Japan reports first case of mysterious children's liver disease as health experts explore possible Covid links

Japan has detected its first probable case of a mysterious liver disease that has so far affected over 170 children, largely in Britain, as health experts explore its possible links to Covid-19.

Japan's Health Ministry said Tuesday that a child had been hospitalized with an unidentified type of severe acute hepatitis — or liver inflammation — in what is thought to be the first reported case in Asia.

As of April 23, at least 169 cases of the disease have been detected in 11 countries globally, according to the World Health Organization. The vast majority of those have been in the U.K. (114), followed by Spain (13), Israel (12) and the U.S. (9). The addition of Japan marks the 12th country to identify a case.

Of those infected, one child has died and 17 have required liver transplants.

The WHO said it is "very likely more cases will be detected before the cause can be confirmed."

Children aged five years old or younger have so far been the most widely affected by the disease, though cases have been detected in children aged one month to 16 years.

Common symptoms including gastroenteritis — diarrhea and nausea — followed by jaundice or yellowing of the skin and eyes.

Health experts are now investigating the likely cause of the outbreak, which was first reported in the U.K. in January 2022, and whether it bears any connection to the coronavirus.

Specifically, they are exploring if a lack of prior exposure to common viruses known as adenoviruses during coronavirus restrictions, or a previous infection with Covid-19, may be related. Alternatively, the genetic make-up of hepatitis may have mutated, resulting in an easier triggering of liver inflammation.

Crucially, experts say there is no known link to the Covid-19 vaccine.

A strain of adenovirus called F41 is so far looking like the most probable cause, according to the U.K. Health Security Agency.

"Information gathered through our investigations increasingly suggests that this rise in sudden onset hepatitis in children is linked to adenovirus infection. However, we are thoroughly investigating other potential causes," Meera Chand, UKHSA's director of clinical and emerging infections, said.

Adenovirus was the most common pathogen detected in 40 of 53 (75%) of confirmed cases tested in the U.K. Globally, that number was 74.

Covid (SARS-CoV-2) was identified in 20 cases of those tested globally. Adenovirus and Covid-19 co-infection was detected in 19 cases.

The new case from Japan tested negative for adenovirus and the coronavirus, though officials have not revealed other details.

Typically, children gain exposure — and immunity — to adenoviruses and other common illnesses during their early childhood years. However, pandemic restrictions largely limited that early exposure, leading to more serious immune responses in some.

Adenoviruses, which present cold-like symptoms such as fever and sore throat, are generally mild. However, some strains can display liver tropism, or a favoring of liver tissue, which can lead to more serious consequences like liver damage.

Just how serious this latest outbreak will be is not yet clear and will depend largely on how much it spreads over the coming months, according Dr. Amy Edwards, an assistant professor of pediatrics at the Case Western Reserve School of Medicine.

"Adenovirus is a ubiquitous virus and it's not seasonal. If this is a more severe form of adenovirus that causes liver disease in children, that's very concerning. But right now it's isolated enough and few enough cases not to jump to conclusions," she told CNBC.

Edwards said health authorities had been placed on alert and would be monitoring the situation.

In the meantime, parents and guardians should be alert to common signs of hepatitis, including jaundice, dark urine, itchy skin and stomach pain, and contact a health care professional if they are concerned.

"Normal hygiene measures such as thorough handwashing (including supervising children) and good thorough respiratory hygiene, help to reduce the spread of many common infections, including adenovirus," UKHSA's Chand said.

"Children experiencing symptoms of a gastrointestinal infection including vomiting and diarrhoea should stay at home and not return to school or nursery until 48 hours after the symptoms have stopped," she added.