An outbreak of high-pathogenicity avian influenza H7N7 virus that took place on 255 poultry farms in the Netherlands during 2003 has been used to provide clues about the current avian influenza H7N9 viruses in China. During the Dutch outbreak 453 humans showed symptoms of illness and 89 were confirmed to have infection with the virus. Some interesting observations from that outbreak:
- Conjunctivitis (inflammation of the membranes surrounding the eyelids) was observed in many of the human cases, as well as in later human infections with H7 influenza viruses. Apparently these viruses replicate well in the eye, which bears alpha-2,3 sialic acid receptors. From there the viruses could reach the nasal cavity via the nasolacrimal duct.
- There was one fatal infection during the Dutch outbreak, and virus isolated from this individual contained the amino acid change E627K in viral protein PB2, which is associated with higher replication of avian influenza viruses in mammals. This change likely arose during replication of virus in the patient as it was not observed in other isolates. The recent H7N9 isolates from China all have the PB2 E627K mutation.
- In the Dutch outbreak there was no evidence for human to human transmission of H7N7 viruses. This conclusion is in part supported by phylogenetic analysis of viral sequences, which showed that during the outbreak the viruses diversified into multiple lineages with human strains at the ends of the trees (Figure; click to enlarge. Credit: Eurosurveillance).
So far the H7N9 virus does not appear to be spreading from human to human. This observation suggests that the virus is widespread in poultry in China, and that there have been multiple introductions into humans. It seems likely that these novel viruses arose relatively recently in China and some time thereafter had to opportunity to infect humans.
The question on everyone’s mind is whether the avian influenza H7N9 viruses will acquire the ability to transmit among humans. On this subject the authors have the following comment:
Although human infections with H7 influenza viruses have occurred repeatedly over the last decades without evidence of sustained human-to-human transmission, the absence of sustained human-to-human transmission of A(H7N9) viruses does not come with any guarantee.
It is possible that during replication in birds or humans, the H7N9 viruses might randomly acquire a mutation that allows for transmission. In the right place at the right time, such a virus could spread through the human population. Alternatively, such a transmission-facilitating mutation might interfere with the overall fitness of the virus, thereby preventing it from spreading. I favor the latter hypothesis because the H7N9 viruses have been transmitting since at least February 2013; they have undergone many replication cycles without such a mutation arising. If the virus has not entered wild birds, culling poultry could eradicate it from China – assuming that it has not gone elsewhere.