The latest epidemic of Asian lineage avian influenza A (Asian H7N9) in China infected almost as many patients as the four previous outbreaks combined and gave rise to a variant less susceptible to available treatments and vaccines, researchers report in an article published in the September 8 issue of the Morbidity and Mortality Weekly Report.
The fifth annual epidemic, which occurred from October 1, 2016, through July 31, 2017, included 759 patients and 281 deaths (37%). The cumulative mortality for all five Asian H7N9 outbreaks since February 2013 is 39% (605 of 1557 patients).
“Although human infections with Asian H7N9 viruses from poultry are rare and no efficient or sustained human-to-human transmission has been detected, when human infections do occur, they are associated with severe illness and high mortality,” write James C. Kile, DVM, from the Influenza Division at the National Center for Immunization and Respiratory Diseases at the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia, and colleagues. “Continued vigilance is important to identify changes in the virus that might have epidemiologic implications, such as increased transmission from poultry to humans or transmission between humans.”
The fifth outbreak involved all but three of 33 provinces, regions, and municipalities in China. All infected patients had either lived in or traveled to these areas, and most (90%) infections were associated with poultry exposure, especially at live bird markets.
The authors note, however, that the increase in infections was the result of wider geographic spread and higher prevalence of Asian H7N9 viruses among poultry, not increased poultry-to-human or human-to-human transmission.
Notably, during the fifth epidemic, some Asian H7N9 strains acquired mutations that turned them from low-pathogenic to high-pathogenic strains. Twenty-seven patients were infected with high-pathogenic avian influenza strains that had acquired mutations.
Sequencing data from 166 viral isolates collected during the fifth epidemic uncovered genetic markers of resistance to amantadine and rimantadine. In addition, researchers found significant changes in surface proteins among some of the high-pathogenic avian influenza strains, making them less similar to previous candidate vaccine viruses.
These factors contributed to the CDC’s decision to classify Asian H7N9 as the virus with the highest potential pandemic risk among all viruses evaluated using the Influenza Risk Assessment Tool.
The government of China is trying to minimize Asian H7N9 spread by promoting large-scale farming and centralized slaughtering, improving poultry product handling, and routinely closing, cleaning, and disinfecting live poultry markets and enhancing surveillance for influenza-like illness.
The authors write, “CDC has partnered with China CDC, and other China government organizations, United Nations organizations, and surrounding countries to enhance surveillance and laboratory capacity to detect and respond to Asian H7N9 in animals and humans.”
One coauthor holds two US patents for influenza vaccines. The other authors have disclosed no relevant financial relationships.
Morb Mortal Wkly Rep. 2017;66:928-932. Full text
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