Nonenveloped (Naked) RNA Viruses

Published on 18/02/2015 by admin

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Last modified 18/02/2015

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Nonenveloped (Naked) RNA Viruses

I Picornaviridae

• This family of small viruses with a naked, icosahedral capsid and single-stranded (+) RNA genome includes the enteroviruses and rhinoviruses.

Microinjection of picornavirus or calicivirus genome (+RNA) into cells is sufficient for infection.

Picornavirus, calicivirus, and reovirus are resistant to detergent and acid and are transmitted by fecal-oral route.

A Enteroviruses: poliovirus, Coxsackie A virus, Coxsackie B virus, echovirus, and hepatitis A virus (see Chapter 27)

• The enteroviruses are acid stable, can survive in the gastrointestinal (GI) tract, and are transmitted primarily by the fecal-oral route.

1. Pathogenesis (Fig. 23-1)

23-1 Pathogenesis of picornaviruses. All of the enteroviruses can be spread by the fecal-oral route but cause different diseases depending on the target tissue that is infected. Some enteroviruses, like the rhinoviruses, can be spread by respiratory means and cause symptoms of the common cold. HFMD, hand-foot-and-mouth disease.

Virulence of enterovirus lytic infection; certain viruses are neurotropic

• Initial replication occurs in the mucosa and lymphoid tissues of the pharynx and tonsils and subsequently in the GI tract.

• Viremic spread of virions to target tissues depends on tissue specificity (tropism) of the virus.

a. Poliovirus binds to receptors on muscle cells and neurons, causing disease in the central nervous system (CNS).

b. Coxsackie viruses and echoviruses have a broader tissue tropism, causing disease in the CNS, lungs, heart, pancreas, and other tissues.

• Cytolytic replication in target tissues causes direct tissue damage.

2. Diseases due to enteroviruses (Box 23-1)

BOX 23-1 Enterovirus Infections

Quick Cases

Poliomyelitis: A 15-year-old Burmese boy experiences fever, headache, nausea, and a stiff neck for 24 to 36 hours. His condition improves for several days, and the same symptoms return, accompanied by increasing weakness and paralysis of the legs. His medical history indicates that he had not been vaccinated against poliovirus.

Coxsackie A Virus

Herpangina: During summer vacation, a 7-year-old child develops a sudden fever accompanied by loss of appetite, sore throat, vomiting, and pain on swallowing. Examination shows discrete vesiculopapular lesions on the tongue and roof of the mouth.

Acute hemorrhagic conjunctivitis: A young girl has swollen eyelids with redness, congestion, and pain in her eyes. Several other children who attend the same nursery school show similar symptoms.

Coxsackie B Virus

Epidemic pleurodynia: A 13-year-old boy suddenly develops a fever and severe paroxysmal chest pain, which lasts for 4 days. He also complains of headache, fatigue, and aching muscles.

Coxsackievirus or Echovirus

Aseptic (viral) meningitis: A 9-month-old girl has a fever and skin rash and is suffering from nausea. She appears listless and has difficulty moving her head from side to side. CSF analysis shows normal glucose, no bacteria, and an increase of proteins and lymphocytes. Within 1 week, the infant is fully recovered.

• Poliomyelitis

a. Usually caused by poliovirus but also Coxsackie A virus

Trigger words:

Poliovirus: asymmetric flaccid paralysis, fecal-oral, major disease, minor disease, picornavirus

Coxsackie and echoviruses: picornavirus, fecal-oral, Coxackie B (for Body), myocarditis, pleurodynia; Coxsackie A virus: hand foot and mouth disease, vesicular lesions.

b. Abortive poliomyelitis (minor illness)

c. Nonparalytic poliomyelitis (aseptic meningitis): fever, headache, sore throat, stiff neck, pleocytosis of cerebrospinal fluid (CSF)

d. Paralytic poliomyelitis (major illness): same symptoms as nonparalytic polio plus flaccid paralysis resulting from destruction of lower motor neurons

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