PICORNAVIRIDAE

• STRUCTURE: Smallest of RNA viruses, ss (+) RNA, non-enveloped, linear, infectious. No cap but there is a viral protein linked to the 5' end.

  • Rigid capsule makes fecal-oral transmission possible

  • A large number of infectious viral particles is produced. Humoral immunity is crucial to fighting these.

  • Non-enveloped structure makes it a good GI pathogen, able to survive in GI tract.

• REPLICATION: Cytoplasmic, RNA-Dependent RNA Polymerase.

  • Entire genome is immediately translated into a polyprotein.

  • Polyprotein is cleaved into several small constituents:

    • Viral RNA-Dependent RNA Polymerase

    • Structural proteins

  • RNA Dependent RNA Polymerase then makes a minus-strand RNA out of the plus-strand RNA.

  • The minus-strand RNA is then used as a template for making additional (plus-strand) viral particles.

• ENTEROVIRUSES:

  • CONDITIONS:

    • Stable at pH 3, thus it can replicate in stomach.

    • Optimal temp is 37C, favoring GI tract.

  • EPIDEMIOLOGY: Highly communicable. Fecal-oral transmission, respiratory, person-to-person.

    • Peak occurrence in summer and fall.

    • Incubation period of 3-5 days for localized infections, or longer for generalized illnesses.

    • Virus is tropic for lymphoid tissue in small intestine and pharynx.

  • General Manifestations: Usually asymptomatic or associated with mild illness. Even though GI tract is a major site of replication, GI symptoms are rarely seen (except as a consequence of generalized illness).

  • IMMUNITY: Acquired immunity is lifelong and serotype specific. Humoral and mucosal immunity is important, to counteract free viral particles.

    • Enteroviruses that infect the gut produce a good IgA response, whereas viruses that infect only the upper respiratory tract do not produce as good a response.

  • POLIOVIRUS:

    • STRUCTURE: 3 serotypes.

      • Ig-Superfamily cell-surface receptor protein has been identified.

    • DIAGNOSIS:

    • MANIFESTATIONS: Only 1 in 200 infections come down with Paralytic Poliomyelitis.

      • Infection of anterior horn cells of spinal cord (spinal poliomyelitis) or pontine nuclei (bulbar poliomyelitis).

      • Symptoms: Myalgia, generalized weakness, followed by flaccid paralysis.

      • Post-Polio Syndrome (PPS): Strange symptoms, thought to be due to nerve cell attrition, very late in the course of disease. Fatigue, muscle weakness, and respiratory difficulty.

    • VACCINE: Sabin (live, oral) and Salk (dead). Most remaining cases of Polio today are residuals of the live attenuated vaccine.

      • The Sabin Oral Polio vaccine should not be given to people with HIV. Other live vaccines (measles, mumps, HBV, influenza) can be given.

      • Enhanced Polio Inactivated Vaccine has been developed recently. Will be given as the first shot -- inactivated vaccine, in order to prevent patient from getting polio from vaccine. Subsequent booster shots will then be the oral polio vaccine.

  • COXSACKIEVIRUSES:

    • COXSACKIEVIRUS A: 26 serotypes.

      • MANIFESTATIONS: Usually just mild respiratory disease.

        • ASEPTIC MENINGITIS:

          • Viral Meningitis occurs in Summer and Fall -- not Winter and Spring as in Mumps virus.

          • Coxsackie A&B, and Echovirus account for 80-90% of all cases of viral aseptic meningitis.

          • Important to distinguish it with partially treated bacterial meningitis: lumbar puncture will show lymphocytes (not PMN's), and a normal glucose (not low glucose).

        • HERPANGINA: Severe febrile, vesicular pharyngitis -- vesicles or nodules on soft palate.

        • Acute Hemorrhagic Conjunctivitis. Very contagious pain and swelling of eyelids. Occurs in Coxsackie A24.

        • HAND-FOOT-AND-MOUTH DISEASE: Vesicular rashes. Coxsackie A9 and A16.

        • Hemolytic Uremic Syndrome (HUS): Can be seen in strains of Coxsackie A and B, and Echovirus. Also due to E. Coli strains.

    • COXSACKIEVIRUS B: 6 serotypes.

      • MANIFESTATIONS: The leading viral cause of myocarditis and pericarditis.

        • Myocarditis and Pericarditis.

        • Aseptic meningitis.

        • Pleurodynia: Sharp muscle pain. Peak age of incidence in adolescent -- older than most enteroviral infections.

        • Newborn Disease: Myocarditis, encephalitis, hemorrhagic hepatitis. Also found in Echovirus. Non-immune neonate (no maternal antibodies) is susceptible, and can be fatal.

  • ECHOVIRUS: Enteric Cytopathic Human Orphan Virus

    • STRUCTURE: 30 serotypes. Called "orphan" because it wasn't associated with any disease state. Generally non-pathogenic.

    • MANIFESTATIONS: Asymptomatic or mild respiratory disease, and can cause aseptic meningitis and HUS, severe diseases of newborn.

  • ENTEROVIRUSES 68-72: Newer numbered enteroviruses.

    • ENTEROVIRUS-70:

      • MANIFESTATIONS: Acute Hemorrhagic Conjunctivitis. Very contagious pain and swelling of eyelids. Also seen with Coxsackie A24.

    • ENTEROVIRUS-71:

      • MANIFESTATIONS:

        • Hand-foot-and-mouth Disease: Herpetiform lesions in hand, foot, and mouth.

  • ENTEROVIRUS-72: HEPATITIS-A

    • STRUCTURE: Single stranded RNA, non-enveloped, stable to ether, temperature.

    • TRANSMISSION: Fecal-oral, sexual. Found in shellfish, where viral particles can become concentrated.

    • MANIFESTATIONS: Unlike the other picornaviruses, it is not cytocidal.

      • Damage to liver is due to hypersensitivity, cytotoxic T-Cells -- not due to viral cytopathic effect.

      • As opposed to HBV and HCV, HAV is only acute -- there is no persistent infection.

    • DIAGNOSIS: Look for Anti-HAV IgM to diagnose acute infection.

      • AST will be high early on.

      • Anti-HAV IgM, increases slowly, and drops off after a few weeks.

      • Anti-HAV IgG starts afer 5-6 weeks and persists for life.

    • VACCINE: Available, and given to people traveling to endemic areas. Three doses.

• RHINOVIRUS:

  • STRUCTURE: more than 100 serotypes. "Rhino" = nose.

    • ICAM cell-surface receptor protein has been identified.

  • CONDITIONS:

    • Labile at pH 3, thus it is inactivated in the stomach.

    • Optimal temp = 33C, thus it likes to grow in the nose.

  • IMMUNITY: Acquired immunity is poor.

  • DIAGNOSIS: It isn't cultured, but must distinguish it from bacterial otitis media or pharyngitis. Culture for Strep-A.

  • MANIFESTATIONS:

    • Common Cold: 30-50% of cases, occurring throughout year and peaking in summer.

Category: Microbiology Notes