An international outbreak of severe hepatitis in young children has health officials worldwide seeking an explanation and scrambling to provide the best treatments.

The outbreak was first spotted in the United Kingdom, where 74 children have become ill so far this year. Cases have also been found in Spain, Ireland, Denmark and The Netherlands, according to the Associated Press, which said the Centers for Disease Control and Prevention is investigating nine cases in Alabama in children ages 1 to 6.

The World Health Organization is also investigating cases.

The cases are “mysterious” because none of the children tested positive for the common strains, which are hepatitis A, B, C and E.

Health experts are particularly interested in whether an adenovirus — particularly the adenovirus-41 strain — is responsible for the infections. That type of virus typically causes gut inflammation.

“Mild hepatitis is very common in children following a range of viral infections, but what is being seen at the moment is quite different,” Graham Cooke, a professor of infectious diseases at Imperial College London, told the wire service. He said some of the children in the United Kingdom have required specialized care and a few needed liver transplants.

Alabama health officials said none of the children who became ill with liver inflammation have underlying health conditions.

The liver is crucial to digestion and filters all the blood that leaves the stomach and intestine. It serves, too, as a giant filter that cleans toxins from the blood and breaks down nutrients for use. It also helps fight infection. Hepatitis can lead to jaundice, diarrhea and stomach pain. Liver failure can be serious, even deadly.

Health officials say the outbreak is not related to the pandemic or the COVID-19 vaccine. None of the sickened children have been vaccinated.

Instead, they think it might be caused by an adenovirus, which typically causes a cold. While many of the children have tested positive for adenovirus, “so far, the evidence is too thin to resolve the mystery, researchers and physicians say,” according to a Science.org report.

The outbreak was first noted in late March after Scottish investigators reported a highly unusual cluster of 3- to 5-year-old children admitted to the Royal Hospital for Children in Glasgow with signs of severe liver inflammation and no known cause.

Health officials also speculate that lockdown during the pandemic may have left very young children “immunologically naive,” which means they are more vulnerable because they haven’t been exposed to many viruses, which would normally help build a stronger immune system.