A mysterious and potentially deadly form of acute hepatitis is infecting hundreds of children around the world, leaving experts puzzled and scrambling to find the cause.
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What has confused medical experts the most is that some countries - including Australia - have been yet untouched by the outbreak, despite having very similar vaccination rates and health environments.
According to the World Health Organisation (WHO), between April 5 and May 26 this year, 650 probable cases of acute hepatitis, of unknown cause from 33 countries have been reported.
President of the Australian Centre for Hepatitis Virology and Westmead Institute for Medical Research Group Leader and Senior Researcher Dr Thomas Tu said hepatitis is defined as "any disease that causes an inflammation of the liver."
Dr Tu himself lives with Hepatitis B. He said the underlying cause of that inflammation could be completely different things, but generally, hepatitis refers to the liver being affected and damaged in some way.
The cause of this 'mystery' acute hepatitis, in children under 16, is still under investigation.
"So right now we don't know the actual cause evasion, it could be genetics, it could be immune mediated, it could be environmental - like a toxin, or something infectious," Dr Tu said.
"Or more importantly, it may be the combination of many of these together.
"We have many hypotheses that are basically guesses at the moment, that are the start of scientific investigation."
Dr Tu said it's difficult finding the cause of liver disease and it originally took 15 years to find the cause of Hepatitis C.
He said it should also be known that it is fairly regular have at least some cases of hepatitis with unknown cases year to year.
The WHO has said the acute hepatitis cases we're seeing are "more clinically severe," and a higher number of infected children end up developing acute liver failure.
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The link between this acute hepatitis and the COVID-19 virus and adenoviruses (a group of viruses that cause a range of respiratory and intestinal illnesses like common colds and gastro infections) is one being explored.
"What we can do at the moment is rule out some things, in particular, it's not due to the COVID-19 vaccine because the majority of the kids affected by this disease have not had the vaccine," Dr Tu said.
A joint report by the WHO and the European Centre for Disease Prevention and Control (ECDC) found that out of 63 cases, 84.1 per cent were unvaccinated.
The report also found that "as severe hepatitis can take some time to develop after the onset of the first symptoms and as investigations take time, there may be a delay in the reporting of cases".
Some of the countries affected include the United States of America, The United Kingdom, France, Japan and Singapore.
To date, no cases have been reported in Australia.
Dr Tu said we don't know why there have been cases in several other similar countries but not in Australia. Experts don't yet know the role that childhood immunisation plays in the staving of infections.
According to a report by the WHO, approximately 75.4 per cent of cases are in children five and under.
A small percentage of those affected (14.1 per cent) were admitted to an intensive care unit.
Out of the 650 probable cases at the time of publication, at least 38 (6 per cent) children have required transplants, and nine (1 per cent) have died.
And 60.8 per cent tested positive for adenovirus, while of the 188 cases PCR tested for COVID-19, 12.2 per cent tested positive. Of those serology tested (26 cases), 73.1 per cent tested positive for COVID-19.
According to the WHO, laboratory testing has excluded hepatitis A,B,C,D,E viruses in the infected children.
The Gastroenterological Society of Australia (GESA) say this acute hepatitis is a rare occurrence.
GESA encourages people to practice thorough hand washing, covering your mouth and nose when coughing or sneezing, and contacting your health professional if you have concerns about your child's health.
Dr Tu said it will require greater funding into research to find the cause and cure of this acute hepatitis.
"We may only be seeing the tip of the iceberg," he said.
"We should be investing a lot more into this research to shorten that time and to be able to translate that knowledge into vaccines and cures that you see today for COVID."
