WASHINGTON (AFP) - The US Centres for Disease Control and Prevention is investigating 109 childhood cases of a mysterious form of hepatitis, including five reported deaths, the agency said on Friday (May 6).

"Investigators both here and abroad and around the globe are working hard to determine the cause," said Jay Butler, deputy director for infectious diseases for the CDC.

Ninety per cent of the cases have involved hospitalisation, with 14 per cent requiring liver transplants. The majority fully recovered.

The CDC issued a health alert last week notifying doctors and public health authorities to be on the lookout for similar cases, and began examining case histories extending back to Oct 1, 2021.

More than half the cases tested positive for adenovirus 41 - a virus that is normally associated with gastroenteritis, but not hepatitis in otherwise healthy children.

"Because of the link to adenovirus, I would call that top of the list of viruses of interest," said Butler.

"But we don't know if it is adenovirus itself that is causing the cases, or is there an immune reaction to this particular strain of adenovirus."

Environmental factors are also being examined - such as the presence of animals in the house, as well as whether other pathogens, such as Covid-19, may have played a role.

Adenovirus cases might also be rebounding after Covid-19 lockdowns stopped the spread for a few years, or the adenovirus might have evolved into a newer, more dangerous strain.

But the CDC has said it doesn't believe Covid-19 vaccinations are to blame.

Nine cases in Alabama that were investigated in depth were among children with a median age of two years old, too young for Covid-19 vaccination.