Other virus vectors?
H7N9 cases to 16 April 2013 (red circles), and population densities of humans (A; density in 2010), pigs (B), chickens (C) and ducks (D) in China and Asia in general. (Livestock densities are modeled numbers of animals per square kilometer standardized to 2006 national totals. Note different scale for pigs.) H7N9 case locations courtesy of EMPRES, FAO, Rome. Human population from ASIAPOP.
Source: Map supplied by T. P. Robinson, G. R. W. Wint, G. Conchedda, T. P. Van Boeckel, V. Ercoli, E. Palamara, G. Cinardi, L. D’Aietti and M. Gilbert (2013). Unpublished data. (T.P.R., Fonds National de la Recherche Scientifique, Brussels; G.R.W., University of Oxford, UK; G.Co., V.E., E.P., G.Ci. and L.D'A., FAO, Rome; T.P.V.B. and M.G., Université Libre de Bruxelles, Brussels.)
The H7N9 virus has mutations that mean that it spreads from birds to humans more easily than does H5N1. Proximity between bird and mammal populations could also give the virus opportunities for further adaptation to mammals, including humans. An international team of researchers compiled maps for Nature showing the population densities of chickens, pigs, ducks and humans in many parts of China and throughout Asia. They calculate that 131 million people, 241 million domestic chickens, 47 million domestic ducks and 22 million pigs live within a 50-kilometer radius of each of the 60 H7N9 human cases that had occurred up to 16 April.
There has been no evidence of sustained human-to-human spread of H7N9 so far. But what if it did happen? In what Jeremy Farrar, director of the Oxford University Clinical Research Unit in Ho Chi Minh City, Vietnam, describes as an "extraordinary effort", he and other international researchers, including some from China and the WHO, have worked together in the past weeks to rapidly analyze airline-passenger data for China. The resulting maps and data may give an idea of where the zones of immediate highest risk worldwide might be.
One map supplied to Nature by the researchers shows, they note, that eastern China — the epicenter of the current H7N9 outbreaks — is one of the world's busiest hubs for airline traffic. "A quarter of the global population outside of China lives within two hours of an airport with a direct flight from the outbreak regions, and 70% if a single connecting flight is included," the researchers explain.
As Peter Horby, director of the Oxford University Clinical Research Unit in Hanoi, Vietnam, and colleagues point out in a World View in Nature this week, "The first human case of H7N9 outside mainland China is perhaps only a matter of time. Then the public-health and clinical community will need to assess, carefully and quickly, whether it represents a single imported case of animal-to-human transmission, an animal epidemic that has spread abroad, or the international spread of a partially or fully human-adapted virus.”