Abstract
Highly pathogenic avian influenza H5N1 viruses are circulating in many countries. We recently discovered that these viruses have been transmitted to pigs on multiple occasions in Indonesia. To investigate whether avian H5N1 influenza viruses adapted to mammals through their introduction into pigs, we examined the growth of avian and swine isolates in cell culture and compared their pathogenicity in mice. We found that swine isolates were less virulent to mice than avian isolates, suggesting that the viruses became attenuated during their replication in pigs. Continuous surveillance of H5N1 viruses among pigs is clearly warranted.