A Toronto hospital says it has detected seven cases of severe acute hepatitis of unknown origin in children over a six-month period, as global experts race to figure out what’s causing the unexplained illness.

A SickKids Hospital spokesperson said the hospital has come across cases in seven children that met Public Health Ontario’s case definition. The cases were identified between Oct. 1, 2021 and April 30, 2022.

In an interview with CP24 on Monday afternoon, SickKids head of infectious diseases Dr. Upton Allen said that at this point it is too early to say whether the number of cases represents an increase on what the hospital may have encountered in previous years.

“What we're focusing on now is trying to determine the answers to two questions. Number one is are the cases that we're seeing now truly representative of a new signal, a signal that is different over the last six months or so compared to what we would have seen a year ago? Two years ago? Three years ago?” he said. “Secondly, are they different in terms of causation? We don't have the answers to those two key questions as of yet.”

Public Health Ontario defines the cases as transaminitis, or liver inflammation, alongside jaundice and gastrointestinal dysfunction in children up to age 16, with no presence of the known Hepatitis viruses A through E.

Other symptoms include lethargy, fever, dark urine, pale stools, loss of appetite and nausea.

The infection is confirmed through blood, respiratory or stool testing.

It’s been hard to discern these cases from others because they often present with symptoms somewhat typical of COVID-19, and then the usual viruses that cause hepatitis are not found in their blood.

“The challenge with this breakout of new cases is that the typical hepatitis viruses are not being found in these children,” pediatrician Dr. Dina Kulik told CP24 earlier on Tuesday.

“Some of these kids tested positive for COVID, some had previous documented COVID-19 infections, and many kids are testing positive for another typically benign virus known as adenovirus that doesn’t typically cause hepatitis or really severe illness at all.”

The World Health Organization says it is now aware of 348 cases of the unexplained hepatitis in kids around the world, with one death reported and 17 liver transplants required so far in response.

Upton said that the cases at SickKids have involved children between the ages of two and 14. None of the children are currently being cared for in hospital, he said.

“The most important thing is, first of all, this is rare. So it's important to recognize that and it's not so far any of the recognized causes of hepatitis. But the investigations continue worldwide to get a better understanding of whether or not we're looking at a new pathogen or an old pathogen that is behaving differently in individuals for whatever reason,” he said. 

Multiple causes of the hepatitis cases are under investigation. British authorities have said a rare adenovirus may be to blame.

Meanwhile, researchers in India and Israel have found data that suggests previous COVID-19 infection might be the culprit.

In a statement provided to CP24 on Tuesday, the Public Health Agency of Canada said that it is aware of the cases of severe acute hepatitis at SickKids and is monitoring the situation.

It did not say whether there have been any other cases reported at other Canadian children’s hospitals.

“At this time, we do not know if we are seeing an increase in the number of cases of acute hepatitis of unknown cause in children. More information is needed to assess the situation and any potential risks to people in Canada,” the statement notes. “We are working closely with provincial, territorial, and international partners on this evolving event. We are prepared to carefully and thoroughly investigate any cases reported to PHAC that meet the national case definition. This will help us to determine the national scope in Canada, and to determine if cases of acute hepatitis in Canada may be related to other cases reported around the world.”