The U.S. Centers for Disease Control and Prevention is investigating 109 cases of severe, unexplained hepatitis in children in 25 states.
According to the CDC, several children needed transplants, and five children have died so far. More than 90 percent of the children affected needed to be hospitalized.
The hepatitis cases may be linked to a worldwide outbreak.
CNN reported that Dr. Jay Butler, the CDC's deputy director of infectious diseases, said some of the common causes of viral hepatitis have been considered but were not found in any of the cases.
Adenovirus has been detected in more than 50 percent of the cases, although Butler said its role in the outbreak isn't clear. According to the CDC, adenovirus type 41 commonly causes pediatric acute gastroenteritis, which typically presents as diarrhea, vomiting, and fever; it can often be accompanied by respiratory symptoms.
The investigation comes after a cluster of unusual cases of hepatitis was reported in an Alabama children's hospital. Five children at the hospital tested positive for adenovirus and had severe liver damage. Three of the children had acute liver failure.
While two of the patients needed transplants, none of them died.
A review of cases over the winter found a total of nine cases of adenovirus type 41 were discovered at the Alabama children’s hospital.
In a Wednesday briefing, Dr. Philippa Easterbrook with the World Health Organization's Global Hepatitis Program said that since the first cases were reported in the United Kingdom on April 5, there are 228 probable cases reported from 20 countries, with more than 50 cases now under investigation.
One death has been reported, and around 18 children have needed liver transplants.
Easterbrook said only six of the countries are reporting more than five cases. The other 14 countries are reporting less than five and, in some cases, just one or two cases.
The Virginia Department of Health released a statement saying none of the U.S. cases were in Virginia, but that the department will continue to monitor and follow up on reports.
"VDH released a clinician letter asking healthcare providers to report suspected cases to LHDs," said Larry Hill with the VDH. "We have not confirmed any reports meeting our surveillance definition but will continue to monitor and follow-up on reports, as well as partner with CDC on this evolving investigation."