Viruses in Cambodian bird flu cases identified as endemic clade

The cases reported last week had raised concerns they were caused by a new strain of H5N1, clade 2.3.4.4b

The viruses that infected two people in Cambodia with H5N1 avian influenza have been identified as an endemic clade of bird flu circulating in the country, the U.S. Centers for Disease Control and Prevention (CDC) said.

The cases reported last week had raised concerns they were caused by a new strain of H5N1, clade 2.3.4.4b, which emerged in 2020 and has caused record numbers of deaths among wild birds and domestic poultry in recent months.

But work so far suggests this is not the case.

Preliminary genetic sequencing carried out in Cambodia led its health ministry to identify the viruses as H5 clade 2.3.2.1c, which has circulated in Cambodia among birds and poultry for many years and has sporadically caused infections in people, the CDC said in a statement on Saturday.

"Yes, this is an older clade of avian influenza that had been circulating around the region for a number of years and while it has caused human infections in the past, it has not been seen to cause human-to-human transmission. However, that doesn't mean that the threat is any less," said Erik Karlsson, director of the National Influenza Center of Cambodia and acting head of virology at the Institut Pasteur du Cambodge, which sequenced the virus.

He added that the response needed to be coordinated and swift to prevent any further spread and to limit exposure to any common source.

An investigation into the source and to detect any additional cases is ongoing, the CDC said, adding that so far there had been no indication of person-to-person spread.