Public-health experts have warned that a lack of surveillance may be allowing the 2009 pandemic H1N1 flu virus, dubbed 'swine flu', to go undetected in pigs, which raises the risk that the virus could circulate freely between humans and pigs, making it more likely to develop into a deadlier strain.
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.
According to a report in Nature News, some experts say that the H1N1 flu virus is an artifact of patchy to non-existent flu surveillance in pigs.
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," said 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," said Smith.
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," said 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," he added.
"Surveillance for swine flu is not something that has been high on the agenda of government services," said Steve Edwards, chairman of the OIE-FAO Network of Expertise on Animal Influenza (OFFLU). "It is seen as a farming-industry problem," he added.