POLIO VIRUS


History:
  • Poliovirus - first identified in 1909 by inoculation of specimens into monkeys. The virus was first grown in cell culture in 1949 which became the basis for vaccines.


Viral properties:
  • 3 serotypes of poliovirus (1, 2, and3) but no common antigen.
  • Have identical physical properties but only share 36-52% nucleotide homology.
  • Humans are the only susceptible hosts.
  • Polioviruses are distributed globally. Before the availability of immunization, almost 100% of the population in developing countries before the age of 5.

Pathogenesis:
  • The incubation period is usually 7 - 14 days.
  • Following ingestion, the virus multiplies in the oropharyngeal and intestinal mucosa.
  • The lymphatic system, in particular the tonsils and the Peyer's patches of the ileum are invaded and the virus enters the blood resulting in a transient viraemia.
  • In a minority of cases,the virus may involve the CNS following dissemination.
  • It affect by following staging:
  1.  Multiplication:with in incubation period and in intestine and it enter by mouth
  2.  Initial viraemia: from intestine it spred to lymphatic system and cause viraemia by entering in blood
  3. Second viraemia: who fail to control second stage there occures.virus migrate to CNS and can cause CNS disturbance


Clinical manifestation:
There are 3 possible outcomes of infection:
  • Subclinical infection (90 - 95%) - inapparent subclinical infection account for the vast majority of poliovirus infections.
  • Abortive infection (4 - 8%) - a minor influenza-like illness occurs, recovery occurs within a few days and the diagnosis can only be made by the laboratory. The minor illness may be accompanied by aseptic meningitis
  • Major illness (1 - 2%)(paralytic poliomyelities) - the major illness may present 2 - 3 days following the minor illness or without any preceding minor illness. Signs of aseptic meningitis are common. Involvement of the anterior horn cells lead to flaccid paralysis. Involvement of the medulla may lead to respiratory paralysis and death.

Laboratory infection:
Virus Isolation
  • Mainstay of diagnosis of poliovirus infection
  • poliovirus can be readily isolated from throat swabs, faeces, and rectal swabs. It is rarely isolated from the CSF
  • Can be readily grown and identified in cell culture
  • Requires molecular techniques to differentiate between the wild type and the vaccine type.
Serology
  • Very rarely used for diagnosis  since cell culture is efficient. Occasionally used for immune status screening for immunocompromised individuals. 


Prevention:
No specific antiviral therapy is available. However the disease may be prevented through vaccination. There are two vaccines available.

Intramuscular Poliovirus Vaccine (IPV)
  • Consists of formalin inactivated virus of all 3 poliovirus serotypes.
  • Produces serum antibodies only: does not induce local immunity and thus will not prevent local infection of the gut.
  • However, it will prevent paralytic poliomyelitis since viraemia is essential for the pathogenesis of the disease.
Oral  Poliovirus Vaccine (OPV)
  • Consists of live attenuated virus of all 3 serotypes.
  • Produces local immunity through the induction of an IgA response as well as systemic immunity.
  • Rarely causes paralytic poliomyelitis, around 1 in 3 million doses.


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