ABOVE: Ribbon diagram of the protein coat of an adeno-associated virus © ISTOCK.COM, Yabusaka Design

The mysterious outbreak of acute pediatric hepatitis first recognized in April has now affected more than 1,000 children in at least 35 countries and continues to defy explanation. Numerous hypotheses have been offered, but a pair of preprints now suggests two viruses may work together to cause the disease, particularly in children with an immune-related genetic variant. The research has not yet been published in a peer-reviewed journal.

Alasdair Munro, a pediatric infectious disease researcher at University Hospital Southampton in the UK, is among several experts expressing excitement about the hypothesis on Twitter. “Looks like a pretty major breakthrough in the investigation of paediatric hepatitis with unknown cause,” he writes in a thread about the results. “Everything seems to fit, will be interesting to see if further examination can confirm this as the cause — hopefully putting to bed some of the debates,” he adds.

The two teams behind the preprints—one led by scientists at University College London’s Great Ormond Street Institute of Child Health and the other by researchers with the Medical Research Council at the University of Glasgow Centre for Virus Research—sequenced samples from children with acute hepatitis cases as well as from matched controls. In both studies, adeno-associated virus 2 (AAV2) was detected in cases that required transplants as well as the vast majority of hospitalized cases, but was detected in very few of the matched healthy controls or hepatitis cases with known causes.

AAV2 requires the assistance of another virus in order to replicate in human cells. Both teams found such viruses—either an adenovirus (predominately adenovirus 41) or human herpesvirus 6B—in nearly all the children with unexplained hepatitis. Each of these viruses has, on their own, previously been associated with the disease, but the new data “suggests that AdV 41 (or another helper virus) is necessary but not sufficient,” Angela Rasmussen, a virologist at the University of Saskatchewan who didn’t work on either preprint, tells STAT.

The Glasgow team also looked for potential causative factors in the children themselves, finding that 89 percent of the children with acute hepatitis—but only 16 percent of Scottish children in general—carry an allele of the HLA-DRB1 gene that was previously associated with autoimmune hepatitis.

image of tweet showing prevalence of HLA mutation in children with and without hepatitis
Emma Thomson (@emcat1) on Twitter

In a Twitter thread, University of Cambridge virologist Charlotte Houldcroft says that if the findings can be replicated in a larger cohort, “This would explain A. the timing of the surge in cases in children, B. why not all children develop this rare complication and C. why the risk [isn’t] evenly distributed around the world (because of both pathogen exposure factors AND differences in genetic background).”

That said, many questions remain. “I think the important things to keep in mind here are that this is correlation, not causation,” Rasmussen tells STAT. “More work will be required to establish this as the cause, including figuring out the mechanism.”