January 2022, Doctors in Scotland have noticed a worrying trend: a spread of severe hepatitis cases among children aged one to five. The children developed gastrointestinal symptoms — abdominal pain, diarrhea, vomiting — followed by jaundice. Seeing this type of acute hepatitis (a broad term that basically describes inflammation of the liver) in previously healthy young children is very unusual and cause for concern.

By 5 April, Scottish health authorities had recorded 11 cases. Unable to determine their cause, they notified the World Health Organization, launching a global investigation for health authorities to find answers.

Across Europe – Denmark, France, the Netherlands, Ireland, Romania and Spain – as well as Israel and the US, cases were detected immediately. On April 12, the European Centre for Disease Prevention and Control instructed its hepatitis network to keep a close eye on more cases that fit the description. Since then, the number of cases has continued to grow. The UK has now reached 114 cases, with 10 children needing liver transplants. In total, at least 190 cases have been recorded in at least 12 countries. A child has died.

But experts are still not sure what caused these cases.

Hepatitis can be caused by exposure to toxins or drugs (eg, liver damage from an overdose of paracetamol). But toxicology screening did not reveal any plausible explanation.

So could a viral infection be the culprit? The acute form of viral hepatitis is usually caused by infection with one of five hepatitis viruses: A, B, C, D, and E. It’s not uncommon to have cases of hepatitis following infection with one of these viruses — but in children, it usually happens when they’re immunosuppressed. “It is unusual that it occurs so frequently in generally healthy children in such a short period of time,” said virologist Conor Bamford of Queen’s University Belfast. However, no tests have been carried out in any of the children. to hepatitis virus, so it can be ruled out.

But one common virus suspect does keep showing up in testing: adenovirus. This common family of viruses is one of the leading causes of the common cold, with around three-quarters of sick children in the UK testing positive for the virus. Specifically, adenovirus F41 has been singled out and detected in blood tests from multiple children, although other adenoviruses have also been detected. “The strongest evidence is adenovirus because it’s just one of the most consistent things they’ve ever seen,” Bamford said.

But what doesn’t add up is the fact that adenovirus infections are usually very mild. Although adenoviruses can cause hepatitis in rare cases, it is not known to them. One theory is that a mutated adenovirus may be circulating, which could explain why the reaction was worse than usual.

There’s another guess: “It’s actually the same virus we’ve always had, but it’s less likely to spread because of lockdowns and less human-to-human interaction,” Bamford said. The idea is that by reducing exposure to the virus throughout the pandemic, children don’t have the chance to bolster the immunity they would normally get by smearing each other’s germs on the playground — meaning they could develop more severe disease when they are eventually exposed . However, Some evidence from the UK showed that adenovirus never really stopped spreading during the pandemic, so it was unclear to what extent children were really exposed.