Antigenic and genetic diversity among swine inf luenza viruses in Europe

H3N2 and ‘‘avian-like’’ H1N1 subtypes have circulated in European pigs since the midto late 1970s. Following reassortment to acquire six internal genes of the H1N1 viruses in the early 1980s, the H3N2 viruses have evolved to produce antigenically distinguishable variants. Sw/Finistere/127/ 99, isolated from a pig in January 1999, was shown to be closely related in antigenic and genetic characteristics to contemporary human A/Sydney/5/97-like viruses, and may represent the emergence of another ‘‘human-like’’ virus in European pigs. The H1N1 viruses circulating in pigs in France have evolved gradually over the past 20 years, but are in general closely related to early isolates such as Sw/Finistere/2899/82. An antigenically distinct variant, represented by Sw/Ille et Vilaine/1455/99, represents a significant drift from other recent French H1N1 viruses. Most of the H1N2 reassortant viruses isolated in France and Italy since 1997 were shown to be genetically similar to the H1N2 viruses which have become prevalent in the U.K., but to exhibit significant variation in antigenicity. Genetic reassortment between H1N2 and H1N1 viruses has also contributed to the recent increase in diversity of viruses circulating in pigs in Europe, and emphasizes the potential difficulties in controlling influenza by vaccination. Since most of these swine viruses are resistant to amantadine, they provide an increasing pool of amantadine-resistant M genes for potential incorporation into future human viruses. D 2001 Elsevier Science B.V. All rights reserved.