Serological and molecular methods for surveillance of influenza A virus

Abstract

The natural reservoir for influenza A viruses is birds, and numerous outbreaks of highly pathogenic avian influenza viruses have been documented. There is a risk of novel subtypes originating from birds infecting humans, and the question of migratory birds as long-distance vectors for highly pathogenic avian influenza viruses has also been raised. Areas where migratory flyways meet and birds nest have been suggested as hot spots for influenza A viruses to mix. In our study we found no evidence of recent genetic mixing at Point Barrow in Alaska.

In order to know which subtypes currently circulate, influenza surveillance in domestic and wild birds is crucial. Detection of viral RNA from bird faeces is commonly used. However, detection of antibodies against influenza A virus provides useful information after birds have ceased to shed virus. We evaluated a pseudoparticle neutralization test, based on highly pathogenic avian influenza virus hemagglutinin. Our results show that the test can be used for detection of H5 and H7 specific antibodies, which offers an alternative to using standard neutralization tests where live virus is required.

Influenza virus with new genetic material from birds has caused several human pandemics during the 20th century. In 2009, the A(H1N1)pdm09 virus emerged. The receptor binding structure of the virus, the hemagglutinin, was phylogenetically closely related to the virus of the 1918 Spanish influenza. During the 2009 pandemic the elderly population was only mildly affected, possibly due to pre-existing cross-reactive neutralizing antibodies. Using a pseudoparticle neutralization assay we were able to investigate neutralizing antibody crossreactivity patterns in different age groups against H1 influenza viruses from 1918, 1934, 1999, and 2007.

A significant difference between age groups in antibody titers against the 1918 and 1934 viruses was observed. Individuals over the age of 90 had the highest levels of neutralizing antibodies against the 1918 virus, while those aged 71-90 had the highest levels against the 1934 strain. The 1918 virus is antigenically similar to the 2009 virus and antibodies against the 1918 virus may have protected against the 2009 virus. We also tested the sera for presence of neuraminidase inhibiting (NI) antibodies against the A(H1N1)pdm09 virus.

The results revealed a strong correlation between NI antibodies and age. NI antibodies did, however, not appear to significantly influence the neutralizing titers in a long-incubation neutralization assay. Antibodies targeting the neuraminidase may prevent severe illness and could together with pre-existing cross-reactive neutralizing antibodies have contributed to the mild outcome in the elderly during the 2009 pandemic.