Nearly 200 cases of severe, unexplained hepatitis have been reported in children worldwide over the last few months, including at least nine cases in the U.S., according to news reports.
Here's what we know about the mysterious outbreak:
What is hepatitis?
Hepatitis is inflammation of the liver. It can be caused by viruses, drugs such as alcohol, prescription medications, toxins or certain medical conditions, according to the U.S. Centers for Disease Control and Prevention (CDC) (opens in new tab). Most often, hepatitis is caused by a viral infection — specifically, the hepatitis viruses A, B, C, D and E. In the U.S., hepatitis A, B and C are the most common causes of viral hepatitis, according to the CDC.
Hepatitis can cause swelling and damage to the liver, which can affect how well this vital organ functions, according to the National Institutes of Health (opens in new tab).
Why are the cases unusual?
Cases of severe hepatitis, like the ones in the current outbreak, are unusual in children. In addition, hepatitis viruses — the usual culprit for liver inflammation — have been ruled out in these cases, Live Science previously reported.
Although doctors sometimes see cases of unexplained hepatitis in children, they are now seeing a greater than expected number in places where the outbreaks have been reported. For example, more than a dozen cases of unexplained hepatitis in children were reported in Scotland during March and April, when the country typically sees fewer than four such cases per year, Live Science previously reported.
Where have cases been reported?
So far, about 190 cases have been reported worldwide, according to the European Centre for Disease Prevention and Control, Reuters reported (opens in new tab). This includes more than 100 cases in the United Kingdom, which was the first country to identify cases. About a dozen other countries have since reported cases, including the U.S., Israel, Denmark, Ireland, the Netherlands, Spain, Italy, Norway, France, Romania and Belgium, according to the World Health Organization (WHO) (opens in new tab). The cases have mostly occurred in children under age 10.
What are the symptoms in children?
A classic symptom of hepatitis is jaundice, or yellowing of the skin or the whites of the eyes, which has been seen in many of the affected children, according to the WHO. Many of the children also experienced gastrointestinal symptoms, including abdominal pain, diarrhea and vomiting. Most cases did not have a fever, WHO officials said.
Other symptoms of hepatitis can include fatigue, loss of appetite, dark urine, light-colored stools and joint pain, according to the CDC.
Seventeen of the affected children have required liver transplants and at least one has died, according to the WHO.
What's causing the outbreak?
The cause of the cases is still unknown. But officials suspect that the cases are linked with adenovirus infections. Adenoviruses are a family of viruses that can cause a range of symptoms, including cold-like symptoms, fevers, pneumonia, diarrhea and pink eye, according to the CDC (opens in new tab).
All nine cases of unexplained hepatitis in the U.S. tested positive for adenovirus type 41, which usually causes gastrointestinal symptoms in children, according to the CDC.
Although adenovirus type 41 has previously been linked with hepatitis in children with weakened immune systems, this particular virus "is not known to be a cause of hepatitis in otherwise healthy children," according to the CDC alert (opens in new tab) about the cases.
Most of the affected children in the U.K. (75% of those tested) have also tested positive for adenovirus, according to a statement from the U.K. Health Security Agency (opens in new tab). In those cases, preliminary tests for the type of adenovirus also point to adenovirus type 41, but U.K. officials said that further tests are needed for confirmation.
SARS-CoV-2, the virus that causes COVID-19, is not thought to be behind the cases. None of the U.S. cases have tested positive for COVID-19, and U.K. officials have also not found a connection.
Originally published on Live Science.