Poliovirus is spread by the "fecal-oral" route, which, despite its
unsavory name, is a common route of microbial infection. The virus can
be isolated from human feces and sewage. In areas where raw sewage enters
a watershed without treatment, polio can be found in rivers, lakes, and
streams. When a susceptible person drinks water from one of these sources
(possibly from the kitchen tap when local water supplies are not treated
properly), the virus enters his digestive tract.
After surviving the harsh, acidic conditions of the stomach, the virus
infects
the cells lining the intestine (the "gut mucosa"). Each round of replication
produces thousands of new virus particles, or virions, which are then carried
through the intestine and released into the sewage system to start the
cycle over again. In addition to untreated drinking water, the virus appears
to spread through contact, especially among children, whose hands are often
contaminated.
Polio infection is frequently accompanied
only by minor symptoms. In some cases, though, the virus enters the central
nervous system after replicating in the gut and bloodstream, and this can
result in paralysis of one or more limbs, or death. Even though many infections
are asymptomatic, the efficiency with which polio is transmitted under
the right conditions can lead to epidemics of infantile paralysis, such
as those seen in American cities in the first half of the 20th Century.
If a large enough portion of the population of a city is exposed to polio,
even the small percentage of infections which lead to paralysis will produce
large numbers of casualties, making it a major concern for public health
authorities. Epidemics of paralytic poliomyelitis still occur in some lesser-developed
countries today, but the World Health Organization
(WHO) has embarked on a campaign to eradicate
polio by the year 2000.
Despite its long history, polio has had
its most noticeable effect on humanity within the past century. Epidemics
of poliomyelitis have been characterized as a "disease of development,"
meaning that, ironically, major outbreaks seem to accompany an improvement
in sanitation and living conditions. In highly unsanitary circumstances,
virtually all children are exposed to the virus during infancy, when infection
with polio is most likely to be asymptomatic, and these babies then acquire
lifelong immunity to the disease. When older children or adults are infected,
they are more likely to be paralyzed or killed by the virus. As a society
improves its sanitation (a transition which helps eliminate a number of
other diseases), individuals are likely to be exposed to polio later in
life, if at all, so the paralytic disease starts to occur in sporadic epidemics.
Because the wealthy are the first to benefit from improved sanitation,
they are often the first to experience these epidemics. Franklin Roosevelt,
perhaps the most famous polio survivor,
was a victim of this phenomenon.
Before the 1950s, there was no effective treatment for poliomyelitis,
and ignorance about its route of transmission often hampered attempts to
control it. As part of the "War on Polio," researchers
Albert Sabin and Jonas Salk, taking different approaches, developed effective
vaccines against the virus, and a widespread immunization campaign rapidly
brought it under control in developed countries. The Salk vaccine is an
injection of chemically killed virus, which "teaches" the immune system
to recognize the virus and eliminate it. This vaccine confers lasting,
but not always life-long, immunity. Sabin's vaccine is given orally, and
contains attenuated, live viruses of each of the three polio serotypes.
The attenuated virus replicates in the patient's intestinal tract and induces
immunity, but is not virulent enough to cause paralysis (except
in rare cases).
| Salk Inactivated Polio Vaccine (IPV) | Sabin Oral Polio Vaccine (OPV) |
|---|---|
| Delivered by injection, requires trained personnel. | Delivered orally, does not require extensive medical training. |
| Confers immunity, but the patient can still act as a carrier. | Confers life-long immunity and prevents the patient from acting as a carrier. |
| No risk of vaccine-associated paralysis | Slight risk of vaccine-associated paralysis. |
| Vaccinates only the patient who receives it. | May be transmitted to others for "secondary vaccination". |
Table 1: Comparison of Polio Vaccines
The live vaccine, in addition to confering life-long immunity, has other advantages over the Salk injection (see Table 1). Because of the danger of vaccine-associated paralysis, the CDC has recently changed the protocol for childhood vaccinations to use a combination of the two vaccines. In other parts of the world, where money for health care is scarce, the Sabin vaccine is preferred, and it is this vaccine which has made the eradication of polio a realistic goal.