Researchers say they have strong new evidence that a virus is involved in a rare and puzzling polio-like condition that began affecting children in the U.S. about five years ago. The researchers hope their work will lead to a better test for the paralyzing condition, called acute flaccid myelitis (AFM), which has been diagnosed in more than 500 kids since 2014.
The scientists used an experimental method to pull evidence of viral infections from the spinal fluid of 42 AFM patients. This technique turned up antibodies specific to enteroviruses, a group of viruses that includes the poliovirus. They also include the leading suspect for causing AFM, an enterovirus called EV-D68. The test also found evidence of other viral infections.
It’s not definitive evidence that EV-D68 has caused most or all of the recent cases of AFM, said Dr. Michael Wilson, an assistant professor of neurology at the University of California, San Francisco, who helped lead the research team. But it’s a big step toward proving enteroviruses are the culprits.
“These results offer a roadmap for rapid development of enteroviral cerebrospinal fluid antibody assays to enable efficient clinical diagnosis of enterovirus-associated AFM in the future,” Wilson and colleagues wrote in a preliminary report published in the preprint journal Biorxiv. Wilson stresses that the report, which includes work from 17 different labs across the country and Canada, has not yet been subjected to peer review.
“This paper looked for and found immune responses to enterovirus in spinal fluid. Finding evidence of antibodies in spinal fluid in response to the virus is an important first step toward a diagnostic test for AFM and a path toward treatment,” the federal Centers for Disease Control and Prevention, which took part in the research, said in a statement to STAT. If one virus is found to be responsible, that could fuel arguments for developing a vaccine to prevent infection.
Doctors who have treated patients with AFM have long suspected that EV-D68 and perhaps other, similar viruses are causing the illness. But it’s been almost impossible to find the evidence they need to say so conclusively, because the condition appears after an infection has cleared up.
“By then the virus is long gone,” Wilson said in a telephone interview. “A lot of these kids have been sick for a week or two before they develop their neurological symptoms.”
Acute flaccid myelitis first started causing concern in 2014, when it was diagnosed in 120 patients in 34 states. Since then, the number of cases has risen in an every-other-year pattern. The CDC confirmed 22 cases in 2015, 149 cases in 2016, 35 cases in 2017 and 232 last year. So far this year, nine cases have been confirmed.
The pattern seemed to line up with outbreaks of enterovirus, especially EV-D68, which causes respiratory symptoms. Neurological complications are known to sometimes follow viral infections, but AFM specifically affects gray matter in the spinal cord. Patients, virtually all of them young children, suffer symptoms ranging from muscle weakness to widespread paralysis.
Because the spinal cord is affected, the CDC wanted to find evidence that the virus was invading tissue there. But tests only occasionally found this evidence.
“People were afraid that something was going on that we didn’t understand,” Wilson said.
Wilson, who investigates rare neuroinvasive conditions, is part of a network of physicians trying to get to the bottom of AFM. The CDC has also since set up a task force to find its cause.
What they needed was a test for past infections – one that would show the most recent infection, and perhaps show whether one single virus was responsible for the damage, or whether a co-infection was to blame.
Wilson’s team adapted a method developed by Harvard Medical School geneticist Stephen Elledge and colleagues, which uses bacteriophages — a type of virus that infects bacteria — to fish out antibodies from blood.
Bacteriophages are good for this work because they can be engineered to display various peptides on their surface, like lures on a fishing line. Wilson’s team created phages that carried pieces of all the viruses known to infect vertebrates, as well as viruses carried by mosquitoes. This created a test for multiple viral infections, using only a small amount of spinal fluid.
“You incubate it with a patient’s spinal fluid, and let the antibodies in the spinal fluid bind to the partner peptide,” Wilson said.
The researchers found evidence of many viral infections, but mostly enteroviruses. Comparison tests in spinal fluid taken from 58 children with other neurologic diseases did not turn up the same antibodies.
“These results provide further evidence for a causal role of non-polio enteroviruses in AFM,” they wrote.
Elledge praised the study. “It is a very good paper and demonstrates clearly and convincingly what others had some data for that were not conclusive, that AFM is likely to be caused by enteroviruses,” he said.
There’s a lot of work left to do, Wilson said. The team will have to establish thresholds for how much enterovirus antibody is needed to link it with the AFM symptoms. “You really only develop a robust antiviral response in the spinal fluid when you have an invasive viral infection,” Wilson said.
But now they have a new test to work with. “It was a pain to build,” Wilson said. “But now that it’s built, it’s very high throughput. We can run 200 to 300 samples at a time.”
That means they can use it to test for other neuroinvasive viruses, such as West Nile Virus and the varicella virus that causes chickenpox and shingles.