(NBC News) — Doctors at Cincinnati Children’s Hospital in Ohio say they have treated at least six cases of severe hepatitis in previously healthy young children aged 18 months through 10 years. One required a liver transplant.

These cases add to a growing number of puzzling reports of young children worldwide who are developing acute hepatitis without a known cause.

Dr. Jorge Bezerra, director of the Pediatric Liver Care Center at Cincinnati Children’s Hospital, said that his team has seen an increasing number of such patients since December.

“We have seen six,” Bezerra told NBC News Wednesday. “Today, we’re going to be seeing a seventh patient,” he said. “They’re still coming.” The children treated for the liver disease at Cincinnati Children’s are all from Ohio.

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