Little known adeno-associated viruses may be key to unexplained hepatitis in kids, studies say

Scientists in the United Kingdom say they've found "strong circumstantial evidence" linking an obscure type of sidekick virus called adeno-associated virus to unexplained cases of liver damage in children around the world.

The research also suggests that affected children may share a genetic susceptibility, though it's not clear what role it plays.

Since October, over 1,000 children worldwide, more than 350 of them in the United States, have been diagnosed with hepatitis, or swelling of the liver, with no known cause. Most of the children are young -- under the age of 5. Many were healthy before they suddenly got sick. In the US, at least 20 have needed liver transplants, and 11 have died, according to the latest updates from the CDC.

About half have tested positive for adenovirus 41, a stomach bug that commonly gives kids vomiting, diarrhea and flu-like symptoms but has never been known to cause hepatitis.

Most of the children who tested positive had low levels of adenovirus in their blood and no adenovirus infection in their livers, leading many doctors to suspect another cause such as Covid-19, or a complicating factor like genes or an environmental exposure.

In two new studies, scientists say they may have found a smoking gun in adeno-associated virus 2, which is different from advenovirus 41.

Adeno-associated virus 2 was present in high amounts in liver tissue and blood in nearly all the children with unexplained hepatitis in the studies. And it was only rarely found in healthy kids, those with only adenovirus infections but no liver damage, or those who had liver damage with a clear cause.

STRONG LINK BETWEEN CASES

Adeno-associated viruses are tiny bits of DNA they can't copy themselves until they're in the presence of another virus, usually adenovirus or herpes. They belong to a family of viruses called dependoparvoviruses.

Virologists say they're kind of like remoras, the fish that attach themselves to sharks and eat the leftovers of their meals.

"A remora is not like the best analogy, but it's sort of like that. It's got to have a host virus," said Alex Greninger, assistant director of the clinical virology laboratory at the University of Washington.

Gary Ketner, a molecular microbiologist at Johns Hopkins University who specializes in the study of adenoviruses, describes them as "very small, DNA-containing viruses. They have just two genes. And by themselves when they infect cells, they're unable to grow because they don't encode all of the functions required for the replication. So they depend on a cells being infected at the same time with a helper virus, and the helper is usually adenovirus."

Because they're thought to be mostly inert on their own, adeno-associated viruses are being studied as vehicles for gene therapy.

Beyond that, scientists say, not much is known about them. Researchers say this finding has them scratching their heads.

"They are viruses that merit about one line in very small print in the largest textbooks, so I've never come across them as a cause of disease," said Will Irving, a virologist at the University of Nottingham.

DETAILED INVESTIGATIONS UNCOVER CLUES

In first study, led by researchers at University College London, scientists performed deep genome sequencing on liver tissue samples from five children who needed transplants because of unexplained hepatitis. They did the same for blood samples from other children with unexplained hepatitis who never became so ill that they needed a transplant.

The investigators then compared those results to testing on blood and liver samples from a variety of control groups: healthy kids, those who were hospitalized for unrelated reasons, kids who had good immune function and poor immune function both with and without adenovirus infections, and those with Covid-19 infections and hepatitis caused by other things. In total, there were 23 cases of unexplained hepatitis in the study, compared against 136 controls.

Without exception, the researchers found high levels of adeno-associated virus 2 in the the liver tissue children who needed liver transplants because of unexplained, sudden hepatitis. But it was only present in the liver tissue from one child in the control group.

In blood, adeno-associated virus 2 was present in 94% of samples from children with unexplained hepatitis and was found at high levels in 91% of samples.

In contrast, researchers found adeno-associated viruses in only 6% of children with normal immune function who were hospitalized for unrelated reasons and 31% of immunocompromised controls.

In the second study, led by researchers from the Glasgow Center for Virus Research, adeno-associated virus was detected in the blood and liver samples of all children with unexplained hepatitis but in none of samples from healthy children, those who had adenovirus infections without hepatitis or those with hepatitis from a known cause.

When adeno-associated viruses were present, herpes and adenoviruses -- which are needed for adeno-associated viruses to copy themselves -- were frequently found, too.

In a further analysis, researchers looked at changes to genes that make up the human leukocyte antigen, or HLA, complex. HLA genes code for markers on the surface of cells that help regulate immune function.

Nearly 90% of children with unexplained hepatitis had the same form of a change to their HLA genes, suggesting a genetic vulnerability affecting how their immune systems work. By comparison, this same change is found in only about 15.6% of Scottish blood donors.

The new studies were posted online as preprints, ahead of scrutiny by outside experts and publication in medical journals.

MORE STUDIES UNDERWAY

The researchers say their studies can't prove cause and effect. They aren't sure that adeno-associated viruses are directly damaging the liver. It could be that they're a marker for something else that's going on. At best, they write, this is "strong circumstantial evidence."

They note that many of the hepatitis cases followed the Omicron wave, which struck many countries in the winter and spring. Although some of the children tested positive for Covid-19 before or during their admissions for hepatitis, the researchers couldn't find any evidence of active coronavirus infection in the blood or liver of infected children. Instead, they say, the pandemic may have contributed to these cases indirectly, affecting children's immunity when people began mixing again after periods of relative isolation.

A larger study by the UK Health Security agency is in the works, and researchers say its results should help clarify the significance of the new findings.