The World Health Organization (WHO) confirmed Tuesday that 348 probable cases of hepatitis of unknown origin had been identified, amid studies into the potential role of adenovirus and Covid-19 infections.

The World Health Organization said the leading hypotheses remain those involving adenovirus. Adenoviruses are commonly spread by close personal contact, respiratory droplets, and surfaces. They are generally known to cause respiratory symptoms, conjunctivitis, or even digestive disorders.

Hepatitis cases have been reported in 20 countries, with 70 additional cases from a further 13 countries which are pending classification as tests await completion. Only six countries are reporting more than five cases, with more than 160 being reported in Britain.

Important progress with the further investigations

Philippa Easterbrook, from the WHO's global hepatitis program, told a press conference that "over the last week, there's been some important progress with the further investigations and some refinements of the working hypotheses." 

Easterbrook pointed out that Britain had been coordinating a comprehensive set of studies looking at the genetics of the children affected, their immune response, viruses, and further epidemiological studies.

WHO was first informed on April 5 of 10 unexplained hepatitis cases in Scotland, detected in children under the age of 10.

The US Centers for Disease Control and Prevention said Friday it was investigating 109 such cases, including five reported deaths.

70% of the cases tested positive for adenovirus

"At present, the leading hypotheses remain those which involve adenovirus -- with also still an important consideration about the role of Covid as well, either as a co-infection or a past infection," Easterbrook explained.

Further testing in the past week confirmed that around 70% of the cases tested positive for adenovirus, with sub-type 41 -- normally associated with gastroenteritis -- the prevalent sub-type, she added.

Testing has also shown that around 18% of cases actively tested positive for Covid-19.

Incidental infection or not?

The scientist said that within the week, there should be data from Britain on a case-control study comparing whether the detection rate of adenovirus differs from that in other hospitalized children.

"That will really help hone down whether adeno is just an incidental infection that's been detected, or there is a causal or likely causal link," Easterbrook underlined.

WHO refers to the outbreak of severe liver inflammations as acute hepatitis of unknown origin among young children. Three children in Indonesia have died from the disease. Some cases have caused liver failure and required transplants.