Infection with adenovirus, a common childhood virus, is the leading hypothesis for recent cases of severe hepatitis of unknown origin in children that have led to at least six deaths, U.S. health officials said on Friday.

The Centers for Disease Control and Prevention (CDC) said it is continuing to investigate whether 180 cases identified in 36 states and territories since last October represent an increase in the rate of pediatric hepatitis or whether an existing pattern has been revealed though improved detection.

The agency in April issued a nationwide alert for doctors to be on the lookout for children with hepatitis, which can cause liver damage and lead to liver failure.

Dr. Jay Butler, CDC deputy director for infectious diseases, said on a conference call that around half of the children diagnosed in recent months were also infected with a type of adenovirus, a virus that causes the common cold, but the agency is still investigating the exact cause of the illness.

"Evidence is accumulating that there is a role for adenovirus, particularly adenovirus-41," he said.

Butler said one theory is that pandemic mitigation measures may have limited exposure to adenovirus, leading to a "catch up" in infections as social distancing and other efforts were eased.

Hepatitis linked to this type of adenovirus has almost exclusively been associated with immunocompromised children, but many of the cases first reported to the CDC did not have such conditions.

The CDC is also investigating whether COVID infection may be playing a role, as well as other pathogens, medications and risk factors.

Compared to pre-pandemic rates, the agency said it has not seen an overall increase in the incidence of severe hepatitis in children, which remains rare with about 1,500 to 2,000 cases identified in a typical year.

CDC officials said they are continuing to work with counterparts in Europe, particularly the UK, which has identified at least 175 cases of acute hepatitis in children.