Enterovirus D68 and acute flaccid myelitis-evaluating the evidence for causality

Abstract

Increased circulation of enterovirus D68 in 2014 and 2016 temporally and geographically coincided with increases in cases of acute flaccid myelitis, an uncommon condition of paralysis due to lesions in the anterior horn of the spinal cord. The identification of enterovirus D68 in respiratory specimens from cases of acute flaccid myelitis worldwide further supports an association, yet the absence of direct virus isolation from affected tissues, infrequent detection in cerebrospinal fluid, and the absence, until recently, of an animal model has left the causal nature of the relationship unproven. In this Personal View we evaluate epidemiological and biological evidence linking enterovirus D68 and acute flaccid myelitis. We applied the Bradford Hill criteria to investigate the evidence for a causal relationship and highlight the importance of comprehensive surveillance and research to further characterise the role of enterovirus D68 in acute flaccid myelitis and pursue effective therapies and prevention strategies.

Figure 1:. Acute flaccid myelitis cases and enterovirus D68 circulation in the USA from 2014 to 2016

Shaded areas represent periods in which enterovirus D68 circulation was identified in the USA during 2014 to 2016, although absence of active surveillance precludes quantification of prevalence.^,– Bars represent the monthly number of confirmed acute flaccid myelitis cases in the USA reported to the Centers for Disease Control and Prevention with onset from August, 2014, to December, 2016 (adapted from Centers for Disease Control and Prevention).

Figure 2:. Experimental mouse model of enterovirus D68 paralytic disease

Several 2014 strains of enterovirus D68 caused permanent paralysis in neonatal mice by intracerebral (ic) inoculation. One strain tested in further detail, US/MO/14–18947, caused paralysis by multiple routes of inoculation, in inverse order of disease frequency: intramuscular (im, 100%), intracerebral (ic, about 50%), intraperitoneal (ip, about 5%), and intranasal (in, about 3%). During the course of enterovirus D68 infection, whole spinal cords were removed from paralysed animals. (A) Infected spinal cords inoculated into cell culture resulted in cytopathic effect. Media from cell culture inoculated into naive mice produced paralytic disease, as per Koch’s postulates. (B) Spinal cords taken over the course of infection showed enterovirus D68 VP2 protein within motor neurons of the anterior horn and subsequent loss of infected neurons. (C) Cell culture assays of affected spinal cords demonstrated presence of infectious virus. Spinal cords tested by qRT-PCR and metagenomic deep sequencing confirmed the presence of enterovirus D68.