruses ||RNA ||Meningitis, etc. ||Many ||No ||– ||– |
|Hepatitis A virus ||RNA ||Hepatitis ||1 ||Yes ||+ ||– |
|Rotavirus ||RNA ||Diarrhea ||Several1 ||No ||+ ||– |
|Norovirus ||RNA ||Diarrhea ||Many1 ||No ||– ||– |
|Adenovirus ||DNA ||Diarrhea ||41; of which 2 cause diarrhea ||Unknown ||– ||– |
This virus causes poliomyelitis.
The host range is limited to primates (i.e., humans and nonhuman primates such as apes and monkeys). This limitation is due to the binding of the viral capsid protein to a receptor found only on primate cell ...