Hsu Tien-lai, director-general of the COA's Bureau of Animal and Plant Health Inspection and Quarantine, said humans remain the most probable source of the infection because the new flu strain had only existed in humans prior to now.

The infection was discovered among a herd of 160 pigs raised on a farm in Taitung County's Guanshan area after the animals came down with coughs, runny noses and diarrhea.

As the animals were bred and raised on the farm and had never left it since being born, it sparked immediate speculation that the virus could have been passed from an infected worker on the farm to a pig before spreading through the herd.

The county's Public Health Bureau announced yesterday, however, that the farm's six workers had all tested negative for the virus.

Based on the results, the bureau's director-general Lu Chiao-yang said the virus could have been transmitted to the animals by other agents, including birds.

But Hsu disagreed, arguing that just because the workers tested negative for the virus did not mean they had never been infected.

The Centers for Disease Control (CDC) will conduct more sophisticated tests on the workers' specimens, and the results will be available in two to three days, Lu said. Meanwhile, CDC Deputy Director-General Chou Jih-haw said a "human-to-pig" transmission is the most probable scenario in this case, although the transmission might not have been through direct contact.

The pigs could have been infected by eating food that was contaminated with the secretion of an infected patient, Chou said.

Since the A(H1N1) outbreak began earlier this year, there have been reports of the infection in pigs, cats, turkeys and martens, and the means of transmission were either from humans to animals or between animals of the same species, he said.