 |
 |
|
| Archive : Spring 2006 |
POLIO IS ALMOST GONE, BUT WILL IT
EVER BE?
Eradication plans have been in place
for decades // Two vaccines have saved millions of
lives // More countries become virus-free each year
// But several doors to infection remain wide
open.
Still a Scourge
 By Wendy Orent
|
|
|
S ome of these kids have been vaccinated
10 or 12 times,” says Olen
Kew, “and they still get paralytic
polio.” For 21 years, Kew,
chief of the molecular virology laboratory at the
Centers for Disease Control and Prevention in Atlanta, has been involved
with the global effort to eradicate polio. His voice cracks in frustration
as he recounts the virus’s continued elusiveness even as the campaign,
launched by the World Health Assembly in 1988, draws nearer its goal.
Eradicating poliomyelitis has been more difficult
and taken more time than almost anyone anticipated. Initially the World
Health Assembly set 2000 as the year of eradication. Now the best guess
is sometime in 2007. Wild (as opposed to vaccine-derived) poliovirus is
circulating in Nigeria, India, Pakistan and Afghanistan, in descending
order of known cases. The virus has spread from Nigeria and India to cause
renewed infection in formerly polio-free countries. Making eradication
all the more difficult, highly evolved oral vaccine-derived polioviruses
have been found in several other countries and, in 2005, even spread within
one community of unimmunized people in the United States.
Still, the campaign has made dramatic gains. When
it began, an estimated 350,000 to 500,000 children per year worldwide
were contracting acute flaccid paralysis, and many died. In 2005 there
were only about 1,900 cases. China, with more than a fifth of the world’s
population, has been polio-free since 1994. Bangladesh, Egypt and Niger,
where the disease had been widespread, have gone more than a year with
no cases. Type 2 poliovirus, one of three serotypes—related yet
distinct strains that don’t confer cross-immunity—was eradicated
in 1999. By most estimates, the quest to rid the planet of polio has saved
more than 9 million children.
Yet the poliovirus continues to circulate. And no
matter how intently scientists study this foe, many mysteries remain,
including why epidemic polio has become such a problem in the first place.
Though the virus has probably been with us for centuries or millennia—an
Egyptian carving dating as far back as 1580 B.C. shows a priest with a
withered leg and a dropped foot, suggestive of paralytic polio—there
are no records of epidemic paralytic disease before the nineteenth century.
And in 1952, only three years before Jonas E. Salk’s poliovirus
vaccine was approved, more than 57,000 children contracted polio in the
United States. Why then? And why does polio, an intestinal virus, produce
paralytic disease at all?
In his classic A History of Poliomyelitis, Yale epidemiologist
John R. Paul makes what has become a well-accepted case for the reason
polio reached epidemic proportions during the late nineteenth and early
twentieth centuries. Societies were once awash in poliovirus, Paul suggests,
and children were exposed as infants, while still protected by maternal
antibodies. But beginning in the 1800s, improvements in hygiene, water
quality and sewer systems reduced that exposure. As more children failed
to develop immunity, terrifying polio epidemics broke out in the United
States and Europe.
But if those outbreaks resulted from improved hygiene,
why does polio today occur in filthy, crowded locales with high birth
rates, poor hygiene and bad water quality? According to Paul’s hypothesis,
such conditions ought to leave infants exposed to poliovirus at a young,
less vulnerable age. So how can polio be associated at once with dirt
and cleanliness, with poor and good water systems?
One possibility could be the role of breastfeeding
in providing immunity. The maternal antibodies in breast milk lend some
protection against infection while it is in the gut, says Eckard Wimmer,
a polio virologist at the State University of New York in Stony Brook.
Breastfeeding diminishes the chance that the virus will leave a child’s
intestines and invade the nervous system. Adults who have been infected
with polio have high levels of antibodies, some of which will be passed
on through the placenta and the milk of a nursing mother.
Before the nineteenth century, children may well
have been exposed to constantly circulating poliovirus while still protected
by maternal antibodies—both after birth and through breastfeeding,
which often lasted two years or more. But exclusive breastfeeding rates
dropped drastically during the late nineteenth and early twentieth centuries,
bottoming out during the 1940s and ’50s in Western countries. The
same thing happened in the developing world, and despite some recent improvement,
rates remain low, according to Claire Hajaj of UNICEF. Though it’s
difficult to prove a link between the decline in breastfeeding and the
rise of epidemic polio—and the virus’s continued presence
in countries where breastfeeding is rare—the timing and what we
know about the disease suggest there could be some connection.
|
Next page | Pages: 1 2 3
Photo by Giocomo Pirozzi/UNICEF
|
|
|
|
