Public health experts have warned that the swine flu virus — influenza A(H1N1) — may be circulating undetected in pigs because global surveillance of the animals is so poor.
Animal health organisations have rejected the idea that pigs are responsible for the virus's spread on the grounds that there is no supporting evidence.
But now experts are saying that this could be because of lack of surveillance, which may have allowed the flu strain to persist and evolve in pigs for years.
If the virus can circulate freely in herds and pass easily back and forth to humans, the chances of it reassorting into a deadlier strain persist, researchers say.
Flu viruses in pigs tend to cause mild illness and there is no obligation for farmers to report cases. Just one herd, in Alberta, Canada, has been confirmed to have influenza A(H1N1) but "it is highly likely that more pigs are infected in more places", says Jimmy Smith, head of livestock affairs at the World Bank.
A paper published in Nature last week concluded that the strain has circulated and evolved undetected in pigs for many years. And UK scientists have shown that pigs can easily become infected with, transmit and shed influenza A(H1N1) into the environment.
"There is abundant genetic evidence that the name [swine flu] is appropriate," says a Nature editorial.
Animal health scientists tend to work for government agencies that naturally want to protect livestock and the meat trade, the editorial says — leading to "conflicts of interest, as well as some policy positions that border on denial".
The editorial calls for increased funding for surveillance and international support for the OIE/FAO Network of Expertise on Animal Influenza (OFFLU), which has been coordinating labs that research human disease arising from animals.
Nature doi:10.1038/459894b (2009)
Nature doi:10.1038/459889a (2009)
Nature doi:10.1038/nature08157 (2009)
Nature doi:10.1038/nature08182 (2009)