Cases of acute hepatitis have been reported in countries around Europe, following a mysterious spike in cases of severe liver disease in children in the UK.
The UK’s public health agencies have been investigating 74 cases of hepatitis - an illness causing liver inflammation - in children since January.
Health experts are trying to determine the causes of the cases, as the usual viruses causing the illness were not detected.
The World Health Organization (WHO) and the EU’s health agency issued warnings to other countries to be on the alert and asked them to report any cases of unknown origin.
The European Centre for Disease Prevention and Control (ECDC) said on Wednesday that additional cases of hepatitis of unknown origin have been reported in Denmark, Ireland, the Netherlands, and Spain.
There have also been nine cases in children between the ages of one and six in the state of Alabama in the US, who also tested positive for adenovirus.
What is hepatitis?
Hepatitis is inflammation of the liver, which is a vital organ for processing nutrients, filtering blood, and fighting infection.
Inflammation can affect the liver’s function, and the illness can vary in severity depending on the cause.
While some types of hepatitis are mild and don't require treatment, other forms of the disease can become chronic and be fatal.
Hepatitis is often contracted through the hepatitis virus, of which there are five main types: A, B, C, D, and E.
Unexplained cause behind the spike in hepatitis cases
The UK’s Health Security Agency (UKHSA) said on April 12 that it was investigating the cause of the disease, which was in most cases affecting children aged two to five.
Some of the children had suffered acute liver failure and a "small number have required liver transplantation".
What is puzzling authorities is that evidence of the main hepatitis viruses were not found in the UK cases, and scientists and doctors are now considering other possible causes, including COVID-19, other viral infections and environmental factors.
The UKHSA said "there is no link to the COVID-19 vaccine," adding that none of the confirmed cases in the UK have been vaccinated.
It said one of a number of potential causes is adenoviruses.
"Adenoviruses are a family of common viruses that usually cause a range of mild illnesses and most people recover without complications," it said.
ECDC said investigations are ongoing in all countries reporting cases, and “at present, the exact cause of hepatitis in these children remains unknown”.
"Toxicological investigations are ongoing but an infectious etiology is considered more likely given the epidemiological picture and the clinical features of the cases," the agency said.