Zoonotic infections with avian influenza A viruses and vaccine preparedness: a game of "mix and match"

Abstract

Various direct avian-to-human transmissions of influenza A virus subtypes upon exposure to infected poultry have been previously observed in the past decades. Although some of these strains caused lethal infections, the lack of sustained person-to-person transmission has been the major factor that prevented these viruses from causing new pandemics. In 2013, three (A/H7N9, A/H6N1, and A/H10N8) novel avian influenza viruses (AIVs) yet again breached the animal-human host species barrier in Asia. Notably, roughly 20% of the A/H7N9-infected patients succumbed to the zoonotic infection whereas two of three A/H10N8 human infections were also lethal. Thus, these events revived the concerns of potential pandemic threats by AIVs in the horizon. This article reviews the various human incursions with AIV variants and provides insight on how continued circulation of these viruses poses perpetual challenge to global public health. As the world anticipates for the next human pandemic, constant vigilance for newly emerging viruses in nature is highly encouraged. With the various numbers of AIVs demonstrating their capacity to breach the animal-human host interface and apparent limitations of current antivirals, there is a need to broaden the selection of pre-pandemic vaccine candidate viruses and development of novel alternative therapeutic strategies.

Keywords: Avian influenza A virus; Humans; Influenza vaccines; Pandemic; Pathogenicity; Transmission.

Fig. 1

Transmission and spread of avian influenza viruses from wild to domestic birds and potential spill to humans. Wild aquatic birds could carry avian influenza viruses (AIVs) over long distances. Ducks play important role in the transmission of AIVs from migratory waterfowls to domestic poultry. Newly introduced AIVs cause occasional poultry outbreaks, as in the case of highly pathogenic avian influenza H5 and H7 viruses, resulting to mass culling that leads to huge economic losses. Some AIVs become established in domestic poultry where they co-circulate and reassort with locally endemic strains producing novel virus progenies. Similarly, these newly formed virus variants find their way back to migratory birds through ducks. Direct human infections with AIVs typically occur during outbreak settings due to extensive exposure to infected poultry usually. These cases, although sporadic, could provide an opportunity for the virus to evolve into one that could sustain efficient human-to-human transmission. Meanwhile, introduction of AIVs into pig herds could further hasten up the genetic evolution of AIVs through adaptation and/or reassortment which could generate strains with potential threat to animal and human health.