Experts are still mystified by hundreds of young children around the world being hit by severe liver damage.

There are a number of theories about what has caused the hepatitis cases and some believe the most likely explanation lies with a fairly common stomach bug – despite it not being known to cause liver problems in otherwise healthy youngsters.

Yet though that illness, known as adenovirus 41, was detected in the children’s blood, it has not been found in their diseased livers.

Eric Kremer, a virus researcher at France’s Institute of Molecular Genetics of Montpellier, said: ‘There’s a lot of things that don’t make sense.’

With health officials in at least 12 countries looking into the perplexing evidence, the main questions they are said to be asking are these:

• Has adenovirus 41 been surging and therefore caused more cases of a previously undetected problem?
• Are children more susceptible due to lockdowns that sheltered them from the viruses kids their age usually experience?
• Is there some mutated version of the adenovirus causing this? Or some other not-yet-identified germ, drug or toxin?
• Is some kind of haywire immune system reaction, perhaps caused by a past Covid-19 infection, impacting their response to the virus?

The US Centres for Disease Control and Prevention (CDC) is among those investigating the illnesses.

Officials last week said they are now looking into 180 possible cases across the US.

Most of the children were hospitalised; at least 15 required liver transplants and six died.

More than 20 other countries have reported hundreds more cases in total, though most cases have been in the UK and US.

Symptoms of hepatitis – or inflammation of the liver – include fever, fatigue, loss of appetite, nausea, vomiting, abdominal pain, dark urine, light-coloured stools, joint pain and jaundice.

The scale of the problem became clear last month – though disease detectives have been working on the mystery for months.

What are the symptoms of hepatitis in children?

Symptoms vary from child to child, but there are some key telltale signs to look out for.

They are as follows:

• Flu-like symptoms
• Yellowing of the skin or the whites of the eyes (jaundice)
• Fever
• Nausea or vomiting
• Loss of appetite
• Stomach pain or discomfort
• Diarrhoea
• Joint pain
• Sore muscles
• Itchy red hives on the skin
• Clay-coloured stools
• Dark-coloured urine

You should visit your GP if your child has any persistent or troublesome symptoms that you think could be caused by hepatitis.

Read more about hepatitis in children here.

Conventional causes of liver inflammation in otherwise healthy children – the viruses known as hepatitis A, B, C, D and E – did not show up in tests.

The children came from different places and did not seem to have common exposures.

But adenovirus 41 did frequently show up.

More than half of the US cases have tested positive for adenovirus, and in the small number of cases tested to see which kind was present, number 41 came up every time.

Dr Jay Butler, the CDC’s deputy director for infectious diseases, suggested that adenovirus’ repeated presence suggested it is playing a role but it is unclear what.

The viruses have previously been linked to hepatitis in children, but mostly in those with weakened immune systems.

Dr Umesh Parashar, chief of the CDC group focused on viral gut diseases, suggested recent genetic analysis has shown no evidence that a single new mutant version of the virus is to blame.

Adenovirus infections are not systematically tracked in the US, so it is unclear if there has been a new wave of infections.

But adenoviruses are so common that researchers are split on their presence.

‘If we start testing everybody for the adenovirus, they will find so many kids’ that have it, said Dr Heli Bhatt, a paediatric gastroenterologist who treated two Minnesota children with the liver problems.

She added that not finding a cause is quite common, with experts estimating that roughly a third of acute liver failure cases are unexplained.

Many of the cases recently added to the total were earlier ones that were re-evaluated, with about 10% of the US cases occurring in May, according to Dr Butler.

Other doctors believe the phenomenon may in fact have been going on for years.

Covid-19 vaccination has been ruled out because ‘the vast majority of these children are unvaccinated’, Dr Butler explained.

But past infection with the coronavirus itself might be factor, scientists say.

However, only 10% to 15% of the children with the mysterious hepatitis had Covid-19, health officials say.

Yet it remains possible that coronavirus particles lurking in the gut could be playing a role, Petter Brodin, a paediatric immunologist at Imperial College London, said.

In a piece earlier this month in the medical journal Lancet, Mr Brodin and another scientist suggested that a combination of lingering coronavirus and an adenovirus infection could trigger a liver-damaging immune system reaction.

‘I think it’s an unfortunate combination of circumstances that could explain this,’ he said.

Dr Markus Buchfellner, a paediatric infectious diseases doctor at the University of Alabama at Birmingham, was involved in the identification of the first US cases in the autumn.

He branded the illnesses weird and concerning.

But six months later, ‘we don’t really know exactly what we’re dealing with’, he added.

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