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About viruses and viral disease
Amy returns to TWiV to discuss her work on the identification of cross-reactive antibody responses among diverse enteroviruses, and the implications for our understanding of viral pathogenesis and seroprevalence studies.
Hosts: Vincent Racaniello, Dickson Despommier, Rich Condit Brianne Barker, and Amy Rosenfeld
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The Centers for Disease Control and Prevention thinks that viruses play a role in the childhood paralysis called acute flaccid myelitis (AFM). The finding of antibodies to enterovirus D68 (EV-D68) in the cerebrospinal fluid of patients with AFM strengthens the link between infection with this virus and AFM.
Acute flaccid myelitis (AFM), which mainly occurs in children, involves weakness of the arms and legs and may include other symptoms such as inability to breathe or swallow. There have been outbreaks of AFM in the US in 2014, 2016, and 2018, mainly in the late summer and early fall. The outbreaks of AFM are associated with infection with enteroviruses.
AFM was first defined in 2014 after reports of limb weakness in children across the US during an outbreak of respiratory disease caused by EV-D68 (pictured). AFM may also be associated with infections caused by other viruses, such as enterovirus A71, Coxsackievirus A16, West Nile virus and adenovirus.
Of the 233 patients with confirmed AFM in 2018, EV-D68 was the most frequently detected virus, mainly in respiratory samples. Confirming a viral etiological agent of AFM would be greatly strengthened by finding virus in the cerebrospinal fluid (CSF). However in 2018 only two samples of cerebrospinal fluid from AFM patients were found to be positive for viral RNA: one for EV-D68 and one for EV-A71. There may be multiple reasons for the failure to detect viral RNA in CSF, including clearance of virus by the time AFM is diagnosed, or very low levels of viral genomes.
An alternative to searching for viral genomes in CSF is to identify anti-viral antibodies, which may be produced during infection of the brain and spinal cord. Peptide microarrays were used to detect antibodies against enteroviruses in CSF of AFM patients. In this method, 160,000 unique peptides from the capsid proteins of all known human enteroviruses were spotted on slides. CSF samples were incubated with the slides and binding of antibodies to peptides was quantified. The results revealed antibodies to an 18 amino acid peptide of capsid protein VP1 in serum and CSF of 11/14 AFM patients and 3/26 controls. This amino acid sequence is known to be conserved among diverse human enteroviruses.
More importantly, antibodies to a 22 amino acid sequence in VP1 of EV-D68 were identified in CSF of 6/14 AFM, and in 8/11 serum samples. No immunoreactivity to this peptide was detected in samples from 26 controls.
Detection of antibodies in the CSF is not diagnostic of CNS infection because antibodies are known to pass from blood to CSF. The normal ratio of antibodies in blood to CSF is 250:1. Consequently, for diagnostic purposes, both serum and CSF should be analyzed for the presence of antibodies.
The authors of the study acknowledge this issue, noting that they do not have sufficient simultaneously collected serum and CSF samples to exclude that antibodies they detected in the CSF simply came from the blood. They note that of the 16 control patients with serum antibodies to enteroviruses, only 3 also had such antibodies in CSF. Furthermore, in a study of unexplained encephalitis in India, of 77 children with serum antibodies to enteroviruses, only 3 also had EV antibodies in CSF. For these reasons the authors do not believe that the anti-EV-D68 antibodies they have detected in CSF have come from the blood.
How can it be definitively proven that EV-D68 causes AFM? With an outbreak of AFM expected in the late summer of 2020, there should be ample opportunity to obtain numerous clinical samples – nasopharyngeal wash, serum and CSF – to examine for the presence of EV-D68 antibodies and viral genomes.
On episode #367 of the science show This Week in Virology, two Coynes join the TWiV overlords to explain their three-dimensional cell culture model of polarized intestinal for studying enterovirus infection.
You can find TWiV #367 at www.microbe.tv/twiv.
On episode #331 of the science show This Week in Virology, the TWiVÂ team discusses the possible association of the respiratory pathogen enterovirus D68 with neurological disease.
You can find TWiV #331 at www.microbe.tv/twiv.
On episode #328 of the science show This Week in Virology, the TWiVocateurs discuss how the RNA polymerase of enteroviruses binds a component of the splicing machinery and inhibits mRNA processing.
You can find TWiV #328 at www.microbe.tv/twiv.
An outbreak of respiratory disease caused by enterovirus D68 began in August of this year with clusters of cases in Missouri and Illinois. Since then 691 infections have been confirmed in 46 states in the US.
The number of confirmed infections is likely to increase in the coming weeks, as CDC has developed a more rapid diagnostic test. Previously it was necessary to amplify the viral genome by polymerase chain reaction, followed by nucleotide sequencing to determine the identity of the agent. The new test utilizes real time, reverse transcription PCR which is specific for the EV-D68 strains that have been circulating this summer.
Since its discovery in California in 1962, EV-D68 has been rarely reported in the United States (there were 26 isolations from 1970-2005). Beginning in 2009 it was more frequently linked to respiratory disease outbreaks in North America, Europe, Asia, and Africa. It seems likely that the virus was always circulating, but we never specifically looked for it.
The current EV-D68 outbreak is the largest ever reported in North America. Enterovirus infections are not rare – there are millions every year in the US – but why EV-D68 has been so frequently isolated this year is unknown. One possibility is that the CDC, after the initial outbreak in August 2014, began looking specifically for the virus.
Sequence analysis of the EV-D68 viral genomes indicate that 3 different strains are involved in the US outbreak. These viruses are related to EV-D68 strains that have previously circulated in the US, Europe, and Asia. The sequences are available at GenBank as follows: US-IL-14/18952, US-KY-14/18951, US-MO-14/18950, US-MO-14/18949, US-MO-14/18948, US-MO-14/18947, and US-MO-14/18946.
Most of the illness caused by EV-D68 in the US has been respiratory disease, mainly in children. Five of the 691 confirmed EV-D68 cases were fatal, but whether the virus was responsible is not known.
There have also been some cases of polio-like illness in children in several states associated with EV-D68. In Colorado the virus was isolated from four of 10 children with partial paralysis and limb weakness. Previously there had been one report of an association of EV-D68 with central nervous system disease. In this case viral nucleic acids were detected in cerebrospinal fluid. EV-D68 probably does not replicate in the human intestinal tract because the virus is inactivated by low pH. If the virus does enter the central nervous system, it may do so after first replicating in the respiratory tract, and then entering the bloodstream.
There are no vaccines or antivirals to prevent or treat EV-D68 infection. Most infections will resolve without intervention save for assistance with breathing. As the fall ends in North America, so will infections with this seasonal virus.