The Centers for Disease Control and Prevention is asking physicians nationwide to be on the lookout for unusual cases of severe hepatitis in children. The agency issued a health advisory on the matter Thursday.
Nine cases have been reported in Alabama, and an additional two have been identified in North Carolina, according to those states' health departments.
Dozens of such cases have also been identified in the United Kingdom, Denmark, Spain and the Netherlands, the European Centre for Disease Prevention and Control announced Tuesday.
Hepatitis refers to inflammation of the liver, a condition that can result in diarrhea, abdominal pain, nausea and vomiting. Some of the children in Alabama developed jaundice, and blood tests showed signs of elevated liver enzymes.
Several children in that state became so ill that they needed a liver transplant. No deaths have been reported. All were ages 1 through 6 and were healthy previously, without any underlying conditions.
Bailey Pennington, a spokesperson for the North Carolina Department of Health and Human Services, said two "school-aged" children in that state developed severe hepatitis and have since recovered.
"No cause has been found and no common exposures were identified," Pennington said in a statement.
In Europe, cases have generally been seen in children ages 2 through 5.
Viruses are often the cause of liver inflammation, particularly the hepatitis type A, B, C, D and E viruses. All clinical labs in the U.S. are required to report those viruses when they're discovered, so health authorities can work to stop outbreaks.
So far, however, all of the usual hepatitis viruses have been ruled out.
Investigators also say neither Covid-19 nor the Covid vaccines have anything to do with the hepatitis cases.
"None of the children in the cluster tested positive for Covid-19 disease. None had previously reported Covid-19 disease," said Dr. Karen Landers, a health officer for the Alabama Department of Public Health, adding, "None of the children received Covid-19 vaccine."
Increasingly, signs are pointing to a virus not usually associated with hepatitis: adenovirus type 41. According to the CDC, this particular type of virus is known to cause vomiting and diarrhea in kids, as well as respiratory symptoms akin to the common cold.
"While there have been case reports of hepatitis in immunocompromised children with adenovirus type 41 infection, adenovirus type 41 is not known to be a cause of hepatitis in otherwise healthy children," the CDC said in its alert.
Five of the nine children in Alabama tested positive for the virus. Their cases occurred from October to February.
The CDC's health advisory urged "clinicians who may encounter pediatric patients with hepatitis of unknown etiology to consider adenovirus testing and to elicit reporting of such cases to state public health authorities and to CDC."
A spokesperson for the California Department of Public Health said officials there are "following the situation closely" and "will be working with health care providers to detect cases in California."
"Noninfectious causes of hepatitis or cases where a virus is not recognized may not be routinely reported," said AnneMarie Harper, a spokeswoman for the Colorado Department of Public Health and Environment. "We will be reaching out to Colorado health care providers to share information and actively monitor for possible cases in Colorado consistent with these reports."
Idaho's state epidemiologist, Dr. Christine Hahn, also said her team is reaching out to pediatric infectious disease physicians and pediatric gastroenterologists.
"So far they are reporting no cases," she said. "Stay tuned."