The most recent introduction of an avian influenza A virus without rea
ssortment into mammals occurred in 1979 when H1N1 strains could be iso
lated from diseased pigs in northern Europe. This newly introduced avi
an virus formed a stable lineage in pigs and, in the meantime, spread
all over Europe. In 1991 highly pathogenic H1N1 strains closely relate
d to a contemporary swine virus were isolated from turkeys of a breedi
ng farm near Bremen, Germany, Outbreaks in several farms in Germany, F
rance, and the Netherlands indicate that the ''avian-like'' swine viru
ses can easily be reintroduced into an avian population causing severe
economical losses. (C) 1994 Academic Press, Inc.