A UC San Francisco-led research team has detected the immunological
remnants of a common seasonal virus in spinal fluid from dozens of
patients diagnosed with acute flaccid myelitis (AFM)—a polio-like
illness that causes permanent, sometimes life-threatening paralysis in
young children. The findings provide the clearest evidence to date that
AFM is caused by an enterovirus (EV) that invades and impairs the
central nervous system.
The study was published October 21, 2019 in
Nature Medicine.
AFM, which begins with cold-like symptoms and progresses to limb
weakness and paralysis in a matter of days, was first documented in
2012. Since then, AFM outbreaks have occurred every other year, with
more than 500 confirmed cases recorded so far. But because scientists
have had trouble pinpointing a cause, AFM has been the subject of
contentious debate within the medical community.
Mounting evidence implicated EVs as the likely culprit—specifically
the so-called D68 and A71 strains of the virus. EV outbreaks are common
and normally cause nothing more severe than cold-like symptoms or the
rash-producing hand, foot and mouth disease.
Scientists started to notice, however, that EV outbreaks coincided
with spikes in AFM. They also found that respiratory samples from
children diagnosed with AFM often tested positive for EVs. Plus,
laboratory studies found that these strains caused paralysis in mice.
But many experts remained skeptical of the enterovirus hypothesis,
instead proposing that AFM is an autoimmune disorder or is caused by
some other, as-yet-undiscovered virus. These EV skeptics argued that
that the evidence linking the virus to AFM was circumstantial, because
the virus could not be found in 98 percent of AFM patients who had their
spinal fluid
tested. They maintained that until there was ample evidence of the
virus invading the human nervous system, the link between EVs and AFM
remained unproven.
“People were hung up on the fact that enteroviruses were rarely
detected in the cerebrospinal fluid of AFM patients. They wanted to know
how someone could get neurologic symptoms with no virus detectable in
their central nervous system,” said Michael Wilson, MD, associate
professor of neurology, member of the
UCSF Weill Institute for Neurosciences,
and senior author of the new study. “If we could detect something
specific to a virus in in the spinal fluid of AFM patients, we would
feel more secure claiming that the neurologic symptoms of the disease
are virally mediated.”
The group first searched for the virus directly in spinal fluid using advanced
deep sequencing
technologies,
but this sort of direct detection of the virus failed, as it had
previously. Therefore, to find evidence of the missing virus, Wilson and
his collaborators—researchers at the Chan Zuckerberg Biohub, the
Centers for Disease Control and Prevention, the California Department of
Public Health, the University of Colorado, Boston Children’s Hospital
and the University of Ottawa—used an enhanced version of a virus-hunting
tool called VirScan, first developed at Harvard Medical School in the
laboratory of Stephen J. Elledge, Ph.D.
VirScan, which is a customized version of a Nobel Prize-winning
technique called phage (rhymes with “beige”) display, allowed Wilson’s
team to probe the spinal fluid of AFM patients for signs of an immune
response against enterovirus and thousands of other viruses
simultaneously.
“When there’s an infection in the spinal cord, antibody-making immune
cells travel there and make more antibodies. We think finding
antibodies against enterovirus in the spinal fluid of AFM patients means
the virus really does go to the spinal cord. This helps us lay the
blame on these viruses,” said Ryan Schubert, MD, a clinical fellow in
UCSF’s Department of Neurology, a member of Wilson’s Lab, and lead
author of the new study.
The researchers created molecular libraries consisting of nearly
500,000 small chunks of every protein found in the over 3,000 viruses
known to infect vertebrates (including humans), as well as those that
infect mosquitoes and ticks (an effort to rule out disease transmission
through their bites). They then exposed these molecular libraries to
spinal fluid obtained from 42 children with AFM and, as a control, 58
who were diagnosed with other neurological diseases. Any chunks of viral
protein cross-reacting with any antibodies present in the spinal fluid
would provide evidence for a viral infection in the central nervous
system.
Antibodies against enterovirus were found in the spinal fluid of
nearly 70 percent of AFM patients; less than 7 percent of non-AFM
patients tested positive for these antibodies. Furthermore, because
spinal fluid from AFM patients did not contain antibodies against any
other virus, every other known virus could be eliminated as a possible
culprit. These results were confirmed using more conventional lab
techniques.
“The strength of this study is not just what was found, but also what
was not found,” said Joe DeRisi, Ph.D., professor of biochemistry and
biophysics at UCSF, co-president of the Chan Zuckerberg Biohub, and
co-author of the new study. “Enterovirus antibodies were the only ones
enriched in AFM patients. No other viral family showed elevated antibody
levels.”
Though the study provides the most robust evidence so far that
enteroviruses cause AFM, many questions around AFM and these viruses
remain unanswered. For example, though the AFM-causing enterovirus
strains—EV-D68 and EV-A71—were identified decades ago, they only
recently seemed to have gained the ability to cause paralysis, with the
D68 strain in particular responsible for the most severe cases of AFM.
“Presumably there are changes that are causing the virus to be more
neurovirulent, but no one knows for sure what they are,” Schubert said.
“Because the virus is found in such low amounts, if at all, it’s hard to
zero in on the differences between an A71 virus that causes routine
hand, foot, and mouth disease and one that causes AFM.”
Also, because enteroviruses are extremely common, scientists are
still trying to figure out why fewer than 1 percent of infected children
get AFM, and they’re also trying to understand why children are the
only ones affected. “We don’t know for sure why children get paralysis
and adults don’t,” Schubert said. “The thinking is that young children
have low immunity to the virus that increases as they get older, so we
see the most severe effects in children around the age of two. But more
work needs to be done to understand AFM.”
For study co-author Riley Bove, MD, answering these unresolved
questions is a deeply personal mission. Bove, an assistant professor of
neurology and member of the UCSF Weill Institute for Neurosciences, is
the mother of a child who was diagnosed with AFM.
In the summer of 2014, Bove’s entire family came down with what
seemed to be a severe cold. Everyone recovered except Bove’s then
four-year-old son. Just days after the onset of the cold-like symptoms,
he started experiencing difficulty breathing. Soon, he was paralyzed
from head to toe and had trouble breathing on his own.
Today, Bove’s son is a thriving nine-year-old, but she says the
physical and emotional effects of AFM will be with him the rest of his
life. “For every family with a child diagnosed with AFM, the long-term
consequences of the disease remain the top issue,” she said.
Bove hopes that the new study will lead to a scientific consensus
around enterovirus as the cause of AFM, since this a key step on the
road to improved diagnostics and the development of a vaccine for the
illness.
“Public health education is important, but it’s not enough to prevent AFM,” Bove said. “The
virus is too common to avoid. A vaccine is the only way to meaningfully prevent the disease.”
For now, there’s no way to prevent or treat AFM. But if it follows
the biennial pattern first established after the 2012 outbreak, AFM
cases may spike again next year.
“We’re all holding our breath for 2020,” Schubert said.
More information: Pan-viral serology implicates enteroviruses in acute flaccid myelitis,
Nature Medicine (2019).
DOI: 10.1038/s41591-019-0613-1 ,
https://nature.com/articles/s41591-019-0613-1
