One child has died of a new form of acute hepatitis, after reports of a mystery strain have emerged from around the world.

The World Health Organisation (WHO) said on Saturday there have been at least 169 reported cases among children across 12 countries.

Britain had the most infections with 114 cases. Spain reported 13 cases, with Israel and the United States reporting 12 and nine respectively. Denmark, Ireland, the Netherlands, Italy, Norway, France, Romania and Belgium also had cases.

WHO said all reported cases occurred in children ranging in age from 1 month to 16 years. Seventeen of the children infected had to have a liver transplant.

The cause and origin of the acute hepatitis are as yet unknown. Further confusing health professionals, none of the common hepatitis viruses A, B, C, D or E has been detected in any of the cases, suggesting an entirely new strain.

The United Nations agency reported many of the cases presented with symptoms including abdominal pain, urine and faecal discolouration, diarrhoea, vomiting and jaundice. Many of the children also had increased levels of liver enzymes, which caused severe liver inflammation.

While scientists are still unsure what is causing the virus, many theorise that an adenovirus may be behind the infections. Adenovirus is a very common virus that causes common health issues such as the common cold, croup and pinkeye. Usually most adenovirus infections tend to be mild.

WHO reported that adenovirus had been detected in at least 74 of the cases. Covid was also detected in 20 of the cases, with 19 having both adenovirus and Covid together.

Adenoviruses have been recorded to have caused hepatitis in immunocompromised children in the past, but never before in healthy children.

Public Health Scotland's director, Jim McMenamin, told Reuters that work was under way to establish if the severe hepatitis was the result of a mutated adenovirus, or even two viruses working "in tandem".

A molecular epidemiologist at Nottingham Trent University, Dr Conor Meehan, said it was possible the hepatitis could result from an interaction of both adenovirus and coronavirus infecting the same child.

"Alternatively, it could be caused by a totally different virus that hasn't been detected yet," he said.

Scientists have ruled out any connection between the acute hepatitis and the Covid-19 vaccine, saying the majority of children infected in Britain were not vaccinated.

While adenovirus is one hypothesis of the cause of the mystery hepatitis, WHO warns that it has not been previously linked to such clinical presentations.

If adenovirus is found to be the main cause, Meehan suggests there may be preventive measures parents and children can take.

"Adenoviruses spread through the air and via touch," he said.

"The main preventive measure is proper handwashing, by kids and adults alike, along with good respiratory hygiene, such as coughing into your elbow."

WHO recommends visiting a healthcare professional should children display symptoms.

Australia's Department of Health spokesperson told news.com.au that while there was no increase in cases of children hospitalised with unexplained acute hepatitis in the country, they were carefully monitoring the situation.

"Clinicians are being alerted to the issue and provided with advice through state and territory networks," the spokesperson said.

- Madeleine Bower, news.com.au