An outbreak of severe hepatitis in children has continued to grow, with new cases now identified in the U.S., the World Health Organization (WHO) reported this weekend.

The WHO published its first report on the outbreak on April 15, when cases were isolated in the United Kingdom. At the time of that report, there were a total of 10 cases in children between the ages of 11 months and 5 years old.

No cause has been identified as of yet. As of last week at least 169 cases of acute hepatitis, or liver inflammation, of unknown origin had been reported in children between the ages of 1 month and 16 years old. While still concentrated in the U.K., additional cases have appeared in Spain, Israel, the U.S., Denmark, Ireland, the Netherlands, Italy, Norway, France, Romania, and Belgium. At least one death has been reported, according to the WHO.

The common viruses associated with acute viral hepatitis—hepatitis A, B, C, D, and E—have not been identified in any case. Adenovirus, a class of common viruses that typically cause self-limiting infections in humans, has been identified in at least 74 cases, according to the report. The WHO is also investigating connections between the outbreak and COVID, which was identified in 20 cases.

Beyond acute hepatitis and elevated levels of liver enzymes, symptoms in the reported cases have included abdominal pain, diarrhea, vomiting, and jaundice. Adenoviruses, which typically spread from person to person, most often cause respiratory symptoms like coughing, sore throat, and runny nose. Depending on the specific type, those affected can also develop gastroenteritis, pinkeye, and bladder infection, among other symptoms.

There are more than 50 adenovirus types that can infect humans, according to the WHO’s report. Of the cases that had access to molecular testing, 18 were identified as involving adenovirus type 41, which has never before been closely linked with severe hepatitis. It most often causes diarrhea and vomiting.

“While there have been case reports of hepatitis in immunocompromised children with adenovirus infection, adenovirus type 41 is not known to be a cause of hepatitis in otherwise healthy children,” said the WHO in its report.

The high incidence of adenovirus infections associated with the hepatitis outbreak could simply be due to better testing, according to the WHO. “While adenovirus is currently one hypothesis as the underlying cause, it does not fully explain the severity of the clinical picture,” said the WHO.

The WHO said that hypotheses relating to side effects of the vaccine are not supported, since most affected children had not been vaccinated. In the U.S. children under 5 are still not approved for the vaccine.

The U.S. Centers for Disease Control and Prevention (CDC) has previously linked COVID with Multisystem Inflammatory Syndrome in Children (MIS-C), a condition characterized by inflammation of different organs. A paper published in the scientific journal Hepatology in late 2020 identified hepatitis as a common symptom in children afflicted with MIS–C.

“Further work is required to identify additional cases, both in currently affected countries and elsewhere,” the WHO said in regard to proper public health response to the hepatitis outbreak. “Common prevention measures for adenovirus and other common infections involve regular hand washing and respiratory hygiene.”