The top agency for health in farm animals on Monday said Mexico's outbreak of deadly influenza was unleashed by a pathogen mixed from bird, human and hog viruses and branded the term "swine flu" as wrong and harmful to pig farmers.
The World Organisation for Animal Health (OIE) said the pathogen was "not a classic human virus... but a virus which includes [in] its characteristics swine, avian and human virus components."
"The virus has not been isolated in animals to date. Therefore, it is not justified to name this disease swine influenza," the Paris-based OIE said in a statement.
It said that science would show whether the virus was circulating among farm animals and the outcome should determine whether countries were justified in banning pig imports.
"Currently, only findings related to the circulation of this virus in pigs in zones of countries having human cases would justify trade measures on the importation of pigs from these countries," it said.
In an interview with AFP, OIE Director General Bernard Vallat described the virus as a "cocktail" of four different strains.
"The background of these strains has been reconstituted," he said.
"The avian strain is of American origin, and of the two swine strains, one is American origin and the other appears to be Asian. The human strain is American."
He added: "There is no proof that this virus, currently circulating among humans, really is of animal origin. There is no element to support this."
Vallat argued that "it would be really unfair to penalise pig farmers, who depend on their output for their livelihood, by talking about a risk which is not at all proven."
The OIE noted that past epidemics of human influenza epidemics with animal origin had been named after their geographical origin, such as Spanish flu or Asian flu.
"It would be logical to call this disease 'North American influenza'," it suggested.
Vallat said that "no-one, so far" had been able to show how or where the novel strain of virus had brewed.
Pigs are well-known crucibles for mixing viruses, able to harbour strains of flu that normally are specific to pigs, birds and humans.
When present in the same animal, these viruses are able to swap genes as they replicate, which can result in a new strain and leap the species barrier to humans.
However, the strain may further mutate as it is transmitted among humans, and, in addition, humans may infect pigs rather than the other way round.