Polio Virus

Question 1

Objectives:

1. Recognize the three forms of polio and the associated symptoms
2. Discuss the advantages/disadvantages of the inactivated and live polio vaccines; distinguish how each offers protection
3. Recall the WHO polio eradication program

Question 2

What is polivovirus?

Answer 2

This is the causative agent of poliomyelitis, a flaccid paralysis

Question 3

A note about the eradication of Polivovirus

Answer 3

Poliovirus has been essentially eradicated from much of the world due to highly successful vaccines

1. declared polio-free if no wild cases during 3-year period
2. 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

3. also absent in Australia, most of Europe and China
4. less than 1,000 worldwide cases per year

Question 4

Polio remains endemic in what regions?

Answer 4

Afghanistan, Pakistan, India, and Nigeria

Question 5

What is the structure of Poliovirus?

Answer 5

nonenveloped icosahedral capsid

7.5 kb ss RNA (+) genome

three serotypes

Question 6

What family and genus does poliovirus belong to?

Answer 6

Family Picornaviridae, genus Enterovirus

Question 7

How does poliovirus spread?

Answer 7

fecal-oral, most commonly in summer months (humans are the only reservoir)

Question 8

How does polio act in humans?

Answer 8

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

Question 9

How does poliovirus infection present

Answer 9

Most infections are subclinical, but three types of disease can result:

1. abortive poliomyelitis (4-8%)
2. aspectic meningitis (nonparalytic poliomyelitis; 1-2%)
3. paralytic poliomyelitis (0.1-1%)

Question 10

Describe abortive poliomyelitis

Answer 10

a. nonspecific febrile illness of 2-3 days
b. no involvement of CNS

Question 11

Describe aseptic meningitis (nonparalytic meningitis)

Answer 11

Febrile illness of 2-3 days plus stiff neck and a CNS infection (but limited)

Question 12

Describe paralytic poliomyelitis

Answer 12

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

Question 13

Who mostly gets polio infections?

Answer 13

children under 5 yo

1. adult infections are often the result of improved sanitation which delays exposure to the virus
2. risk of paralysis increases with age

Question 14

How is polio diagnosed?

Answer 14

Specimens for virus isolation are taken from stools, rectal swabs, throat swabs and cerebrospinal fluid, depending upon the stage of infection

žNeutralization assays are performed to confirm polio

Vaccine strains can be distinguished from wild poliovirus using strain-specific polyclonal sera (specialized labs only)

Question 15

What vaccines are available for polio?

Answer 15

Salk inactivated polio vaccine (IPV)

Sabin live oral polio vaccine (OPV)

žSince 2000, only IPV has been recommended for US vaccinations

Question 16

The polio žVirus is best inactivated with:

Answer 16

formaldehyde or chlorine

žVirus is resistant to detergents and ethanol

Question 17

Describe the Salk IPV

Answer 17

žLicensed in 1955

žAdvantage: formalin-inactivation prevents reversion to virulence

žDisadvantages: Four subcutaneous injections; Protects against paralysis but not spread of wild virus

žDo not use if allergic to neomycin, streptomycin, or polymyxin B

Question 18

Describe the Sabin live OPV

Answer 18

žLicensed in 1963

žAdvantages: Cheap; 8 cents/dose; Oral administration (three doses); Protects against paralysis and spread of wild virus and all 3 serotypes

žDisadvantage: can revert to virulence during preparation or during shedding by recipients since it is liver

Question 19

žIn 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?

Question 20

WHO’s Polio Eradication Program had targeted global eradication by 2000

Answer 20

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