30 June 2009
Patchy pig monitoring may hide flu threat

Egypt has seen mass culling of pigs. Public-health experts are warning that a lack of surveillance may be allowing the 2009 pandemic H1N1 flu virus to go undetected in pigs. This raises the risk that the virus could circulate freely between humans and pigs, making it more likely to reassort into a deadlier strain, they say. Pig surveillance is largely the remit of animal-health organizations, agriculture ministries and the farming industry. Their main concern tends to be that any reports of the pandemic virus in pigs might provoke overreactions such as the mass culling of pigs that took place in Egypt, or trade bans on pigs and pork. Within minutes of the World Health Organization (WHO) announcement on 11 June that swine flu had become a pandemic, Bernard Vallat, director-general of an intergovernmental trade body, the World Organisation for Animal Health (OIE), had reiterated that trade sanctions were unjustified. "So far the role of animals has not been demonstrated in the virus's epidemiology or spread," he asserted.

But some experts say that is an artefact of patchy to non-existent flu surveillance in pigs. In a paper published last week (G. J. D. Smith et al. Nature advance online publication doi: 10.1038/nature08182; 2009), Gavin Smith, a flu geneticist at the University of Hong Kong, and his colleagues concluded that "the lack of systematic swine surveillance allowed for the undetected persistence and evolution of this potentially pandemic strain for many years". The virus originated from a mixture of swine flu strains, and pigs are an "obvious" part of the epidemiology of the new virus, says Smith. Yet the number of swine-flu sequences in the international GenBank database is about a tenth of that for avian flu viruses. Circulation of the virus between pigs and humans is "definitely a possibility", he adds. The pandemic virus has so far been found in pigs from just one farm, in Alberta, Canada, where it spread throughout the herd. But no one has been able to pin down how the herd became infected. Scientists at the Veterinary Laboratories Agency in Weybridge, UK, have shown that pigs can easily become infected with the virus, and readily transmit it between themselves and shed it into the environment. Past pandemic viruses have also gone on to become endemic in pig populations. "It's absolutely surprising that a virus this contagious in both humans and swine, and which has been reported in humans in 76 countries, has only been reported in one swine farm in Canada," says Jimmy Smith, head of livestock affairs at the World Bank in Washington DC, and a member of the organization's flu task force. "It is highly likely that more pigs are infected in more places."

Nature
June 30, 2009

Original web page at Nature