Where have the new cases been reported?

In early April, Britain became the first country to notify the W.H.O. of a cluster of unexplained hepatitis cases in children. The cases were unusual because they occurred over a short period of time in otherwise healthy children, and because clinicians quickly ruled out any of the common hepatitis viruses as the cause. They did not identify any patterns in travel, diet, chemical exposures or other risk factors that might explain the outbreak, according to the U.K. Health Security Agency’s briefing.

Since then, Austria, Belgium, Denmark, France, Germany, Ireland, Israel, Italy, Japan, the Netherlands, Norway, Poland, Romania, Spain and the United States have reported similar cases, the E.C.D.C. said.

In the United States, Alabama recorded nine cases between October and February. Three of the children developed liver failure, and two required liver transplants, the Centers for Disease Control and Prevention noted in a recent report. All of the children either recovered or are recovering, the agency noted.

“The two that received the transplant are actually doing quite well,” said Dr. Henry Shiau, a pediatric transplant hepatologist at the University of Alabama at Birmingham and the Children’s of Alabama hospital.

The cases prompted the C.D.C. to issue a nationwide alert, asking health care providers to keep an eye out for similar cases.

Illinois and Wisconsin have since announced potential cases. North Carolina, Delaware, Minnesota, California, New York, Georgia and Louisiana have also identified, or are investigating, possible cases, state officials told The New York Times.

What are the symptoms?

In many of the cases, children developed gastrointestinal symptoms, including vomiting, diarrhea and abdominal pain, followed by a yellowing of the skin or eyes, known as jaundice. They also had abnormally high levels of liver enzymes, a sign of liver inflammation or damage.