Polio Virus Flashcards
Objectives:
- Recognize the three forms of polio and the associated symptoms
- Discuss the advantages/disadvantages of the inactivated and live polio vaccines; distinguish how each offers protection
- Recall the WHO polio eradication program
What is polivovirus?
This is the causative agent of poliomyelitis, a flaccid paralysis
A note about the eradication of Polivovirus
Poliovirus has been essentially eradicated from much of the world due to highly successful vaccines
- declared polio-free if no wild cases during 3-year period
- absent from US since 1977, Western Hemisphere since 1994
20 cases in Dominican Republic and Haiti; 4 cases in Amish community in central Minnesota- both derived from revertant of oral vaccine, spread thru under-immunized population
- also absent in Australia, most of Europe and China
- less than 1,000 worldwide cases per year
Polio remains endemic in what regions?
Afghanistan, Pakistan, India, and Nigeria
What is the structure of Poliovirus?
nonenveloped icosahedral capsid
7.5 kb ss RNA (+) genome
three serotypes
What family and genus does poliovirus belong to?
Family Picornaviridae, genus Enterovirus
How does poliovirus spread?
fecal-oral, most commonly in summer months (humans are the only reservoir)
How does polio act in humans?
Incubation of 7-14 days followed by:
initial replication in lymphoid tissue of pharynx and gut; virus begins to appear in throat and feces
secondary spread is via viremia, virus crosses blood-brain barrier to infect the CNS
motor neurons are destroyed due to virus replication
How does poliovirus infection present
Most infections are subclinical, but three types of disease can result:
- abortive poliomyelitis (4-8%)
- aspectic meningitis (nonparalytic poliomyelitis; 1-2%)
- paralytic poliomyelitis (0.1-1%)
Describe abortive poliomyelitis
a. nonspecific febrile illness of 2-3 days
b. no involvement of CNS
Describe aseptic meningitis (nonparalytic meningitis)
Febrile illness of 2-3 days plus stiff neck and a CNS infection (but limited)
Describe paralytic poliomyelitis
This is a febrile illness, with lapse of symptoms, and then flaccid paralysis that can attack all 4 limbs or brain stem. Usually there is gradual recovery over 6 months, but any lingering paralysis after 6 months is permanent
d. 5-10% fatality rate among paralyzed patients
e. post-polio syndrome: increasing weakness later in life
Who mostly gets polio infections?
children under 5 yo
- adult infections are often the result of improved sanitation which delays exposure to the virus
- risk of paralysis increases with age
How is polio diagnosed?
Specimens for virus isolation are taken from stools, rectal swabs, throat swabs and cerebrospinal fluid, depending upon the stage of infection
Neutralization assays are performed to confirm polio
Vaccine strains can be distinguished from wild poliovirus using strain-specific polyclonal sera (specialized labs only)
What vaccines are available for polio?
Salk inactivated polio vaccine (IPV)
Sabin live oral polio vaccine (OPV)
Since 2000, only IPV has been recommended for US vaccinations
The polio Virus is best inactivated with:
formaldehyde or chlorine
Virus is resistant to detergents and ethanol
Describe the Salk IPV
Licensed in 1955
Advantage: formalin-inactivation prevents reversion to virulence
Disadvantages: Four subcutaneous injections; Protects against paralysis but not spread of wild virus
Do not use if allergic to neomycin, streptomycin, or polymyxin B
Describe the Sabin live OPV
Licensed in 1963
Advantages: Cheap; 8 cents/dose; Oral administration (three doses); Protects against paralysis and spread of wild virus and all 3 serotypes
Disadvantage: can revert to virulence during preparation or during shedding by recipients since it is liver
In the past, US children were first vaccinated with the inactivated vaccine and then given a “booster” with the oral vaccine. What was the rationale behind this strategy?
WHO’s Polio Eradication Program had targeted global eradication by 2000
1) employs OPV for National Immunization Days that target children (introducing a new bivalent (serotypes 1 and 3) OPV (bOPV) that appears to be more effective)
2) has resulted in a 99% reduction in yearly cases since 1988
NOTE: some controversy due to exclusive use of OPV
