The origins of influenza viruses
Influenza virus genomes are a patchwork quilt of genes from different sources
From Neumann G et al. Nature 2009;459:931–9
Analysis of its genetic make-up suggests that it first appeared some time between autumn and Christmas 2008, so was almost certainly circulating before it was first detected in Mexico.
Where it first made the leap from pigs to humans is unclear; its closest relative in pigs is a Hong Kong sample from 2004. However, this is a ‘cousin’, distantly related to influenza A(H1N1)pdm09, so it cannot be said with certainty where the pandemic virus emerged.
Asia may be a ‘hotspot’ for jumps across species as large numbers of people live close to livestock such as pigs and poultry. China, for example, accounts for half the world’s pig consumption – around 1.7 million pigs every day.
More generally, influenza A(H1N1)2009pdm shows how influenza viruses can evolve through the mixing of DNA from multiple sources. This occurs most readily in organisms such as pigs that can be infected by viruses that infect other species – they are a ‘melting pot’ in which new genetic recipes can be concocted.