LOS ANGELES, April 27 (Xinhua) -- More than a dozen cases of unusual, severe hepatitis in children have been identified in the United States, and researchers of the U.S. Centers for Disease Control and Prevention (CDC) are investigating adenovirus as a possible cause.

       At least three cases have been identified in the state of Illinois in addition to the 11 cases reported earlier in the states of Alabama and North Carolina.

       The cases are unusual because the viruses that typically cause hepatitis, aka inflammation of the liver, have been ruled out, according to the CDC.

       The first U.S. cases were identified in October 2021 at a children's hospital in Alabama that admitted five children with significant liver injury, including some with acute liver failure, without known cause, who tested positive for adenovirus, according to the CDC.

       The new cases identified in Illinois were also "potentially linked to a strain of adenovirus," according to the state's public health department.

       The CDC is currently investigating adenovirus as a possible cause for these severe hepatitis cases, but saying researchers are still learning more, including ruling out other possible causes and identifying other possible contributing factors.

       No known epidemiological link or common exposures were found among these affected children, according to investigators.

       The CDC issued a nationwide health alert and asked physicians across the country to be on the lookout for symptoms and to report any suspected cases of hepatitis of unknown origin to their local and state health departments.

       Besides the United States, an increase of pediatric hepatitis cases without a known cause were also reported in Europe.

       At the latest count released by the World Health Organization (WHO), there were 169 cases of the severe hepatitis across 12 countries in kids 1 month to 16 years old, as of April 21.

       Seventeen children have required liver transplantation, and at least one death has been reported, according to the WHO.

       The "vast majority" of affected children did not receive the COVID-19 vaccine, so the WHO does not believe it has anything to do with side effects from the vaccine.