WASHINGTON — Chinese and US researchers said on Dec 29 they have found how changes in H9N2, a flu virus that has plagued Chinese poultry farms for decades, helped create the novel avian H7N9 influenza that has killed at least 115 people since 2013 and raised a pandemic concern.

The results, published in the US journal Proceedings of the National Academy of Sciences, underscored the need for continued surveillance of flu viruses circulating on poultry farms and identified changes in the H9N2 virus that could serve as an early warning sign of emerging flu viruses with the potential to trigger a pandemic and global health emergency.

H9N2, which has low pathogenicity for avians, was first identified in chicken farms in the south China’s Guangdong province in 1994. Genetic analysis indicated H7N9’s six internal genes all originated from H9N2. However, the H7N9 virus is more highly pathogenic in humans than H9N2.

To investigate the H9N2’s role in the genesis of the H7N9 virus, researchers at the China Agricultural University, Jude Children’s Research Hospital in Memphis, Tennessee, and other research organizations used whole genome sequencing to track the evolution of the H9N2 chicken viruses mainly in farm chickens in China between 1994 and 2013.

The analysis showed that the genetic diversity of H9N2 viruses fell sharply in 2009 and that from 2010 to 2013, an H9N2 virus emerged as the predominant subtype thanks to its genetic makeup that allowed it to flourish despite widespread vaccination of chickens against H9N2 viruses.

The dominant H9N2 virus “had changed antigenicity and improved adaptability in chickens,” co-corresponding author Jinhua Liu, professor of the College of Veterinary Medicine at the China Agricultural University, told Xinhua. “That may be one reason for its widespread outbreak from 2010 to 2013.”

The emergence of this dominant H9N2 virus, Liu continued, was the first step in the genesis of the H7N9 viruses because it greatly increased the likelihood of reassortment between H9N2 and other flu subtypes. Reassortment refers to the tendency of flu viruses to swap genes.

Before long, the dominant H9N2 virus contributed its six internal genes to the novel H7N9 viruses by reassortment, which has caused two outbreaks of human infection, including at least 375 known cases and 115 deaths since March 2014.

“In the past, we have paid great attention to highly pathogenic forms of avian influenza virus such as H5N1, while low pathogenic forms are largely neglected,” Liu said. “But H9N2 has acted as the gene donor for H7N9, H10N8, H6N1 and other viruses that can infect humans, and our study suggests that the prevalence and variation of H9N2 in farmed poultry could provide an early warning of the emergence of novel reassortants with pandemic potential.”