Pandemic predictions

Anthony King

The next big influenza pandemic will almost certainly involve an animal strain, predict scientists in Australia and China who carried out the first global survey of flu strains circulating in animals (One Health, doi: 10.1016/j.onehlt.2023.100514).

Previous flu pandemics involved strains that originated at least partly in animals such as birds and pigs. The study scientists write that it is possible to prevent a human influenza pandemic ‘by identifying influenza viruses with pandemic potential in animal species’.

Their study revealed hotspots of virus diversity, which give clues of possible pandemic strains. Poultry was the main host reported in Asia and Africa; wild birds in North America and Europe.

‘The more widespread a virus subtype and the more events in which the virus transmits between species, especially birds and mammals, the more concern,’ notes Michael Ward at the University of Sydney, Australia. ‘Subtype H5N1 fulfils these requirements, and so deserves special attention as a potential pandemic strain.’

Flu strains are subtyped according to two surface proteins, hemagglutinin (H) and neuraminidase (N). Some subtypes of concern are in swine (H1 and H3), some in birds (H5, H7). The survey found that animal outbreaks involving subtypes H5N2, H7N7 and H7N9 were relatively constant, ‘with a slow upward trend during the past decade’.

Circulation of subtypes in the same animal can allow strains to swap RNA segments – reassortment – and generate new strains with pandemic potential. The risk for reassortment was greatest in Asia, North America and Europe. However, many countries in Africa and South America have never reported animal influenza events, which does not mean that such events are not a regular occurrence.

‘Our survey highlighted surveillance gaps,’ notes Ward. ‘Surveillance for animal influenza virus still remains relatively ad hoc and voluntary in many areas of the world,’ he adds.

Some subtypes rarely reported before, such as H5N6 and H5N8, showed a sharp upwards trajectory and deserve attention, the researchers conclude. One surprising finding was the possible replication of H5N1 in aquatic animals.

This is a very useful mapping exercise, comments Marc-Alain Widdowson, Director of the Institute of Tropical Medicine in Antwerp, Belgium. The main message, he notes, ‘is there are lots of different influenza viruses currently circulating in birds and animals worldwide, which results in a constant threat that needs to be monitored.’

Areas where there are lots of wild waterfowl mixing is of concern, as are wet markets where wild birds and domestic birds interact, Widdowson adds. ‘In general, it is tough to predict epidemics and emergence,’ he warns. ‘The real danger is really in the genetic nuances of each subtype and the different strains with specific mutations that may increase transmissibility or disease severity.’

The study searched public databases and the scientific literature to identify 70,472 records of global animal influenza events until 2016, with China accounting for around half of the records.

Image: KATERYNA KON/SCIENCE PHOTO LIBRARY