Polioviruses

Characteristics

picorno virus = small

polio has been around for a long time, but only began to be associated with paralysis in late 1800’s - why?

the risk of paralysis resulting from polio infection increases with age

before advent of sanitation: fecal-oral transmission meant that most people were exposed to the irus early, developed antibodies (became immune), and were spared paralysis because of their young age

sanitation led to less fecal-oral transmission and less early developed immunity Þ children got their first exposure to the virus through direct contact later in life when they were more susceptible to paralysis

two epidemiological patterns

(1) epidemic: summer/fall; industrialized countries; close contact transmission; older children; more paralysis; nasopharyngeal virus
(2) endemic; throughout year; developing countries; fecal-oral transmission; infants/young children; less paralysis

infection vs. disease:

90-95% of polio infections have no recognizable signs of disease
1-2% have significant disease (paralysis)
in families of patients with severe disease, rest of family is generally infected, but don’t have disease

provocation: certain things seem to make paralysis more likely

tonsillectomy or severe exercise during infection Þ more likely paralysis

receiving a shot in the butt Þ more likely paralysis in that leg

History of the vaccine

1909: crushed brainstems of infected monkeys could be used to isolate the virus

three types (I, II, III) were isolated; the types are not cross-immunizing
monkey spinal cord was not a good medium from which to develop a vaccine

1949: administration of pooled IgG provided protection from paralysis

infection of gut could occur but IgG could protect the nervous system (vaccine possible)
antibody is a surrogate for protection against paralysis

1949: poliovirus was demonstrated to be grown in tissue cultures (good medium)

1954: Salk vaccine was proven effective (inactivated virus, administered by injection)

still used in Sweden, The Netherlands
bad for immunocompromised patients

1960: Sabin: OPV (Oral Polio Vaccine); live attenuated virus

easier to administer, but higher risk of paralytic infection from the vaccine

1985: Pan American Health Organization declared campaign to eradicate polio from the Americas

no cases of wild polio (not caused by vaccine) have been isolated since 1991

1994: polio declared eradicated from the Americas (Indian subcontinent and western Africa are still problem areas)

herd immunity: enough people are vaccinated that transmission of the virus is interupted Þ eradication