TOKYO (BLOOMBERG) - An outbreak of acute hepatitis in children has killed one and required liver transplants in at least 17 others across the globe, according to the World Health Organisation (WHO).

While the cause has yet to be determined, investigators are studying an adenovirus from a family of viruses that cause a range of illnesses, including the common cold.

1. When was it first reported and how many have been affected so far?

The first US cases were identified at an Alabama hospital in October 2021, when five children were admitted with liver damage from an unknown cause.

The WHO was notified on April 5 about 10 cases in previously healthy children in Scotland. Three days later, 74 cases had been identified in Britain.

Most of the 169 cases have been detected in Britain - at 114 as of April 21 - followed by 13 in Spain, 12 in Israel, nine in the US and 21 more scattered among Denmark, Ireland, the Netherlands, Italy, Norway, France, Romania and Belgium, according to the WHO.

On April 25, Japan's health ministry said it found one probable case, raising concerns the disease is spreading outside of Europe and the US.

With more extensive searching, it is "very likely that more cases will be detected before the cause can be confirmed and more specific control and prevention measures can be implemented", the WHO said in a statement.

2. What are the common symptoms?

Abdominal pain, diarrhea and vomiting are followed by jaundice, when the skin or the whites of the eyes turn yellow. Laboratory tests show signs of severe liver inflammation, with markedly high liver enzyme readings.

Most of the children did not have a fever.

Other symptoms of hepatitis include fatigue, loss of appetite, dark urine, light-coloured stools and joint pain.

The affected children were one month to 16 years old, with many at age 10 and under.

About 10 per cent, or 17 children, needed a liver transplant. At least one death has been reported, the WHO said.

3. What's causing the disease?

The cause is not yet known. Health authorities and experts are investigating potential triggers, including infection with a known adenovirus that has been detected in 74 of the children.

Some were also infected with Covid-19, though the role of the viruses are not clear. Common pathogens that cause acute viral hepatitis, including hepatitis viruses A, B, C, D and E, have not been found in any of the cases, according to the WHO.

No other risk factors have been identified, including links to international travel.

Additional testing for other infections, chemicals and toxins is underway in the affected countries, which have also initiated enhanced surveillance activities.

It is possible that the severe hepatitis is an existing, though rare, result of an adenovirus infection that is being detected more often now thanks to enhanced testing, the WHO said.

Adenovirus infections have been on the rise recently after falling to low levels during the Covid-19 pandemic, potentially making young children more susceptible.

The potential emergence of a novel adenovirus must also be investigated, the WHO said.

4. What's an adenovirus?

Adenoviruses are common viruses that cause a range of illness, including cold-like symptoms, fever, sore throat, bronchitis, pneumonia and diarrhea, according to the US Centres for Disease Control and Prevention.

There are more than 50 types of adenoviruses that can cause infections in humans, according to the WHO. While they most commonly cause respiratory symptoms, they can also lead to gastroenteritis, conjunctivitis and bladder infections.

The WHO said adenovirus was detected in more than 40 per cent of cases. Among the samples that underwent molecular testing, 18 were identified as adenovirus F type 41.

The findings are perplexing, however, since adenoviruses normally resolve on their own and do not cause the severity of disease seen in the children.

Adenovirus type 41 typically causes diarrhea, vomiting, and fever, often accompanied by respiratory symptoms. It is not known to cause hepatitis in healthy children.