14.11.2014

Yipeng Sun & Jinhua Liu (2014) Protein Cell

H9N2 influenza virus has become endemic in different types of terrestrial poultry in multiple countries on the Eurasian continent, resulting in great economic losses due to reduced egg production or high mortality associated with co-infection with other pathogens. In China, which is regarded as an epicenter of avian influenza viruses, the H9N2 virus has been detected in multiple avian species. The first outbreak of the H9N2 influenza virus in China occurred in Guangdong province of Southern China during November 1992 to May 1994. These H9N2 viruses killed broilers with mortality of 10%-40%, and reduced the laying rates by 14%-75%. The H9N2 influenza virus is now the most revalent subtype of influenza viruses in chickens in China. H9N2 infections occur throughout the whole year, with lower morbidity in the summer. To prevent H9N2 infection in chickens, China implemented long-term vaccination programs in chicken farms as early as 1998. At least over twenty different commercial vaccines are used in China, with the vaccines are frequently updated. However, H9N2 avian influenza viruses continues to persist in chicken populations, even in vaccinated flocks.
H9N2 viruses isolated from chickens in China underwent antigenic drift to evolve into distinct antigenic groups. This antigenic drift might have led to immunization failure and may explain the current prevalence of the H9N2 influenza virus in China. The identification of antigenic sites of H9 is important for monitoring antigenic variants and developing effective vaccines.
Occasional cases of H9N2 infecting humans have been reported in Southern China and Hong Kong, but there has been no evidence for human-to-human transmission. Poultry workers are considered to be at high risk of infection with avian influenza due to their frequent exposure to chickens. H9N2 influenza viruses in poultry also transmitted to pigs, generating variants with novel antigenic and genetic characteristics. Recently, an avian origin H9N2 influenza virus was isolated from dogs in Southern China, and the positive rates of serums from dogs in Southern China were 20.21% in 2010, 28.98% in 2011, and 44.85% in 2012; suggesting the circulation of H9N2 virus among dogs. The enlarged host range of the H9N2 influenza virus underscored the potential public threat. It is noteworthy that H9N2 viruses present are with increasing adaption to both chickens and mammalians. Because of the wide host range of H9N2, the possibility of reassortment between H9N2 and other influenza viruses in the future cannot be excluded. H9N2 influenza virus has been recognized to reassort with multiple other subtypes. The H9N2 viruses were the likely donors of the six internal genes of the H5N1 viruses causing the avian influenza outbreak which transmitted to humans in Hong Kong in 1997, and the other H9N2 viruses provided the six internal genes to H7N9 and H10N8 viruses which emerged in humans in China since 2013. Moreover, H7N9 influenza viruses continued to reassort with circulating H9N2 viruses, resulting in multiple genotypes of H7N9 viruses.