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1994-08-06
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Excerpted from SCIENCE, VOL. 264, 13 MAY 1994
POST-POLIO SYNDROME:
REMEMBRANCE OF VIRUSES PAST
By Richard Stone
Images of polio--FDR in his wheelchair, rows of coffin-like "iron
lungs"--have largely faded into history. Although the disease persists
in parts of Asia and Africa, it was virtually eradicated from the
A United States in the 1950s by vaccines. But the specter of polio
hasn't disappeared entirely from this country. Decades after suffering
varying degrees of paralysis, nearly a third of the 1.6 million polio
survivors have begun to develop puzzling ailments, such as fatigue,
muscle weakness and atrophy, and in some cases difficulty breathing.
Lumped together under the term post-polio syndrome (PPS), these
symptoms are now thought to afflict 500,000 people. But the cause of
PPS remains a mystery--and the intrigue is growing. Two weeks ago, at a
symposium sponsored by the New York Academy of Sciences,* three research
groups announced finding fragmentary genetic sequences resembling polio
virus in some PPS sufferers. That was startling, since most virologists
assumed the virus had disappeared from the bodies of polio survivors
long ago.
The discovery of the virus fragments suggests to researchers such as
virologist Helena Kopecka of the Pasteur Institute that PPS could be the
renewed assault of a lurking foe. But another camp is skeptical,
particularly since only fragments--and no live virus--have been
detected. The virus replicates quickly and bursts through the infected
cell wall, so "it's difficult to conceptualize it harboring itself in a
cell" for 40 years, says Marinos Dalakas, a neurologist at the National
Institute of Neurological Disorders and Stroke (NINDS) and co-chair of
the conference. Dalakas and other scientists suspect PPS results from
the weakening of neurons that were damaged in the original attack.
Polio is caused by three strains of polio-viruses and, on rare
occasions, related viruses such as Coxsackie B. These viruses, all
members of the enterovirus family, have RNA as genetic material and can
infect the central nervous system and attack nerves that regulate muscle
action. When former polio patients began reporting PPS symptoms in the
1980s, the thought of a renewed infection did occur to neurologists.
Unable to culture a live virus from these patients, however, scientists
discounted the notion.
But those skeptics got a surprise in 1991 when a team led by clinical
neurochemist Mohammad Sharief of the National Hospital for Neurology and
Neurosurgery in London found exactly the kind of immune response
produced by an active polio infection in PPS patients. Sharief's team
detected immunoglobin M (IGM) antibodies specific to poliovirus in 21 of
36 PPS patients, and none in 67 controls (New England Journal of
Medicine, 12 September 1991, p. 749).
Kopecka and her colleagues enlisted a molecular bloodhound to sniff
out the actual virus--the polymerase chain reaction (PCR). At the
Academy meeting, the Pasteur group reported that they had used PCR's
ability to amplify minute genetic sequences to detect "polio-like
sequences" of genomic viral RNA in the spinal fluid of five of eight PPS
patients. The match was identical--there were point mutations and other
rearrangements--but Kopecka says the sequences were similar enough to
poliovirus to "favor the persistence of poliovirus for several decades
in PPS patients." And the virus should be active, since enteroviruses
don't normally assume a latent form. Dalakas' lab has produced similar
findings.
It's also possible that a new infection, not a resurgent attack, is
behind PPS. Collaborating with Sharief, a team led by virologist Peter
Muir of the United Medical and Dental Schools of St. Thomas' Hospital in
London announced at the meeting that it had detected RNA that closely
resembles that of Coxsackie B virus in three of 24 PPS patients. They
could not find any such RNA in their control group, which included polio
patients. One possibility, says Muir, is that "people who had polio in
the past may be susceptible to infection from other enteroviruses." And
new infections, in turn, might trigger PPS.
But the cases for new or long-smoldering viral infections have large
holes. For one thing, scientists have been unable to culture any live
enterovirus from a PPS patient. For another, other researchers haven't
been able to detect the kind of strong antibody response that Sharief
saw in PPS patients. Burk Jubelt, a neurologist at the State University
of New York Health Science Center in Syracuse who failed to find a
strong antibody response in his study, told the meeting his work
"doesn't support the theory that poliovirus persistence is the cause of
PPS."
The poliovirus-like fragment, Dalakas says, might be due to the virus
mutating into a less aggressive form inside the neurons, which might
then gradually slough off non-infective viral fragments over the years.
In that case, Dalakas says, "the fragments may mean nothing" in relation
to PPS. Other scientists are even more skeptical of the PCR findings.
"PCR is notoriously fickle when it comes to detecting small amounts of
RNA," says Joseph Melnick, a virologist and authority on enteroviruses
at Baylor College of Medicine in Texas. Melnick argues that the
fragments may very well be false positives.
If Dalakas and Melnick are right, another factor must explain the
syndrome; Dalakas thinks it is nerve-cell attrition. According to this
theory, the initial viral attack kills a number of motor neurons and
weakens some of the surviving nerve cells. As the post-polio patient
ages, these damaged neurons increasingly lose their connections to
muscles, which stop responding. Several groups have seen this reduction
in nerve-muscle connections in PPS patients.
Until a live poliovirus shows up in PPS patients, most neurologists
favor attrition, says Johns Hopkins neurologist Richard Johnson, but he
says he's keeping an open mind until further work is done. And one
crucial experiment is underway: Researchers at NINDS and elsewhere are
using PCR to search for poliovirus directly in spinal-cord neurons from
dead PPS patients. If live polio-virus doesn't show up in these
neurons, then Melnick and other researchers say the virus will have been
banished from Western medicine--for the second time.
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*"The Post-Polio Syndrome: Advances in the pathogenesis and treatment,"
Bethesda, Maryland, 27 to 30 April.