Christopher W. Olsen,

Department of Pathobiological Sciences, School of Veterinary Medicine, University of Wisconsin-Madison, 2015 Linden Drive, Madison, WI 53706, USA

Excerpt-

Antigenic drift variants of these H1N1 viruses were isolated in 1991-1998, but a much more dramatic antigenic shift occurred with the emergence of H3N2 viruses in 1997-1998. In particular, H3N2 viruses with genes derived from human, swine and avian viruses have become a major cause of swine influenza in North America.

In addition, H1N2 viruses that resulted from reassortment between the triple reassortant H3N2 viruses and classical H1N1 swine viruses have been

isolated subsequently from pigs in at least six states.

Finally, avian H4N6 viruses crossed the species barrier to infect pigs in Canada in 1999.

Fortunately, these H4N6 viruses have not been isolated beyond their initial farm of origin. If these viruses spread more widely, they will represent another antigenic shift for our swine population,

and could pose a threat to the world's human population. Research on these novel viruses may offer important clues to the genetic basis for interspecies transmission of influenza viruses.

ARTICLE HERE-
http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T32-456WTFH-1&_user=10&_origUdi=B7581-47W60JD-17&_fmt=high&_coverDate=05%2F10%2F2002&_rdoc=1&_orig=article&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=0f35aad8b1aab4491a6ddba4da301abd