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Picornaviruses
VIRUS:- Picornaviruses: Enteroviruses, Rhinoviruses
GENERAL CONCEPTS:
- Picornaviruses are small, nonenveloped viruses containing a single positive strand RNA genome. They possess an icosahedral symmetry.
- Picornaviruses are divided into two groups; the Enteroviruses (Poliovirus, Coxsackievirus and Echovirus) and the Rhinoviruses. There are about 63 serotypes of Enterovirus and more than 100 serotypes of Rhinovirus.
- Picornaviruses commonly produce subclinical infections; acute disease may range from minor illness to paralytic disease.
- The Enteroviruses enter via the intestinal tract and attach to receptors on intestinal epithelia. During this alimentary phase, the virus replicates in cytoplasm. They then spread into the lymphatic circulation (lymphatic phase) and then to the bloodstream (viremic phase). The viremic phase generally marks the end of the infection but an occasional neurologic phase can lead to more severe and permanent problems.
DISTINCTIVE PROPERTIES:
- Polioviruses: Poliovirus types 1, 2 and 3 are recognized. Their genome contains a 7000 base positive strand of RNA. These viruses adsorb only to intestinal epithelial cells and motor neuron cells of the central nervous system.
- Coxsackie: These viruses are divided into two groups; A and B. There are 23 serotypes of A, 6 serotypes of B. In humans, Coxsackieviruses produce respiratory disease, herpangitis, "hand, foot and mouth" disease, febrile rashes, pleurodynia, pericarditis, myocarditis, aseptic meningitis and paralytic disease.
- Echoviruses: An acronym for "Enteric Cytopathogenic Human Orphan viruses, the Echoviruses contain 31 serotypes and produce respiratory disease, febrile illness (with or without a rash), aseptic meningitis and paralytic disease.
- Rhinoviruses: This group of viruses are sensitive to acid pH and their optimal growth occurs at 33°. There are over 100 serotypes of Rhinoviruses and they produce the common cold.
PATHOGENESIS:
- Polioviruses: A number of syndromes can be observed, most (90-95%) are subclinical. About 4-8% present with a mild fever, sore throat and headache. Nonparalytic polio occurs in about 1% of cases while paralytic disease affects only 0.1%. Of the paralytic conditions, about 5-10% affect the cranial nerve or medulla while the others are spinal and limited to muscle weakness rather than complete paralysis.
- Coxsackievirus, Echovirus: Infection by Coxsackie- or Echoviruses resembles that produced by Poliovirus. A variety of syndromes are possible, ranging from trivial to severe.
- Rhinovirus: Infection results via the nasopharynx by direct contact. Viral replication leads to inflammation and edema, symptoms of the common cold. Inapparent infection is common.
HOST DEFENSES:
- Enteroviruses: Interferon is effective against these viruses. More specifically, IgA antibodies in the intestine and saliva are protective.
- Rhino: Susceptibility to the Rhinoviruses is dependent on prior exposure. Antibody of the IgA and IgG classes is important. Non-specific defenses including interferon, gastric
acidity and temperature may play a major role in controlling infection.
EPIDEMIOLOGY:
- Enteroviruses: Found worldwide, the enteroviruses are spread via the fecal-oral route. Most illnesses occur in the summer and fall. The virus may be carried in the throat for a week and shed in the feces for several weeks.
- Rhinoviruses: These viruses are spread from person to person, usually by direct contact. Inapparent infections occur in about half. On average, most persons suffer with 2-4 colds per year during the fall and spring months and these represent different serotypes of the virus.
DIAGNOSIS:
- Clinical: Diagnosis of enteroviral infections is usually not possible based on clinical presentation. However, some symptoms (pleurodynia, myocarditis) or conditions (aseptic meningitis) are suggestive. Diagnosis of rhinoviral infections, in contrast, is usually based on clinical presentation.
- Laboratory: Recovery of Enterovirus from the throat or feces is diagnostic. Recovery of Rhinoviruses is simply not practical.
CONTROL:
- Sanitary: Avoidance of contacts is the best means for preventing disease.
- Immunological: Vaccines for Polio have been available for more than 40 years. The Salk vaccine is a trivalent (types 1, 2 and 3), formalin inactivated suspension that is given by injection (parenteral). The Sabin vaccine is an attenuated, live, trivalent, oral suspension that produces intestinal IgA but has rare vaccine-associated paralysis. Vaccines against the other picornaviruses are not practical (too many serotypes).
- Chemotherapeutic: Supportive care is best.
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