The number of probable cases of children with acute severe hepatitis in Japan has reached seven, the health ministry said Friday.

The tally includes the nation¡¯s first case reported April 25, as well as another two confirmed on April 28, and four more reported Friday. The patients are all 16 years old or younger and the ministry has withheld their exact age, location and sex for privacy reasons.

One patient has tested positive for the coronavirus, and one patient tested positive for adenovirus Type 1. The health ministry declined to disclose whether these are two separate people or one person who contracted both viruses. None of the seven has had a liver transplant, the ministry said.

The patients correspond to the World Health Organization¡¯s working definition of a ¡°probable case¡± of the acute, severe hepatitis. The probable cases are defined as hospitalized patients who have seen spikes in their liver enzyme levels, and for whom hepatitis viruses A, B, C, D and E have been ruled out as the cause.

, 169 cases of hepatitis of unknown origin, with one death, have been confirmed in 12 countries so far, with ages ranging from 1 month to 16 years old. Reported symptoms include abdominal pain, diarrhea, vomiting, jaundice and elevated liver enzyme levels.

Of the cases worldwide, adenovirus has been detected in 74 cases, but whether adenovirus, which has over 80 different types, is the cause of the disease is unknown. Most cases of young children with hepatitis had been detected in Europe, with small numbers also reported in the Americas, the Western Pacific region, which includes most of East Asia and Oceania, and Southeast Asia, according to the WHO.

Japanese health ministry officials said it is uncertain at this point whether the number of children afflicted with hepatitis is on the rise. The ministry on April 20 issued a request to municipal governments to report unusual cases of hepatitis they have come across. The seven cases reported so far cover children who were hospitalized between Oct. 1, 2021, and Thursday.

Adenovirus is a common virus that can cause infectious gastroenteritis, often in children 6 years old or younger. It can infrequently cause hepatitis among those who have received organ transplants, those who are undergoing chemotherapy and immunocompromised people, often leading to severe illness and even death.

Tomoya Saito of the National Institute of Infectious Diseases said it has been rare for adenovirus to cause severe hepatitis among healthy individuals, adding that experts are currently trying to come up with various hypotheses based on the reported cases.